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1. filename input Python script to be checked gt if filename format match filename print Looks like a Python file OK else print This file does not have a py extension filename return filename def format info app author s Author 0 s Maintainer 1 s Credits 2 s format app author app maintainer app credits apps File 0 s Version 1 s 2 s Last Modified 3 s format app file app version app status app date out str Application Details 0 AUTHORS 1 format app s author s return out str HHH Main HHH Input get Python filename from user filename while not filename filename get file Store filename in spellbook spellbook file filename CHAPTER 7 WORKING WITH TEXT Open the file for reading script open filename TODO Test if file is valid python 3 0 if not run 2to3 on file print n TODO n Iterate through lines in script for line no line in enumerate script Check that the script calls the correct version of the interpreter if line no 0 and line usr bin env python n print Warning wrong interpreter invoked Check that the script declares Unicode encoding if line no 1 and line coding UTF8 n print Warning no text encoding declaration Next should be a
2. speed s format vars self return stats Once I have the Thing class set up nicely the code in the body of the program becomes very simple I can create instances of the objects I need at the same time as I create an inventory dictionary inventory box Thing 0 1 0 axe Weapon 75 60 50 shield Armour 60 50 23 Then I can print out the entire inventory using only a few lines print n Inventory n for key item in inventory items print key item repr item print Which gives this nicely formatted result 193 CHAPTER 9 M CLASSES gt Inventory box Price Str Speed 0 1 0 Thing 0 1 0 shield Price Str Speed 60 50 23 Armour 60 50 23 axe Price Damage Speed 75 60 50 Weapon 75 60 50 Now we re cooking with gas Emulating Existing Types Python provides a whole host of other magic methods that allow you to emulate existing types In rpcombat py I had originally implemented the Player type as a dictionary in order to facilitate the changeover I wanted my new Player class to refer to its attributes via index notation like dictionaries do so I wouldn t need to rewrite the whole program in order to get it to work with this new class In order to emulate a container type such as a list or dictionary you need to include certain recognizable methods depending on how closely you want to emulate the type Tip If you want to create a type that differs only slightl
3. You should find that the program raises a ValueError This is caused by what looks like a misprint in the data a typographical error has added an extra one to the final percentage Luckily the program spotted it because otherwise the program would overestimate the amount of gains to be made if you betted on this data set Correct the error and save and run the program again You should find that if you place just that one odds on bet you re likely to make a modest win far less than if a 111 percent probability had been able to sneak through Creating Your Own Exception Classes You can create exception classes of your own to match the internal details of your application This might be because you want to subclass existing exceptions to provide your surrounding program with more precise information Alternatively you might want exceptions that more closely match situations in which your program will be used For an example of using very specific exceptions for a particular task look at Listing 10 5 This program checks your house s security not literally of course but you can inform it whether or not you ve left the door open and it will judge your home security accordingly as you ll see Save this to a file called home security py you can run it right away but you should see a SecurityError if you do so Listing 10 5 Defining and Raising Your Own Exception Classes We need to parse a report file so import a module to help
4. gt gt gt a b what is in a that is not in b set bananas oranges gt gt gt b a what is in a and not in b set avocados grapes mangos gt gt gt alb what is in either a or b set apples grapes mangos avocados oranges bananas gt gt gt a amp b what is common between a and b set apples gt gt gt al1 Traceback most recent call last File lt stdin gt line 1 in lt module gt TypeError set object is unindexable As the members of a set are not in any particular order you can t reference them with an index 85 86 CHAPTER 5 M USING LISTS Note You can also create sets using curly braces So you could type b avocados mangos apples grapes mangos You still have to use set to create an empty list because makes a new dictionary see the Dictionaries section Dictionaries Dictionaries are like address books if you know the name of the person you can get all of that person s details The name is technically referred to as a key and any corresponding detail is referred to as the value The key must be of an immutable type that is a string number or tuple the value can be more or less anything you like The dictionary itself is a mutable data type which means you can add remove and modify key value pairs The keys are said to be mapped to objects hence dictionaries are referred to as mappings
5. where internal gt gt gt Nor vice versa the local object isn t changed by set config gt gt gt set config even newer True gt gt gt config new True The interplay of scopes separate collections of variables inside and outside the module demonstrated in Listing 11 3 is quite complex but don t worry the important point is that here we can see the config variable hiding inside the module until we explicitly imported it and you can use this in your own code to hide internal variables from the outside world Also you might notice that Python remembered the module s value of config to such an extent that when you eventually imported it Python didn t revisit the file and decide it was actually True but used the remembered value This means that if you want to reload the content of a module whose text has changed on disk Python will ignore those changes unless you use the advanced technique described later in this chapter Self documenting code is even more important than usual if you re writing something that other people or you in many months time will need to be able to understand at a glance without reading 247 248 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES the whole file If you include documentation strings discussed in Chapter 6 in your modules and in the classes and methods contained in them anyone importing your module into the interactive prompt will have acce
6. curtain width float window width 0 75 20 print t round curtain width 2 curtain length float window height 15 print t t round curtain length 2 Now I need to work out how many widths of cloth will be needed and figure out the total length of material for each curtain in cm still CHAPTER 4 If the length of the curtains is less than the roll width I can turn the whole thing on its side and just use one width of fabric but if the curtains need to be both longer and wider than the roll width then I have a further problem If the extra material required is less than half the roll width I would need to buy an additional width of material at the same length If it is more than half then I would need to buy two additional widths print t t t t t t curtain length lt roll width print t t t t t t t curtain width gt roll width if curtain length lt roll width total length curtain width 2 100 elif curtain width gt roll width widths int curtain width roll width extra material curtain width roll width if extra material lt roll width 2 widths 1 if extra material gt roll width 2 widths 2 print t t t widths total length curtain length widths 100 HE H He HE HE HE else total length curtain length 2 100 print t t t t round total length 2 Finally I need to work out how much it will cost Rounded to two decimal places using
7. 1 9 loop while purchase not no 1 9 1 Display shop stock list with prices 1 9 2 Prompt user to make a purchase 1 9 3 If the item is in stock and the player has enough gold buy it 1 10 Add new player to list of players Combat Set up descriptive lists hits misses damage report life changing 2 Prompt user to enter into combat Prompt user to choose a weapon The weapon must be in player s inventory Default to fist if weapon not available 3 Loop while attacker health gt 0 and target health gt 0 3 1 for player in players 3 1 1 Calculate velocity of blow 3 1 2 Calculate damage inflicted by blow 3 1 3 Print damage report 3 1 4 if attacker health gt 0 and target health gt 0 break 3 2 Print progress report 4 Print winner This translates fairly easily into Python as shown in Listing 5 2 Immediately after the docstring and the magic variables at the beginning of Listing 5 2 is the import random statement We can now use random methods just the same as any built in object The following block are the constants all the salable items in a dictionary called stock each item containing a price damage speed tuple a set containing the items which are some sort of armor and four tuples containing verb phrases for the hits and misses and noun phrases for the damage report and life changing output It s important to have the form of the final output in mind when desig
8. Create command line command ref self self choice replace func command eval command ref GUI output Make clean textbuffer self textbuffer gtk TextBuffer None Create headline style self headline self textbuffer create tag headline weight 700 scale pango SCALE LARGE navigate to start of buffer place self textbuffer get start iter 275 276 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Create pixbuf icon reference icon eval self self choice replace image Insert icon at top of page self textbuffer insert pixbuf place icon Print appropriate text underneath text self choice n n commands getoutput command self textbuffer insert at cursor text itero self textbuffer get iter at line 0 iter1 self textbuffer get iter at line 1 self textbuffer apply tag self headline itero iter1 self textview1 set buffer self textbuffer return The menu item File gt Save is connected to the on save as method which is supposed to create a Save As dialog to specify file to open for writing Fortunately Gtk provides a handy gtk FileChooserDialog method with which to perform this function the dialog retrieves the filename using the get filename method Running the dialog also returns a response which needs to be handled If the dialog receives the OK signal from the user it fetches the output from the Bash commands for each of the headings
9. Inside the box Outside the box pete 2 builtins lt module builtins built in gt file test py fred 3 dave 3 cardboard box lt function cardboard box at Oxb7c4d86c gt name main package None doc Dave the cardboard box n nA simple scope example n 107 108 CHAPTER 6 FUNCTIONS Manipulating Lists and Dictionaries Parameters to a function are passed by value However in the case of objects unlike simple types such as integers and strings if you modify the parts of an object including items in a list or a dictionary the changes are reflected outside the function as well This means that a function can be used to modify a mutable data type such as a dictionary or list Technically such a modification procedure is not a pure function as it neither needs a return value or indeed any input parameters In order to avoid strange side effects a good general policy is to send and return immutable values from functions and keep any modification procedures for your mutable types separate If you send a list or dictionary to a function all you are doing is sending it a pointer to the same global instance of that object just as if you had written local list global list Please review Chapter 5 if you ve forgotten how this works gt gt gt global list gt gt gt def modify list local list local list extend dave pete fred gt gt gt resul
10. You can search for strings from the end of the string as well using string rindex sub start end gt gt gt s rindex a 0 23 10 You can also count the number of times a substring occurs in a string using string count sub start end Start and end values work like slice notation again gt gt gt s count a 3 Strings have an enormous number of methods this section provides just a selection of the most immediately useful ones Using string methods alone it is possible to pretty up the output of your programs considerably be more flexible and possibly even improve the grammar of your application s printout Matching Patterns Using Regular Expressions Sometimes the basic string methods just aren t enough For example you may need to retrieve values that appear within a regular pattern in a piece of text but you don t know what those values are going to be or just have a rough idea of what they should not be This is where regular expressions come in A regular expression regex for short is a pattern that can match a piece of text In its simplest form a regular expression could just be a plain string of ordinary characters that matches itself Regular expression syntax uses additional special characters to recognize a range of possibilities that can be matched These expressions can be used in search and replace operations and to split up text in different way than string split 135 136 CHAPT
11. You may be able to set the executable bit using your file browser look for an option to set the permissions or properties of the file Caution Some tutorials may suggest setting the permissions to 777 or a rwx this is a bad habit to get into because it allows other users to edit and execute your files and therefore creates a potential security problem You may find it useful to open up the permissions of a file in this way to troubleshoot permissions related problems during development but it is wise to close permissions down to a minimum before the application goes live in any public situation You should read your system s documentation to find out how it controls permissions CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Now you can run your program from the command line like this myscript py This works so long as you are in the same directory as the script You can also run the script by clicking or double clicking its icon in your file browser depending on your operating system If you want this script to be accessible to anyone anywhere on the system you need to create a link to the script in one of the directories in the system s PATH To find out which directories are in your PATH echo PATH You can add directories to your PATH if you need to create a location that you have permission to write to PATH HOME bin PATH export PATH echo PATH home tim bin usr local bin
12. data file open rpcharacters rpg w lines for player in players lines append repr player n data file writelines lines data file close return And here s the data file itself rpcharacters rpg life 30 Name Ez gold 7 Gender female armour 0 0 50 Race Pixie Muscle 46 Brainz 37 magic 30 weapon 60 60 50 prot 23 Charm 46 inventory sword Speed 28 Desc Tall and wild life 20 Name Inkon gold 9 Gender male armour 0 0 50 Race Troll Muscle 35 Brainz 45 magic 31 weapon 50 60 55 prot 16 Charm 34 inventory flail Speed 11 Desc Small and clever The second function I created read in is shown in Listing 7 8 It simply has to iterate through the lines in the file which is opened in read only mode and assign the evaluated line to player which is then packed back into the players list Don t forget to close the file each time to prevent accidental corruption of the data 157 CHAPTER 7 WORKING WITH TEXT Listing 7 8 Function to Read in Data from a File def read in print Reading in data data file open rpcharacters rpg for line in data file player eval line players append player data file close print players return Just to complete things for this chapter I ll run
13. desc capitalize def getDesc self return self desc def setGender self gender gender gender lower if gender startswith f self gender female elif gender startswith m CHAPTER 9 E CLASSES self gender male else self gender neuter def getGender self return self gender def setRace self race race race capitalize if race startswith P self race Pixie elif race startswith V self race Vulcan elif race startswith G self race Gelfling elif race startswith T self race Troll else self race Goblin def getRace self return self race def setMuscle self self muscle roll 33 3 def getMuscle self return self muscle def setBrainz self self brainz roll 33 3 def getBrainz self return self brainz def setSpeed self self speed roll 33 3 def getSpeed self return self speed def setCharm self self charm roll 33 3 def getCharm self return self charm def setLife self self life int self getMuscle self getSpeed 2 roll 49 1 2 if 0 lt self life lt 100 pass else self life int roll 33 3 187 188 CHAPTER 9 M CLASSES def getLife self return self life def setMagic self self magic int self getBrainz self getCharm 2 roll 49 1 2 if 0 lt self magic lt 100 pass else self magic int roll 33 3 def getMagic self return self magic def se
14. if gender female sentence phrase replace themself herself elif gender male sentence phrase replace themself himself else sentence phrase replace themself itself CHAPTER 6 M FUNCTIONS return sentence def generate rpc Role Playing Character generator Takes no arguments Returns a new character profile dictionary print print New Character print Create empty profile dictionary profile Name Desc Gender Race Muscle 0 Brainz 0 Speed 0 Charm 0 life 0 magic 0 prot 0 gold 0 inventory Prompt user for user defined information Name Desc Gender Race name input What is your name desc input Describe yourself gender input What Gender are you male female unsure race input What Race are you Pixie Vulcan Gelfling Troll Validate user input profile Name name capitalize profile Desc desc capitalize gender gender lower if gender startswith f profile Gender female elif gender startswith m profile Gender male else profile Gender neuter race race capitalize if race startswith P profile Race Pixie elif race startswith V profile Race Vulcan elif race startswith G profile Race Gelfling elif race startswith T profile Race Troll else profile Race Goblin Generate sta
15. main window1 Window1 window1 run name main main main 281 282 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS This method makes developing new graphic interfaces remarkably quick although you will still need to get your hands dirty with the Gtk library to make the most of what it provides Using Glade and tepache like this you can create sophisticated interfaces able to cope with most requirements you may dream up Jargon Busting Here are the terms from this chapter e Glade This user interface designer for GTK is available at http glade gnome org e GTK This is a cross platform GUI toolkit available at http www gtk org e Signal An event such as a mouse down or hover triggers a signal to a widget to call an appropriately assigned function for the event e tepeche This utility generates a Python framework from a Glade design e Tkinter This is GUI interface building toolkit available at http tkinter unpythonic net wiki FrontPage e Widget This graphical user interface GUD element displays information or provides controls to the user such as a window button or text box Summary Award yourself the rest of the day off or failing that your favorite beverage and a light snack You ve just finished the final chapter in the book We covered a fair amount in this chapter too including four major tools for building GUIs in Python It s clear that Python 3 isn t yet the best languag
16. print tCharm profile Charm tgold profile gold print Prompt user to buy some equipment purchase input Would you like to buy some equipment while purchase done Display shop stock list with prices print print lt SHOP gt for item in stock print t item stock item 0 print lt gt print print You have profile gold gold Prompt user to make a purchase purchase input Please choose an item or type done to quit If the item is in stock and the player has enough gold buy it if purchase in stock USING LISTS 95 96 CHAPTER 5 M USING LISTS if stock purchase 0 lt profile gold print You buy a purchase for stock purchase 0 gold pieces profile gold stock purchase 0 profile inventory append purchase print You have a join profile inventory in your bag print You have profile gold left elif purchase done break else print You don t have enough gold to buy that else print We don t have purchase in stock print You own a join profile inventory Prompt user to enter into combat and choose a weapon print profile Name Are you ready for mortal combat weapon input Then choose your weapon The weapon must be in player s inventory Default to fist if weapon not available weapon weapon lower if weapo
17. print players target Name collapses in a pool of blood print players attacker Name wins the fight print break if trace print print t tmax dam max vel max min vel min print There are several things to be said about Listing 5 2 It is very long and repetitious and by far the longest script you have encountered so far in this book The first thing that needs to be done is to get rid of all unnecessary repetitions This is always true in situations where removing repetition makes the code easier to read Also some of its methods are rather brutal and rely on receiving the right data in the first place How could the error checking and validation be improved to make sure the internal values stay within useful ranges The output is rather gruesome and not particularly grammatical so how could the output be improved Perhaps it could describe a magical battle a war of rhetoric and wit or something silly and fun I m sure you could improve it Development on this version 0 1 of the script stopped as soon as it fulfilled the test requirements that I had set Any improvements new features or fixes mean going back to the pseudocode stage and sketching out the changes as well as setting new tests to check whether the new code works before you do any further coding When you do get that far you need to bump up the version number because you are now working on version 0 2 If you feel adventurous you could
18. var1 lt 6 or var2 gt 7 This returns True if either of the conditions are satisfied Using Assignment Operators You are already familiar with the basic assignment operator which you use when you want to put a value into a variable Python allows you to use some neat typing and time saving tricks with assignments The humble equals sign can unpack sequences which will be covered fully in the next chapter The simplest form of this can unpack a string into its constituent characters gt gt gt char1 char2 char3 cat gt gt gt chart re gt gt gt char2 ra gt gt gt char3 t In addition you can swap the values of two variables using the following technique 53 54 CHAPTER 4 MAKING CHOICES height length length height You can also assign the same value to several different variables X YFZe 1 The assignment operator can be used in conjunction with most arithmetic operators to modify a variable in place For example you could type counter 1 which is the same as counter counter 1 Some other possible combinations are which decrease the value of the variable by the specified amount multiply the value of the variable by the specified amount and divide the value of the variable by the specified amount Understanding Operator Precedence As discussed in the previous chapter the order in which you use operators does matter The best way to avoid confusion or unwante
19. version 0 1 Name 5 un Desc Gender Race un Prompt user for user defined information Name input What is your Name Desc input Describe yourself Gender input What Gender are you male female unsure Race input What fantasy Race are you Pixie Vulcan Gelfling Troll Output the character sheet fancy line lt 4 7 gt print n fancy line print t Name print t Race Gender print t Desc print fancy line n Listing 3 1 is just a fancy version of hello world but the output looks a bit more exciting You can run this from the command line in the same way you will see all the values you enter displayed as a character sheet and formatted by the escape sequences python chargen py www allitebooks com CHAPTER 3 VARIABLES AND DATA TYPES Note If you are in a different directory to your script you will have to supply the full path to its location The one new thing I ve added is the line version 0 1 atthe beginning This is a magic variable the name version with two underscores on either side is a predefined variable with a special meaning to Python s documentation tools For now I m just going to use this to record the version number of this program as I edit and refine the design I will increment this number Next I need to generate some vital statistics to represent the different attributes that the
20. Test values Ez Tall Pixie female Inkon Small Troll male Expected results All statistics should be integers in the 1 99 range CHAPTER 6 M FUNCTIONS Apart from that this script needs play testing Limitations Too many to mention version 0 2 maintainer maintainer website com status Prototype Import modules import random set up constant data stock shield 15 25 50 sword 60 60 50 dagger 25 40 60 halberd 80 75 40 club 15 40 40 flail 50 60 55 hammer 99 100 40 cuirass 30 50 20 armour 101 100 0 lantern 10 5 30 pole 10 5 50 rope 10 5 70 armour types set shield cuirass armour hits hits bashes smites whacks shreds mutilates lacerates annihilates misses misses nearly hits fails to connect with swipes wildly at flails ineffectually at gets nowhere near nearly decapitates themself instead of hits themself on the foot to the amusement of damage report small insult flesh wound deep slash ragged gash savage laceration fractured rib cage smashed up face split skull life changing a scar bruising serious blood loss total debilitation chronic concussion a severed limb multiple fractures an amputated head Preferences Set to True to trace variables trace False reply input H
21. defined 241 242 260 examples of CSV 256 257 datetime 257 258 overview 256 urllib 258 259 flexible importing 244 246 importing 170 172 internals 250 251 overview 241 packages creating 252 254 defined 252 future 259 installing contributed 254 255 reloading dynamically 249 250 standard 254 structuring 246 249 terminology 259 260 using in other programs 243 244 multidimensional lists 83 mutable data type 86 mutable lists 75 mutable sequences 195 mutable value defined 99 mystring center width fillchar method 129 my_string split sep maxsplit 126 N name binding operation 185 namespace 185 defined 220 naming conventions functions 102 natural language defined 6 nesting 15 24 conditional statements 69 quotes 33 NoneType variable 31 None value 31 nonfunctional requirements 14 24 nongreedy expression 136 nongreedy regular expressions defined 159 289 290 INDEX null statement 67 defined 74 number applications creating 43 45 numbers base 16 hexadecimal 41 42 base 8 octal 41 42 base 8 octal and base 16 hexadecimal 41 42 basic mathematical operations 38 39 binary 37 Booleans 37 38 converting 40 41 creating number applications 43 45 dividing 39 40 fractions and flats 40 integers 38 operator precedence 39 overview 37 numerical operators methods implementing 198 numeric types emulating 197 199 O object __contains__ self
22. for result in reversed sorted results print t result print You may guess from the use of double underscores that there is magic afoot If the program has been run directly rather than being imported as a module the magic variable name will contain the value main In this case the main func is called and results are output If the script was imported as a module none of the code would actually be directly available but the functions would all be available if declared public to the calling procedure gt gt gt import style guide gt gt gt help style guide If you actually want the program s output you would have to explicitly call the style guide s main func gt gt gt style guide main func 0 5 1 6499999999999999 2 4199999999999999 1 5 2 6499999999999999 3 4199999999999999 2 5 3 6499999999999999 4 4199999999999999 You will have noticed that it is standard practice to separate function definitions with blank lines the same is true for classes and their methods as you will see in the next chapter It is acceptable to use extra blank lines to group related functions and logical sections of code together You should avoid putting extra whitespace characters immediately inside parentheses before commas colons and semicolons and inside expressions the main exception being that you should put a space either side of operators You should also try to avoid double and triple nested loops try to
23. gt 49 gt 37 True gt gt gt 7 65 6 0 True gt gt gt 5 lt 2 True gt gt gt 7 lt 7 False gt gt gt 23 5 gt 37 75 False gt gt gt 5 gt 5 False gt gt gt 3 2 l 3 2 False You can also create conditional expressions using variables gt gt gt variable 3 0 gt gt gt variable True Caution The single equals sign assigns a value whereas the double equals sign compares two values and returns True if they are equal in value These two operations are commonly confused in the early stages of learning programming If you use the assignment operator when you mean to compare two values you will probably get a syntax error If you use a comparison operator when you intended to assign a value the statement CHAPTER 4 E MAKING CHOICES may well silently fail and you won t find out about it until you get some weird value occurring later on in the execution of your script Also strings can be compared alphabetically using the same comparison operators gt gt gt alpha lt omega True gt gt gt case Case False gt gt gt net lt network True gt gt gt Same Same False Notice that uppercase characters don t count the same as their lowercase counterparts All of the uppercase letters come before any of the lowercase If you are not familiar with this method of sorting the order is laid out in Figure 4 1 Figure 4 1 Case sensitive a
24. gt gt apple in fruits False It is possible to use not in conjunction with in to produce the opposite answer gt gt gt apple not in fruits True You can also use this operator on strings to find out whether x is contained in i e is a substring of string s gt gt gt ana in banana True You can use the operator to combine sequences to make a new sequence gt gt gt pine apple pineapple You can also use the operator on sequences to create multiple copies glued together gt gt gt heart 5 heartheartheartheartheart The best method for joining strings in a sequence is separator join seq It uses an apparently bizarre notation which deserves a little further explanation You will have gathered by now that the different data types all have a different set of operations that can be used with them These operations can be carried out by operators or methods Operators are commands written as a symbol etc or keyword in and or etc between two operands Methods are accessed using a command that follows the variable name immediately after a period dot and terminates in parentheses which may or may not contain further arguments In separator join seq join is the method Methods are like functions that are specific to an object such as a data type in this instance objects are covered in Chapters 9 and 10 For now objects are specific instances of data type
25. gt gt print 13 5 2 39 40 CHAPTER 3 M VARIABLES AND DATA TYPES This is because floor division returns the largest whole number that is less than the result of the fractional division So 13 5 is 2 6 and the largest whole number less than 2 6 is 3 Now consider the remainder the result of multiplied by the divisor gives us the figure to work from to calculate the remainder as it would in all other remainder calculations 13 3 5 remainder Consider another example gt gt gt print 13 5 2 gt gt gt print 13 5 3 Here 2 is the largest whole number less than 2 6 and 3 is calculated as follows 13 2 5 remainder Alternatively you could avoid using this operator with negative numbers Working with Fractions and Floats Fractional numbers are expressed using numbers before and after a decimal point using the float type Like integers these numbers can be positive or negative You don t have to do anything particularly special to assign a float to a variable if you use a decimal point in the number Python will assume that it s a float muscle 2 9 brainz 13 678 speed 0 0 Even if the part after the decimal point is zero the number is considered to be a float Floats can be manipulated using the same operators as integers returning any fractional part as well Converting One Type into Another Python has several convenient built in functions that allow you to convert
26. in lt module gt KeyError foo gt gt gt try foo 225 226 CHAPTER 10 EXCEPTIONS except KeyError as e raise Traceback most recent call last File lt stdin gt line 1 in lt module gt KeyError foo Imagine you re thinking of placing bets on the chances of some events happening But you re a busy person so you want to write a program that takes all the bets for a given day and a minimum likelihood above which you re happy to bet and gives you your total expected winnings if you place all of those bets You already have two existing bits of code for keeping track of how likely it is that things might happen you might be controlling a complicated scientific experiment or predicting the behavior of financial markets However one set uses percentages and the other uses probabilities and you d now like to use them together You could either rewrite one or other set of the existing bits of code or you could write two conversion methods to go from percentage to fractional probability and back But although generally speaking percentages can be greater than 100 percent fractional probabilities can t be greater than 1 So you might want to receive a warning if your combined program tries to convert a percentage over 100 percent into a probability Listing 10 4 shows a script that brings together your two lots of code and tells you how much you will win by placing all of the odds on bets Save this to a fi
27. index stock item 0 The user is prompted to make a choice the choice is added to the current player s inventory and the price is deducted from the player s gold supply Finally the user is prompted to choose a weapon and if that weapon is not in the player s inventory a fist is provided instead Actually what happens is the player gets the relevant tuple from the stock dictionary loaded into the weapon stat The inventory is checked against the armour types set and the resulting tuple gets loaded into the player s armour stat The complete profile is then written out to the player s list The combat takes place continuously inside a while statement that checks both players statistics to make sure neither player is dead Nested within that is a for loop that toggles between the players giving each a chance to play by setting one as the attacker and the other as target It does so by getting the player s index from players using the enumerate function and then turning that into a Boolean value When the player s turn ends the Boolean is reversed with the not operator and assigned to target Now I can use the target and attacker indices to refer to both members of the players list In some places I have to refer to values nested three deep in players whether dictionaries lists or tuples The syntax is fairly straightforward so long as you can remember which structure is nested inside which so making some notes is probably
28. inherited by gtk Hbox child is the widget to be packed and expand and fill are Booleans indicating whether or not the child widget fills the available space in each direction according to the packing direction of the container And padding is the number of pixels between each widget This graphical Hello World example isn t particularly big or clever but it does provide a nice visual representation of how an object oriented graphical application works The principles scale well for small to medium sized applications and the Gtk library provides the majority of the functionality you would expect from a standard windowed application so building bigger applications is simply a question of adding more building blocks Building Complex Interfaces Now you know how to put a tiny window with a couple of buttons in it on the screen The point of creating graphic interfaces is to provide a reassuring user friendly window that the user can point and click within instead of having to follow some arcane command line procedure For the final example in this chapter I m going to show you how to wrap up some command line instructions in a smart Gtk interface My main functional requirement is that the application should display some system information in a window I also want to be able to save that information as a text file I have a Bash script that I ve been using to provide diagnostics for my audio system This script is all well and good for those
29. item method 195 object __delattr__ self name method 197 object __delitem__ self key method 195 object __getattribute__ self name method 197 object __getattr__ self name method 197 object __getitem__ self key 194 object __len__ self method 194 object orientation 181 object __repr__ self function 193 objects defined 124 manipulating lists and dictionaries 108 object __setattr__ self name value method 197 object __setitem__ self key value method 195 octal base 8 defined 46 oct x function 42 open filename mode buffering function 141 open function 142 operands 32 50 defined 74 operating system 6 operations defined 99 operator 32 38 77 defined 46 overloading 192 operator precedence 39 54 55 defined 46 organizing projects 164 os system 268 method 278 overloading 32 built in functions 200 inclusive numerical operators 199 operator 192 rich comparison operators 197 P packages defined 260 parent defined 220 parentheses tuples and 79 plus sign 136 pointer 108 defined 124 polymorphism 184 defined 220 pop method 82 positional arguments 103 defined 124 print function 18 31 38 127 155 privileged information accessing defining functions 101 102 overview 101 passing unknown number of values into function 103 104 sending invitations 102 103 using docstrings to document function 104 program defined 3 7 programming env
30. loop which means that certain statements are executed repeatedly There are two main methods of creating a loop in Python the while and for statements Let s look at the while statement first Looping with the while Statement Listing 4 4 uses the while keyword which will execute the indented statements that follow after the colon while the given condition is True The indented statements after the colon are referred to as the loop body They are indented in the same way as the if statements covered previously in this chapter Listing 4 4 Looping Using a while Statement result 1 while result lt 1000 result 2 print result Recall that result 2is the same as result result 2 so the number printed is doubled during each iteration This is the same as starting with 2 and raising the power by one during each iteration i e 27 23 24 and so on Note An iteration is one repetition of a series of statements such as those found in a loop body To control the number of times the loop is processed it is necessary to specify a conditional expression as long as this conditional expression is True at the beginning of an iteration the loop continues In the preceding example our conditional expression is result lt 1000 So as long as the value of result is less than 1 000 the loop will continue processing Once result reaches 1 024 21 the program will stop processing the loop body The variables used in the co
31. mystring value 0 equals 1 message gt gt gt mystring format x 23 message ok value x equals 23 ok The replacement field names inside the curly braces can also include attributes and element indexes such as someClass data or some list i This field name may be followed by an optional conversion field which is preceded by an exclamation point and can be modified using format specifiers which are preceded with a colon The conversion field is used to force an item to be represented as a particular type regardless of its own type s formatting rules This can either be s which forces a string representation through str or lr which calls repr on the value gt gt gt mystring value 0 s equals 1 r message s gt gt gt mystring format z 37 message well you know value z equals 37 well you know Format Specification Format specification is done using its own built in minilanguage The format specification string is a sequence of characters each with its own special meaning The format specifier field contains a specification of how the value should be presented that is padding alignment sign field width decimal precision and type All these items are optional and follow the colon The first two characters after the colon work together The first character may be any character to be used as a fill character so long as it is followed by one of t
32. refactor the nested blocks as functions to reduce the amount of indentation 169 170 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Importing Modules Modules are imported using the import statement either via the interactive command line or at the start of a script An example would be import foo module gt gt gt import sys This will import everything from the module named sys The sys module contains some very useful dynamic objects e sys argv A list of command line arguments argv 0 contains the path to the script itself e sys stdin The standard input file object as used by input e sys stdout The standard output file object as used by print e sys stderr The standard error object that is used for error messages These objects allow you to pass data in and out of the program via command line arguments as in sys argv or piped in via sys stdin I ll show you how to do this in a moment The objects function and attributes of a module are usually accessed using dot notation so you would write module object to use the object from module in your script for example If you wanted to read in data from stdin you would write sys stdin read which calls the read method on the stdin file like object in module sys The import statement can take a couple of other forms The first is this gt gt gt from sys import argv This will only import the argv object so it can now be accessed wi
33. used to separate words or units of data The list will be split up to maxsplit times so you ll end up with a list that s maxsplit 1 items long If no separator is specified the string will be split up by whitespace characters as if they are words gt gt gt sent4 A much much longer sentence gt gt gt sent4 rstrip sentence split A much much longer You can do this from the end of the string too using string rsplit sep maxsplit gt gt gt sent4 rsplit 2 A much much longer sentence Python has an alternative string splitting method string partition sep which returns a tuple head sep tail The method searches for the separator sep inside the string and returns the part before it the separator itself and the part after it as separate items If the separator is not found the method will return the original string and two empty strings gt gt gt sent3 eXamPLe tEXt gt gt gt sent3 partition eXamPLe tEXt gt gt gt sent3 partition eXamPLe tEXt CHAPTER 7 WORKING WITH TEXT You can do the same thing starting from the end of the string with string rpartition sep gt gt gt s rpartition a at Z a D j Vu a 3 hu Joining Strings or Avoiding Concatenation As I mentioned when I first introduced the string data type using the operator to join strings together is ver
34. validation 25 56 defined 74 values passing unknown number into function 103 104 values method 87 variables 18 25 27 47 assigning values to 28 29 creating 28 29 identifiers selecting keywords 27 28 naming conventions 28 overview 27 overview 27 terminology 45 47 types of dynamic typing 29 31 None value 31 numbers 37 45 overview 29 text fragments joining 31 37 variable scope 105 108 manipulating lists and dictionaries 108 overview 105 107 using global statement 107 vars function 105 107 version number 2 defined 7 views 88 W X Y weak internal use indicator defined 260 weakly internal variables 246 web based services CGI forms 177 178 overview 173 176 weightings variable 244 whence argument 144 while loop flowchart for 67 while statement 69 break keyword 67 68 continue keyword 67 68 nesting conditional statements 69 overview 65 67 whitespace characters 34 35 126 defined 159 widgets 262 282 wildcard character 136 defined 159 windowed applications 261 282 INDEX Glade 279 282 overview 261 PyGTK commands 272 279 complex interfaces 268 269 GUI 269 271 overview 265 267 signal response 267 268 tepache 279 282 terminology 282 Tkinter 261 264 Windows installing Python on 2 write_ out players 157 write w mode 142 writing to files 142 143 Z ZeroDivisionError 232 zero padding 131 ZIP files 255 zip function
35. 228 self documenting 6 10 code 247 self parameter 183 sentinel defined 74 sentinel value 66 separator join seq 77 separators defined 99 sequence packing 80 defined 99 sequences 29 accessing items in 75 76 defined 46 99 determining if items are in 77 79 sets 85 show method 268 side effects defined 124 signals 267 282 simplification 10 single equals sign 50 single stage traceback 235 slices 76 defined 99 software development cycle 5 defined 7 sorted function 89 sorted list method 82 sorting dictionaries 89 lists 82 83 source data 105 defined 124 splitting strings 126 127 stack 82 defined 100 stand alone applications creating 161 164 standard library 251 defined 260 state defined 220 statement 25 29 defined 46 statically typing 45 storing data 156 158 string capitalize method 128 string count sub start end method 135 string expandtabs tabsize method 129 string format args kwargs 129 string format method 155 string join method 127 string literals 29 32 defined 47 string ljust width fillchar method 129 string partition sep method 126 string replace method 145 string rindex sub start end method 135 string rsplit sep maxsplit 126 string rstrip0 method 145 strings 25 75 125 135 advanced formatting 129 133 changing case 128 129 editing 134 finding 134 135 finding patterns at beginning or
36. 29999999999999999 gt gt gt Now import each method from convert into the current scope from convert import gt gt gt convert string lt function convert at 0xb7c1b82c gt gt gt gt weightings 0 20000000000000001 0 29999999999999999 This is simple enough to use but it s untidy practice all of the contents are imported from the module regardless of whether they are used or not and in the second instance some contents might overwrite existing variables Instead you can specify each item you want to import and even what name you want to import it with using variants on the above from syntax gt gt gt Single basic import clause one method from convert import piratify gt gt gt piratify lt function piratify at Oxb7c6b0ac gt gt gt gt Import clause with as sub clause to rename the method This avoids overwriting an existing convert string in the scope from convert import convert string as convertiferize gt gt gt convertiferize lt function convert string at Oxb7cOf82c gt gt gt gt convert itself imports the random module into its own namespace so you can in turn import the random module from convert from convert import random gt gt gt random lt module random from gt gt gt gt Several clauses including renaming can be separated with commas gt gt gt from convert import yarr weightings as weightables gt gt gt y
37. 4 oranges grapes gt gt gt fruits 1 4 2 bananas grapes wa If the starting point i is equal to or greater than end point j the slice you get back will be empty gt gt gt fruits 4 4 2 If you leave out the starting point the slice will start at the beginning of the original sequence and if you leave out the end point the slice will run to the end of the original sequence Indexing and slicing do not modify the original sequence they make a new sequence from the original However the actual individual data items are the same So if you modify an individual item in a sequence you will see the item change in a slice from the sequence as well CHAPTER 5 M USING LISTS Note list alias sequence does not make a copy of the list Instead list alias and sequence are the same i e they are two names references pointing to the same list So if you perform any operation on list alias you will see the change in sequence as well If you do new sequence sequence then operations on new sequence won t affect sequence Beware of this difference If You re Not on the List You Can t Come In Often you will want to find out whether a particular piece of data is in a sequence or not This is achieved in Python using the keyword in which is a Boolean operator that tests for membership in a sequence The test x in s will return True if x is in sequence s otherwise it will return False gt
38. AttributeError return None To people used to looking before they leap Python s approach can feel dangerous LBYL is a very risk averse strategy and implicitly perceives exceptions as risks But exceptions in Python aren t risks they re just another part of the language and so EAFP is entirely suitable in that context Of course you have to be careful to keep your error handling tight in the EAFP example in Listing 10 15 if some object result were itself to raise an exception then handling all exceptions of class Exception would mean your program is ignorant to any problems occurring inside that method As it is an AttributeError from deep within the result method will be ignored in the example To program in a Pythonic way is to follow a number of different interlocking conventions EAFP itself isn t an important one but it embodies many of the other Pythonic conventions These are summarized online in the Pythonic Enhancement Proposal 8 PEP 0008 and are a subject of much discussion among the larger Python community Pythonic conventions are to be encouraged among Python programmers and many of these have reasons that go to the heart of Python s core principles So don t be afraid to use exception raising and handling at the heart of your Python programs your code will only be the more Pythonic for it CHAPTER 10 EXCEPTIONS Jargon Busting Here is some further explanation of the terms presented in this chapter C
39. Create GUI output Make clean textbuffer self textbuffer gtk TextBuffer None Create headline style self headline self textbuffer create tag headline weight 700 scale pango SCALE LARGE navigate to start of buffer place self textbuffer get start iter Create pixbuf icon reference icon eval self save image Insert icon at top of page self textbuffer insert pixbuf place icon Print appropriate text underneath text Saving Information To file s outfile name self textbuffer insert at cursor text itero self textbuffer get iter at line 0 iter1 self textbuffer get iter at line 1 self textbuffer apply tag self headline itero iter1 self textviewl set buffer self textbuffer elif response gtk RESPONSE CANCEL print Save As cancelled dialog destroy return The code for on quit is easy As I have already defined a delete event method this method can be used by the File gt Quit menu entry as well def on quit self widget Quit the application self delete event Quit Quit return Edit gt Copy turns out to be a one liner also The textbuffer has a built in method to transfer its contents to the clipboard named sensibly enough textbuffer copy clipboard self clipboard 277 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS def on copy self widget nun Copy Copies selected information to the clipboard self textbuffer copy clipboard self clipboard re
40. Generate characters while len players lt max players Call character generation function profile generate rpc Go shopping if the inventory is empty shopping profile inventory while shopping shopping buy equipment handbag join with and profile inventory print You own a handbag Choose a weapon print profile Name prepare for mortal combat See if player has any weapons weapon stats stock item for item in profile inventory if item not in armour types if len weapon stats profile weapon weapon stats 0 elif len weapon stats lt 1 profile weapon 0 20 50 else weapon input And choose your weapon The weapon must be in player s inventory Default to fist if weapon not available weapon weapon lower if weapon in profile inventory profile weapon stock weapon else profile weapon 0 20 50 See if player has any armor armour stats stock item for item in profile inventory if item in armour types if armour stats profile armour armour stats 0 else profile armour 0 0 50 CHAPTER 6 M FUNCTIONS print profile Name is now ready for battle Add new player to list of players players append profile Combat print print Then let the combat begin print vel max 23 vel min 1 dam max 23 Loop while more than one player is still alive while len players gt 1
41. I snuck the random module from the standard library into Listing 11 2 To help you find the functionality you need in the standard library a comprehensive and up to date list of all the modules in it is available in Python s own online documentation There s code in there to support string formatting and regular expressions as well as interprocess communication and the operating system There s also a large library containing functions for date and time parsing and formatting and for working with calendars You can process audio and video using multimedia functionality and if you re working with web sites APIs and similar mash up technology there s a long list of modules to interface with such technologies as HTTP FTP and e mail Installing Contributed Packages Eventually you will decide that the core library doesn t provide you with enough functionality and that to write it yourself would take you some considerable time Luckily the thriving distributed community of Python developers is already likely solving the problems you experience The majority of the code they produce is freely available and much of it is made so under open source licenses so you re free to CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES examine what the developers have done and in some circumstances edit it Be warned though any changes you make will probably not be supported by the community certainly not unless you contribute them back an
42. LISTS gt gt gt single tuple item Tuples are normally created by assigning a sequence of comma separated values to a variable which is known as sequence packing Notice that we don t need parentheses in this example because it is unambiguous that it is a tuple gt gt gt t blaha blah2 blah3 The sequence can be unpacked by doing the reverse gt gt gt blah1 blah2 blah3 t Changing the Values in a Tuple You can t That s not what they re meant for Remember when we said they are useful little sealed packets of information They are used mostly in situations where a set of values has to be passed on to another place without worry of having them messed about You can also use them to provide dictionary keys If you want a sequence of data that you can change then you need a list Lists A list is an ordered comma separated list of items enclosed in square brackets Items need not all be of the same type An item can also be another list Lists can be sliced concatenated and indexed the same as any other type of sequence It is possible to change individual items in a list as opposed to immutable strings and tuples Where a tuple is rather like a fixed menu lists are more flexible It is possible to assign data to slices which can change the size of the list or clear it completely Creating a List Lists are easy to create gt gt gt inventory pole another list silver
43. ON THE WEB Enter your name lt input type text name name gt lt input type submit gt lt form gt wed def print footer print lt body gt lt html gt print content print header print lt hi gt It s 0 s lt h1 gt lt p gt Hello 1 s your server is correctly set up to run Python programs lt p gt format now name print form print footer Figure 8 5 shows the first run of this script File Edit View History Bookmarks Tools Help 4 X Gt http localhost cgi bin form cgi i y gt IGl Google It s Friday 14 November 2008 Hello new user your server is correctly set up to run Python programs Enter your name iy tmactcs Done oz Figure 8 5 The results of running this script for the first time CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Jargon Busting These are the new terms from this chapter e Bourne Again Shell Bash Bash is a layer on top of Unix and Linux operating systems that interprets commands for the operating system and allows you to run commands at the command line e Common Gateway Interface CGD This is a technique for processing data and generating dynamic content on the Web e Executable An executable file contains a stand alone application executed by an operating system All applications such as Microsoft Word and Emacs are run using an executable e Home directory On a multiuser operating system
44. Probably the most fundamental change is the rationalization of the string data type In previous versions of Python strings were coded as a sequence of single bytes using the limited American Standard Code for Information Interchange ASCII character set to represent text this 7 bit encoding allows for up to 128 characters including uppercase and lowercase letters numbers punctuation and 33 invisible control characters While ASCII is OK for representing languages that use Latin script such as English and most European languages it is completely useless when it comes to representing the 2 000 basic ideograms of the Chinese language To deal with these sorts of problems the Unicode standard was created to cover all written languages so Python 3 has brought itself up to date by switching to Unicode as its default text encoding The str type is what used to be the unicode type and a new type byte has been introduced to represent raw text strings and binary data Previous versions of Python went through all sorts of contortions in order to deal with text encoding fortunately all you really need to know is that the str type in Python 3 supports international characters by default so you don t have to do anything special if you want to write a string like this gt gt gt S wal Z a D j Vu a Kp 125 126 CHAPTER 7 WORKING WITH TEXT Tip You can use international characters in Python 2 but you would have to
45. Talk Like a Pirate Day September 19 has been an Internet phenomenon since the 1990s On this day many web sites automatically turn their output text into pirate speak which can produce unusual effects depending on the site Listing 11 1 details a skeleton module no piratical pun intended that you will be able to use ina number of different ways to write like a pirate You should save it in the my modules directory and call it strings py Listing 11 1 A Module of String Conversion Methods Module of methods to convert strings to pirate speak def strip punctuation sentence Strips punctuation and all whitespace off the end of a sentence return sentence strip rstrip def piratify str Generic piratify of whole paragraphs by exclaiming everything return str replace replace n n strip def me hearties str Appends me hearties to the end of a stub sentence stub strip punctuation str return stub strip me hearties def yarr str Appends Yarr to the start of a stub sentence and piratifies it stub strip punctuation str return Yarr stub 0 lower stub 1 Make sure you are in the same directory as this file and run the Python interactive prompt At the prompt type the following you should see the results as shown gt gt gt import strings gt gt gt print strings piratify Hello world Hello world gt gt gt print string
46. This round of testing may bring new issues to light In order to help you keep track of what s going on with your code do the minimum necessary to get the new code to pass its tests If the new code fails you should roll back the changes to the point where the code passes again Also if using the function doesn t work don t sweat it reverse the changes and move on There is no rule that says you have to abstract everything into a function The processes of placing code into functions abstraction and turning code that deals with specifics into general purpose code generalization can also be applied to your testing procedures and this is a good thing to do All these test procedures will eventually get moved outside the program into a separate module so abstraction and generalization are also good preparation The rule to remember is to write your tests first and then make sure your existing code still passes before attempting to refactor it Although this might seem like a laborious approach in fact it speeds up the development of new code and provides a mechanism for flagging up which part of the code needs fixing next rather than having to keep all the details in your head This has to be a good thing Next I decided to abstract some of the larger repetitive sections of the program Moving the character generation code into its own function was straightforward This section doesn t require any information to work on as it generates it
47. a keyword and statement in Python 2 x but is now a function It allows you to execute arbitrary strings as if they were Python statements Similar to print the exec statement has been replaced in Python 3 with an exec function gt gt gt list numbers 1 2 3 4 gt gt gt exec for number in list numbers print number If you type the preceding code into an interactive prompt you can see that are actually executing a command string from within the exec function you made a list and then merely iterated over the items in the list with a for loop The exec function is a powerful feature as you can imagine but should be used with care One of the things you should never do is apply exec to the results of the input function or indeed allow your user any direct access to this function or the main namespace of the program The exec function allows you to build code strings on the fly so you can never be entirely sure what the final string will contain that will depend on where the code string gets its constituent parts from For example if the input string was built by the user at runtime issues could arise if the user entered code that wouldn t run or violated security measures A variation of the previous code is shown here gt gt gt gt gt gt list numbers 1 2 3 4 gt gt gt mycode for number in list numbers print number gt gt gt exec mycode In this case all we did was create a text string to execute
48. a module the way Python sees it rather than using the operating system s filename I will cover modules classes and objects in Chapters 9 and 10 gt gt gt help hello world This should give us a result that looks like this Help on module hello world NAME hello world FILE home me 1lib python3 0 hello world py DESCRIPTION Problem Get the computer to output a message Target Users Me Target System GNU Linux Interface Command line Functional Requirements Print out a message User must be able to input some text Testing Simple run test expecting a message to appear expecting message input text Maintainer maintainer website com DATA some text Some Text If you need more assistance you can use the gt gt gt help function to get a basic explanation of most aspects of the Python language The output from the help function provides us with a minimal sort of programmers documentation which is probably more than adequate for this example Larger software projects would produce additional documentation at this stage such as a design document and a user manual The design document is for the benefit of people who will be maintaining the software it should minimally consist of a statement of the problem a top level design the final design possibly with an explanation of how it was arrived at and a data table A data table for the hello world py script from Listing 2 6 would be very simple a
49. adapt the script to allow more player characters to fight by upping the max players count The toggling of attacker and target is rather ungraceful too Perhaps something could be done to reorder the players list so each character gets to fight all the others The methods of hit and damage calculation could also be improved currently the game gives too much weight to the weapon statistics for example One of the ways we could simplify things would be to have containers for commands like the data containers we have explored in this chapter Fortunately we can create these and in the next chapter I ll be introducing you to building your own functions CHAPTER 5 M USING LISTS Jargon Busting As usual here is the roundup of new words in this chapter Arbitrary In this instance anything defined by the programmer is arbitrary Complex data types These are structured or compound types constructed from a sequence of other types of data Constant A constant value does not change during the execution of the program Constrain Ensure that the results of a calculation fall between a specified range Hash A hash is number calculated from a dictionary key to help with storage This number is designed to be smaller than the key to aid efficiency Immutable An immutable value that cannot be edited in place Index An index is a token of an immutable type in square brackets immediately following the variable name used to point to a speci
50. all from scratch so I could copy the whole section as it stood and added return profile to the end def generate rpc Role Playing Character generator Takes no arguments Returns a new character profile dictionary print print New Character print 110 CHAPTER 6 FUNCTIONS Create empty profile dictionary profile Name Desc Gender Race Muscle 0 Brainz 0 Speed 0 Charm 0 life o magic 0 prot 0 gold 0 inventory return profile I then replaced the copied section of code from the main section with profile generate rpc The main body of the program starts here So this is where the flow of execution begins Generate characters while len players lt max players Call character generation function profile generate_rpc The equipment buying section was trickier Because I d put a break statement inside the original while loop the interpreter complained once I had moved it into a separate function because the break statement was now outside the loop Oops The fix for this was to turn the break statement into a return statement that returns the value False It could just as well return any value that evaluates to False so None would also work This forces the flow of execution to break out of the while loop that contains the call for this function The return at the end of the function has to return something it doesn t really matter what
51. also general enough to match all instances of the text we do want Listing 7 2 shows a particularly ungraceful way of doing it With a little ingenuity I m sure you could improve the regular expressions I ve used in Listing 7 2 The results are shown in Figure 7 1 Listing 7 2 txt2html py Convert text file to HTML page import re input filename eval input Enter a filename gt output filename input filename replace txt html title input filename split title title 1 rstrip txt title header lt html gt lt head gt lt title gt o s lt title gt lt head gt lt body gt lt h1 gt 0 s lt h1 gt lt p gt 145 146 CHAPTER 7 WORKING WITH TEXT format title footer lt p gt lt body gt lt html gt Open file input file open input filename text input file read input file close Manipulate text Change anything like Ashpetal or Ashey Petl to Cinderella text re sub Aa sh Pp etl etal lt b gt Cinderella lt b gt text Add lt p gt tags to newlines after periods etc text re sub n 4 lt p gt n lt p gt text Add lt br gt tags to newlines after letters and commas text re sub a z n 1 lt br gt n text Italicise everything between quotes text re sub lt i gt 1 lt i gt text Make everything bold between asterisks text re su
52. and writes them out to a text file displaying a helpful message in the GUI when it s done If the dialog receives the Cancel signal nothing is done Either way dialog destroy is called to get rid of the now extraneous window def on save as self widget Save Opens a dialog asking you where you want to save the information as a plain text file print Exporting aadebug information Create save As dialog amp specify file to open for writing dialog gtk FileChooserDialog title Save Multimedia Info to file action gtk FILE CHOOSER ACTION SAVE buttons gtk STOCK CANCEL gtk RESPONSE CANCEL gtk STOCK OPEN gtk RESPONSE OK dialog set current name aadebug txt response dialog run outfile name dialog get filename print response str response print outfile str outfile name if response gtk RESPONSE OK Write out items a out Multimedia System Information s version headings Kernel CPU RAM Hardware Modprobe Sound Modules Sound Devices Asound a list a out for heading in headings CHAPTER 12 SIMPLE WINDOWED APPLICATIONS command eval self heading func a NNR RK 4S 2K KKK 2S heading commands getoutput command a list append a a out join a list write out System info to file output file open outfile name w output file write a out output file close print aadebug info written out
53. by any means but all I am going to do to finish off this stage of development is add a prompt right at the start to set the number of players The program satisfies the design requirements and runs cleanly without errors so I m going to push version 0 2 out of the door Before releasing version 0 2 into the wild some matters of administration must be attended to Quite a few of changes have been made since the last version so I need to go through all the docstrings and comments and make sure they reflect the current state of affairs I could also create a CHANGELOG file to contain a brief explanation of the changes along with a note of the version number that the changes apply to and a date for the changes a date stamp Once the programming project starts generating additional files it may be sensible to put them all in a separate directory named after the program or you may find it useful to group all your projects together in a common Projects directory in your home folder The full code for rpcombat 0 2 py now is shown in Listing 6 4 Listing 6 4 rpcombat py Refactored usr bin env python rpcombat py Purpose Simple Role Playing Combat Game Target System GNU Linux Interface Command line Functional Requirements The user must be able to generate more than one character profile equip those characters with suitable weapons and model hand to hand combat between characters Testing methods trace table and play testing
54. callback The data passed to this method is printed to stdout def callback self widget data print Hello again s was pressed data This is another callback which exits the application def delete event self widget event data None gtk main quit print Bye return False def init self Create a new window self window gtk Window gtk WINDOW TOPLEVEL Set the title of the window to GUI self window set title GUI 265 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Create a handler for delete event that immediately quits GTK self window connect delete event self delete event Set the border width of the window self window set border width 6 Create a box to pack the widgets into The box is an invisible container which is used to arrange the widgets inside it self box1 gtk HBox False 0 Put the box into the main window self window add self box1 Create a new button with the label Hello self button1 gtk Button Hello Now when the button is clicked the self callback method is called with a text string the Hello button as its argument self button1 connect clicked self callback the Hello button Instead of add we pack this button into the invisible box which has been packed into the window self box1 pack start self button1 True True 0 Always remember this step this tells GTK to actually display the button self button1 show Do th
55. characters will need to interact with the game world This involves tackling the realm of numerical information Working with Numbers First assigning numbers to variables is fairly straightforward muscle 10 brainz 4 As I mentioned earlier if the interpreter encounters a bunch of characters starting with a numeral rather than a letter or a quotation mark it will assume that it is a number This is why you can t start variable names with a number So far so good There are a few things that you need to know before you start trying to do math with computers Using Binary Computers Can Only Count to One All information is stored inside the computer as a series of ones and zeros that is in binary or base 2 Your computer stores and processes data using a huge collection of tiny little switches and these switches can be either off 0 or on 1 This has led to the old programmers joke that there are only 10 sorts of people in the world those who understand binary and those who don t 10 in binary notation represents two rather than ten Using Python you don t really need to know much more than this Bits and Bytes You will frequently encounter the terms bits and bytes in computer literature Bit is short for binary digit and refers to one of these tiny switches that can only hold a yes or no answer It is the smallest possible unit of computer information A byte is theoretically the amount of
56. constructor method GtkInfo init The first thing I do is create some icon images using gtk gdk pixbuf new from file path which creates Gtk picture buffers to hold the images in a format that Gtk and therefore Python can use These will come in handy later I aliased the method as image to save on typing and similarly defined path to be the full path to the gtkinfo directory 269 270 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Creating a clipboard is simple using gtk Clipboard Once the clipboard s created I create a text buffer which will hold the output messages for the right hand pane of the main window display using gtk TextBuffer None The text can be formatted by creating a text tag This tag uses the pango library which also needs to be separately imported Here I am using it to create a simple headline format The next commands get the position of the cursor and place the output items at the cursor position to create the formatted output With all that done I can now start to construct the graphic components First the main window needs creating this is the outer box that provides the title bar and iconify maximize and quit buttons for the application I can set various properties of the main window here and connect any delete event received by the window to my GtkInfo delete event method The compiler will complain if I try to reference a method that doesn t exist yet so I need to create a stub for this met
57. create list of matches using ziply function matches ziply range 0 len players if trace print matches for attacker target in matches life left players target life Calculate velocity of blow velocity calc velocity attacker target if trace print tvel thit tdam tchange print t velocity if velocity gt 0 Print sutable Hit message if velocity gt vel max vel max velocity hit type int 7 velocity vel max if hit type gt 7 hit type 7 if trace print t tHit hit type print players attacker Name hits hit type players target Name end else Print suitable Miss message if velocity lt vel min vel min velocity miss type int velocity vel max if miss type gt 7 miss type 7 if trace print t tMiss miss type print players attacker Name fix gender players attacker Gender misses miss type players target Name End player turn continue 121 122 CHAPTER 6 FUNCTIONS Calculate damage inflicted by blow damage potential damage calc damage attacker target velocity if trace print print t tDamage damage potential damage if damage gt dam max dam max damage Print damage report damage type int 7 damage dam max if damage type gt 7 damage type 7 if trace print t t tDamage damage type change type int 5 damage life left if change type gt 7 change type 7 if trace print t t
58. docstring TODO Turn comments into proper docstring Check for magic variables if line startswith label value line split store magic variables in spellbook spellbook label strip value strip strip Check magic vars are correctly formatted if present version 0 9 0 9 0 9 if label version and not version format match value print Warning dodgy label status Prototype Alpha Beta Release Candidate Stable if label status and not status format match value print Warning dodgy label date 0 3 0 9 01 0 9 0 9 4 if label date and not date format match value print Warning dodgy label maintainer W W W if label maintainer and not email format match value print Warning dodgy label Check rest of lines for s TODO todo FIXME fixme This should be a try statement but they aren t covered until Chapter 11 if todo format match line 151 152 CHAPTER 7 WORKING WITH TEXT task todo format match line label desc task groups 1 todo text 4 s 2 s 0 s line 1 s CK 3 CK format filename line no label desc Now print todo text We won t be needing this anymore script close Fill in some empty variables if not date format match spellbook date spellbook date Now if spellbook author spellbook author s
59. e Services Artist April Milne Distributed to the book trade worldwide by Springer Verlag New York Inc 233 Spring Street 6th Floor New York NY 10013 Phone 1 800 SPRINGER fax 201 348 4505 e mail orders ny springer sbm com or visit http www springeronline com For information on translations please e mail info apress com or visit http www apress com Apress and friends of ED books may be purchased in bulk for academic corporate or promotional use eBook versions and licenses are also available for most titles For more information reference our Special Bulk Sales eBook Licensing web page at http www apress com info bulksales The information in this book is distributed on an as is basis without warranty Although every precaution has been taken in the preparation of this work neither the author s nor Apress shall have any liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the information contained in this work The source code for this book is available to readers at http www apress com www allitebooks com Contents at a Glance Contents ee ee ere ee ee v About the Author ies clit stss cs csissencecvcsion cine uncicnwsnunsenuntand deniuncienviaieeduuicsssdinutucuauedunciusvnisians xi About the Technical Reviewer ssssossense neces xii Chapter 1 Introducing Python cccccccssssssssssssceessssssssnsssseeeesesss
60. either side of an operator e Refining This refers to the process of improving and simplifying a program design e Sentinel This loop condition control variable has a value that is the result of the process needing to be controlled such as user input e Suite A suite is a group of commands executed in a conditional statement and are all indented the same amount e Trace This refers to the process of checking the state of the internal values and conditions of a program over the course of its execution e Trace table This data table is output by performing a trace e Validation This refers to the process of checking that your data is what you expect Summary You now know how to get your programs to actually do something Much of the fundamental action of a program is controlled by the use of comparison operators logical operators assignment operators combined with the arithmetical operators that were covered in the previous chapter and in accordance with the rules of operator precedence You have learned about the fundamental control structures used for decision making if elif else and looping while else and for in else as well as how to get out of loops cleanly using break and continue You have also started to learn how to refine the design and deal with logical errors using trace tables and flowcharting In the next chapter I will introduce the first of several more complex data types the list and show you h
61. end of 138 139 formatting methods 129 joining 127 128 manipulating using regular expressions 139 141 splitting 126 127 string split method 127 145 string strip chars method 134 string strip method 136 string swapcase method 128 stringToSplit split separator max method 78 string upper method 128 str object function 40 strptime method 257 str type 125 styles text checking and correcting 147 154 stylish code overview 165 pythonic approaches 166 169 subgroups 17 subpatterns matching using regular expressions 137 138 suite defined 74 super function 192 symbol table 105 INDEX defined 124 sys argv dynamic objects 170 sys stderr dynamic object 170 sys stdin dynamic objects 170 sys stdout dynamic objects 170 T tags defined 159 tarball defined 159 templates 181 tepache 279 282 tepeche 282 testing software 20 21 text 125 160 applications 145 158 checking and correcting styles 147 154 converting text 145 147 formatting data 154 156 storing data 156 158 files closing 144 finding way around 143 144 opening 141 142 reading from and writing to 142 143 overview 125 regular expressions 135 141 finding patterns at beginning or end of string 138 139 manipulating strings using regular expressions 139 141 matching character sets and alternatives 138 matching subpatterns 137 138 punctuation 139 regular expression object 139 strings 125 1
62. fix style py on rpcombat py again to check I got rid of all those TODO comments python fix style py Python script to be checked gt rpcombat py Looks like a Python file 0K TODO 17 10 2008 TODO rpcombat py line 18 Fix up interpreter call and file encoding Application Details File rpcombat py Version 0 3 Prototype Last Modified 17 10 2008 AUTHORS Author maintainer website com Maintainer maintainer website com Credits In fact there is still one TODO left which I will deal with in the next chapter 158 CHAPTER 7 WORKING WITH TEXT Jargon Busting Let s look at the new terms introduced in this chapter Alphanumeric This refers to letter or number character and is equivalent to the regular expression a zA Z0 9 American Standard Code for Information Interchange ASCID This method of encoding characters on computers uses just 7 bits of information There are 128 values ranging from 0 to 127 of which 95 are printable characters and 33 are invisible control characters ASCII encoding only covers the characters usually found in the English language and a few others that are specifically used by computers Conversion field This is the part of a replacement field after the exclamation point used to force an item to be represented as a particular type regardless of its own type s formatting rules Field names A replacemen
63. girl was very fond of the fields flowers and sheep The sheep had found a hole leading into the garden and the step mother told the little girl that she must stay and watch the hole and not let the sheep through and that she would send her out some dinner When dinner time came she sent out a thimble full of broth a grain of barley a thread of meat and a crumb of bread It was not long before the little girl had finished all that and felt just as hungry as if she had not had anything to eat at all She did not dare to go home as she had been told to stay out all night Cinderella felt very unhappy about her treatment As she cried alone a little black ewe came to her from under the grey stone in the field and wanted to know what was the matter with her So Cinderella told it that she had no dinner and was very hungry The little black lamb told her not to cry but to put her finger into its ear and see what she could find So she put her finger in and got a big piece of bread Then the little lamb told her to put her finger in the other ear So she put in her finger and got a big piece of cheese this provided her with plenty to eat and so she began to cheer up In the evening rather than being tired and hungry as her step mother had expected she was quite bright and cheerful The next day she was told to go out again and her step mother told her that she would not send her any dinner this time But the little lamb came again and gave her so
64. given an expression to evaluate how does the Python interpreter decide which parts go with what and how can you know what its decision is going to be The answer is that there is a predefined order called operator precedence which tells the interpreter which operators bind tightest To see this order you can look in the python documentation at http docs python org 3 0 reference expressions html evaluation order or if you re unsure for a particular pair of operators have a go in the interpreter with some values that will let you distinguish the precedence you can see from the output of 5 4 20 previously that binds tighter than If you want the arguments to be grouped in a different way from Python s default you can get this to happen using brackets so in this example you would type the following gt gt gt charm muscle brainz speed gt gt gt charm 180 Dividing Numbers Division is performed using the operator gt gt gt print 13 5 2 6 gt gt gt print 13 75 4 25 3 23529411765 If you want to do integer division where the fractional part of the answer is dropped also known as floor division you can use the operator instead of The remainder is obtainable by using the modulo operator gt gt gt print 13 5 2 gt gt gt print 13 5 3 You may encounter results you don t expect when doing integer division with negative numbers gt gt gt print 13 5 3 gt
65. gt axe strength 60 gt gt gt axe speed 50 But in practice I d like to be able to assign them all in one go Using Methods The way to do this is to create a method that will take the three values as arguments and assign them to the correct attributes for you Here is a good example of encapsulation In the original version these values were expressed rather cryptically as a tuple with no explanation of what the values meant A suitable method could deal with this tuple automatically gt gt gt class Thing def setValues self price strength speed self price price self strength strength self speed speed def printValues self hice print Price 0 s Strength 1 s Speed 2 s format self price self strength self speed gt gt gt class Weapon Thing print new weapon made new weapon made gt gt gt axe Weapon gt gt gt axe setValues 75 60 50 gt gt gt axe printValues Price 75 Strength 60 Speed 50 The subclass Weapon inherits its methods and attributes from the superclass Thing If you want your Weapon class to behave differently from an ordinary Thing it is possible to override the methods of Thing by defining the methods within the Weapon class definition CHAPTER 9 E CLASSES gt gt gt class Weapon Thing def setValues self price strength speed self price price self strength strength self damage strength self speed speed def printVa
66. gt gold OxFFCCOO The octal number assigned to permissions is an array of information where each bit has a distinct meaning each octal digit sets read write and execute permissions for the user group and others respectively The color value set for gold represents a mix of three values red green and blue which can range from 0x00 to OxFF according to intensity It s important to note that there is only one integer type here what you get when you enter 0x10 0020 or 16 is exactly the same integer If you want to get back the hexadecimal or octal string representation of an integer you can use the following functions e hex x displays integer x in hexadecimal format e oct x displays integer x in octal format gt gt gt X Oxaa 0x33 gt gt gt hex x oxdd gt gt gt 0 0077 0033 gt gt gt oct o 00132 CHAPTER 3 VARIABLES AND DATA TYPES Creating a Number Application In order to take the character generation script any further you will need to be able to compare values and store the results This will require knowledge of conditional statements which I will move on to in Chapter 4 and the use of more complex data types see Chapter 5 so I will return to Cloud Cuckoo Land once you have had time to study these more advanced incantations To demonstrate the use of mathematical data types right now let s consider a simpler problem calculating how much fabric you would need to buy to make a pai
67. help self help menu append self hhelp self hhelp show self help set submenu self help menu self help menu show Don t forget to add them to the menubar 273 274 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS self menubar1 append self file self file show self menubar1 append self edit self edit show self menubar1 append self help self help show Then pack the menubar into place self vbox1 pack start self menubar1 False True 0 self menubar1 show Create the main panels self hbox1 gtk HBox False 0 Create option list panel self scrolledwindow1 gtk ScrolledwWindow self scrolledwindow1 set policy gtk POLICY AUTOMATIC gtk POLICY AUTOMATIC self treeview1 gtk TreeView self treeview1 connect cursor changed self on treeview1 cursor changed self scrolledwindow1 add with viewport self treeview1 self treeview1 show self scrolledwindow1 show self hbox1 pack start self scrolledwindow1 True True 0 Fill it with relevant info self create treelist Create text output panel self scrolledwindow2 gtk ScrolledwWindow self scrolledwindow2 set policy gtk POLICY AUTOMATIC gtk POLICY AUTOMATIC self textview1 gtk TextView self textbuffer self textview1 set wrap mode gtk WRAP WORD self textview1 set editable False self textview1 set left margin 6 self textview1 set right margin 6 self scrolledwindow2 add with viewport self textview1 self textview1 show se
68. idea of what you want to create the problem you wish to solve and a fairly good idea of how to solve it Chapter 2 will take you through the process of designing software known as the software development cycle Usually this is something that you have to learn separately as most programming manuals go straight into the details of the language assuming you will have studied program design at college or in the process of learning some other programming language This can lead you to create code that is difficult to read and understand and makes fixing problems later a nightmare Understanding the principles of software design can dramatically speed up the process of creating new software and help ensure that important details are not missed out In Chapter 3 you start coding those designs in Python You will learn about constructing the basic units of data words and numbers and how to combine and manipulate them In Chapter 4 I will show you how to compare values make choices and deal with errors The subsequent chapters go through the process of refining those designs breaking them down into manageable chunks and expanding your understanding of the Python programming language so you can turn your ideas into fully fledged functional computer programs CHAPTER 1 INTRODUCING PYTHON Creating Your Own Help Files Python is described as self documenting This doesn t mean that it will write your user manual for you but you can add blocks
69. identifying the Problem ssssusssnsnnsannsnnannannnnnannannnnnannanununannannnnnannnnnnannnnnnnnnnnannnnnannnnnnnnnn 10 Creating Your WISN LIST rinsar e EEE 12 Devising aes 0 UO Misa eee 13 Breaking Down the Solution into Steps ccscssssessssessessessessessessessesessassaesaesaesaseaseaes 15 Jargon BUSTING recio sussies casa tecshutesase tues sacs taderecesutesasetucesocstacessectutseasetes 24 E IY scot cct 8 cance ct cna ccence st ccs San ceceesacsectescesecesecnsesceecesecseeas eccreccesettacsecccns 25 Chapter 3 Variables and Data TypeS sssssnnnnennnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 27 Choosing Good Identifiers ca ccicsatecete cescceiccesacesctenededetestdensdedetescdensdensdndedesadebadadeibindesctadadeenes 27 Creating Variables and Assigning VAlUCS cccssssessseseseseeseesesseeseeseesaesassessateaseatens 28 Recognizing Different Types Of Variables cccsesssessessecsesseseseseseseseseseeeeaeeaes 29 Jargon SUNG os tse tees Se tale te deed teeened tects ieaadeasdteandas Geaciaa tansecedcnaadectdeendtansnansnsscnausncneesac 45 www allitebooks com CONTENTS SEa E a T 47 Chapter 4 Making ChoiCeS sssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 49 Comparing VENI S oc cs cata d sa ede dace tach ce cecucae cece hteenx cede cect creer ee eek AANEEN k anaa NEENA 49 Taking Control Of the PrOC SS csssscssscsessseseessessessseeseessessesssesseeso
70. in if len players gt max players players players max players while len players lt max players profile Player Add new player to list of players players append profile write out players Start the game by placing the players in the combat arena arena inventory players arena enter players 0 del reply That s all folks You may notice that the mini command line has a few quirks The way I have done it allows the execution of arbitrary strings which is not good The structure of the code means that you can easily add new Locations Things and Players creating your own adventures In fact you could short circuit the entire setup by putting import rpcombat at the beginning of a new file and designing your adventure from scratch You ll find out more about modules in Chapter 11 Obviously there is still room for improvement and plenty of scope for designing your own games This has been a very condensed taste of what is possible using classes Jargon Busting Here is some further explanation of the terms presented in this chapter e accessor A method that allows the getting or setting of an attribute e attribute A value associated with an object that is referenced by name using dotted expressions e block A block is a section of the text of a Python program that is executed as a unit Modules function bodies and class definitions are all considered to be blocks as is each command you type in the
71. init self master First create a frame This fills the main window of the application frame Tkinter Frame master Everything has to be packed into place before it can be displayed frame pack Create the Hello button self hello button Tkinter Button frame text Hello command self say hi self hello button pack side Tkinter LEFT Create the Quit button self quit button Tkinter Button frame text QUIT fg red command frame quit self quit button pack side Tkinter LEFT def say hi self print hi there everyone CHAPTER 12 SIMPLE WINDOWED APPLICATIONS root Tkinter Tk app HelloTk root root mainloop Listing 12 1 will create a window with two buttons in it as shown in Figure 12 1 In order to understand how it works you will need to run it from a terminal emulator screen Basically two class objects are created root an instance of Tkinter Tk and app an instance of HelloTk with root as its only argument The root object provides a method called mainloop which displays an instance of the window defined in HelloTk and waits for user input in the form of mouse clicks and key pressing Clicking the Hello button will cause a message to be printed on the terminal screen AE Figure 12 1 Hello World Tkinter style To initialize Tkinter you must first create a Tk root widget This is an ordinary window with a title bar and other decorations provided by your wi
72. interactive interpreter e class A template for creating your own data types which bundles up several related pieces of data and defines a bunch of methods for accessing and manipulating that data An instance of a class is called an object 219 220 CHAPTER 9 M CLASSES e encapsulation The grouping together of data and functionality which allows the implementation details to be hidden behind an interface e inheritance A way to form new classes using classes that have already been defined e initialization The process of setting the initial value of an object Initialization is done only once for each object when that object is created e instance An object created from a class definition e interface A set of instructions that an object provides to allow outside control of its contents e method A function that is defined inside a class body If called as an attribute of an instance of that class the method will get the instance object as its first argument which is usually called self e namespace A container that provides a context for names allowing the same name to be used in different subprocedures without causing confusion In Python modules classes functions and methods all form namespaces e parent A class or node of a tree data structure that passes on its attributes and methods to one or more child classes e polymorphism A feature that allows values of different data types to be handled usi
73. into the file it is This will be referred to as the file s current position It s equivalent to 143 144 CHAPTER 7 WORKING WITH TEXT having your finger on the text or to keeping track of the position of the cursor in a text editor The file tell method returns the file s current position gt gt gt out file open story html a gt gt gt out file tell 0 gt gt gt out file write content 522 gt gt gt out file tell 1566 Caution On Windows te11 can return illegal values when reading files with Unix style line endings Use binary mode rb to circumvent this problem It is also possible to set the file s current position using file seek offset whence gt gt gt out file seek 223 0 223 The whence argument is optional and can be 0 1 or 2 0 gives a position relative to the start of the file 1 gives a position relative to the current position and 2 gives a position relative to the end of the file The offset is in bytes again just like the value returned by file te11 For whence values of 1 and 2 the offset must be 0 so you can t perform seeks relative to the end or current position gt gt gt out file seek 0 2 1566 Note Seeks can t be performed on all file objects Closing Files It is a good idea to use file close to close a file after you are finished with it Python will probably notice that you re not using the file anymore and free up the mem
74. is an easy way to do this but I CHAPTER 4 E MAKING CHOICES would end up with strange numbers of spaces in between words and I want only one I have to come up with some method of checking whether the last character was a space For this example I have chosen to set a flag as True if a space is written out and as False if any other character is written I then check whether this flag is set or not before writing out a space character The refined design turns out as follows Enter a text string Loop through characters in text string if character new line elif character if space flag do nothing else add character to output string space flag True elif character not one of 3 lt gt write out character space flag False else if space flag False add space to output string space flag True The else section should catch all the characters in the set of punctuation marks so I want to check whether the last character was a space if not I can add a space to the output In fact the design is slightly repetitious with a little ingenuity it may be possible to merge the elif character andelse suites Enter a text string Loop through characters in text string if character new line elif character not one of 3 lt gt write out character space flag False else if not space flag add space to output string space flag True That s neater
75. it returns so long as it evaluates to True I m only using the return values to control the loop as the buy equipment function is actually a dictionary modification procedure so it doesn t need to receive or return any values for any other reason Then I had a small stroke of inspiration over the course of testing I had started to get annoyed by the unnecessary Would you like to buy some equipment prompt So I replaced it with profile inventory which would evaluate to True if the inventory was empty and False if it contained anything The while loop wrapping the function call now looked like this shopping profile inventory while shopping shopping buy equipment print You own a join profile inventory This change had the desired effect except that it made it possible to exit the loop without having bought anything I replaced the return False statement in the buy equipment function to return profile inventory which did the trick Next I started to wonder what would happen if the generated character didn t have enough money to buy anything I made one further alteration to the breakout return profile inventory and profile gold gt 10 This answers the theoretical question of whether to continue shopping with True if the inventory is empty and enough money is left to buy something otherwise it returns False def buy equipment Purchase an item of equipment P
76. item in self inventory if issubclass type item Player miss counter 0 while len players gt 1 Seed random generator random seed random shuffle players create list of matches using ziply function matches ziply list range 0 len players CHAPTER 9 E CLASSES trace Matches matches for attack targ in matches winner self resolve conflict players attack players targ if winner False miss counter 1 if miss counter gt 6 print n players attack name and players targ name declare a truce n return False elif winner Remove loser from players list del players targ self inventory players break else miss counter 0 Print winner trace Winner players 0 print players 0 name is victorious trace max damage velocity o 1 2 3 format self dam max self vel max self dam min self vel min return def exit self player Commands run when a player exits a Location Takes one argument a Player object Removes Player from Location s inventory If the player in question is Dead gives Game Over message Doesn t return anything either if player name in self inventory self inventory remove player name print You leave the self name else print Game Over return class Shop Location Sub class of Location which allows trading Same as Location but Shop commands allows different game commands The str ou
77. loops cleanly the chief ones being the use of the break and continue keywords If you want to get out of a loop without executing any more statements in the loop body use break If you just want to get out of this particular iteration of the loop continue immediately takes you to the next iteration of the loop The difference between the two is shown clearly in the flowchart in Figure 4 2 While Condition Figure 4 2 Flowchart for a while loop At times you will want the interpreter to recognize a condition but do nothing In this case the pass keyword can be useful it creates a null statement which simply tells the interpreter to move on to the next instruction while True this input int raw input gt 67 68 CHAPTER 4 MAKING CHOICES if this input gt 0 pass else print Input is negative gt 1 Input is negative gt 17 gt Like the if statement while can also be followed with an else suite In this case the else suite is only executed after the loop has finished as long as you didn t break out of it An example of break and continue is shown in Listing 4 6 Listing 4 6 Using break and continue counter 0 sum 0 while True this input float input gt if this input lt 0 if counter print You haven t entered any numbers yet continue break sum this input counter 1 print counter sum 2 1 gt 3 You haven t entered any n
78. memory needed to store a single character this became standardized as 8 bits during the late twentieth century even though modern computers often use more than 8 bits to store characters Using Booleans Truth or Dare It follows from the preceding discussion that the simplest type of numerical value that exists on a computer is one that has only two possible values True equal to 1 or False equal to 0 These 37 38 CHAPTER 3 M VARIABLES AND DATA TYPES true false values are known as Booleans named after a system devised by English mathematician and philosopher George Boole They can be manipulated using logical operators such as AND OR and NOT I ll explain the meaning and use of these operators in Chapter 4 You can assign Boolean values by using the True and False keywords beauty True illusion False Note True and False are always spelled with capital first letters Using Whole Numbers Integers Whole numbers are known as integers in programming terms Integers don t have decimal points but can be positive negative or zero These are used to refer to numbers of things in much the way you d expect like the eggs in the recipe example at the start of the chapter It may be reassuring to know that most of the time you can use numbers like these Performing Basic Mathematical Operations You have learned how to store data inside variables so let s start manipulating that data Integers can be added s
79. mirror 10 gold coins potion Modifying a List New values can be assigned to list items using the assignment operator sequence i 1 x gt gt gt inventory 1 wax gt gt gt inventory pole wax silver mirror 10 gold coins potion You can replace a slice of your list with another sequence using list i j step sequence gt gt gt inventory 2 shield scroll oil CHAPTER 5 M USING LISTS gt gt gt inventory shield wax scroll 10 gold coins oil Here I have specified a slice with no start or end point and a step size of two so every other item in the whole list is affected by this change You can add elements to a list by using the append or extend methods the append method adds individual items the extend method is used to add items from another list gt gt gt inventory append skull gt gt gt inventory shield wax scroll 10 gold coins oil skull gt gt gt inventory extend sword oil gt gt gt inventory shield wax scroll 10 gold coins oil skull sword oil The keyword del can be used to remove entire variables or slices of a list Remember that this only removes references from the list the actual item itself is not affected if they are still being used in other data structures gt gt gt del inventory 4 gt gt gt inventory shield wax scroll 1
80. of raised exceptions 229 230 creating 225 227 229 hierarchy of 224 225 raising 225 227 defined 123 239 handling complex 231 EAFP style 238 overview 221 simple 222 using else clause 232 234 using finally clause 231 232 using tracebacks 234 235 237 using try statement with except clause 222 224 overview 221 terminology 239 240 exec function 172 173 executable files defined 179 execution 17 existing types emulating 194 196 exit function 4 expressions 29 38 defined 46 extend method 81 F field names defined 159 replacement 130 FieldStorage class 177 dictionary 177 file close method 142 144 file extensions 12 file flushQ method 142 144 file readline method 143 file readline size method 142 files text closing 144 finding way around 143 144 opening 141 142 modes and buffers 141 142 reading from and writing to 142 143 file tellQ method 144 file writelines sequence method 143 file write method 144 filter function 84 finally clause defined 239 handling exceptions 231 232 Find function 109 finding strings 134 135 first in first out FIFO structure 82 flags 53 defined 46 flags argument 139 float 40 defined 46 Float Format Specifiers 133 float x function 40 floor division 39 flowchart defined 73 for loop dictionaries 86 formal parameters 102 defined 123 format_info function 148 format specifier 130 defined 1
81. of the specific things you want this software to achieve This requires analyzing the procedure that might be required to perform the job in hand a little more deeply Initially it is quite useful to examine a task that you know how to do It could be something that is currently not automated or computerized The following sections outline some suggestions of things you could do to generate your wish list Talking to Potential Users Interview potential users of your application to find out how they currently perform the task you have in mind Ask them which parts of the procedure could be usefully automated and which factors they want to be able to control Find out which aspects are confusing and how they overcome existing difficulties Some people find it useful to create a short questionnaire even if only you will be answering the questions sometimes this can help to gain some perspective on the problem at hand In the questionnaire you can ask more detailed questions of what your potential users expect the application to look like and how they expect it to behave what file formats they expect to use to read in information from and save it out to particularly covering any unique or special considerations that you can imagine File formats could include plain text XML and HTML for text documents or your users might have images saved as JPEGs or PNGs or WAV audio files that they want to convert to MP3s A file s extension CHAPTER 2 M DESIGN
82. other thing you need to know about strings is that they are immutable this means they can t be changed In practice it is possible to use some fairly simple functions to create a new string with an edited value Creating a Text Application It s time to put all this information into practice on our role playing game example Strings are quite simple to use for the most part you just need to make sure that you enclose your strings in matching quotes properly The top level design for an RPG character description generator script is correspondingly simple Prompt user for user defined information Output the character sheet The minimum information I need is a name for each character I d also like to keep a note of each person s gender and fantasy race and I d like some space for a short description so I have created variables named Name Desc Gender and Race to hold each of those values as a string I can then print out those values in a pretty format using the print function as shown in Listing 3 1 35 CHAPTER 3 E VARIABLES AND DATA TYPES Listing 3 1 chargen py chargen py Problem Generate a description for a fantasy role playing character Target Users Me and my friends Target System GNU Linux Interface Command line Functional Requirements Print out the character sheet User must be able to input the character s name description gender and race Testing Simple run test Maintainer maintainer website com
83. positional first and then keywords In big applications it s best to avoid mixing keyword and positional arguments gt gt gt def collect args b e a 432 c 512 args kwargs print a b c e print args ee print kwargs gt gt gt collect args 486 648 432 512 576 682 768 first 729 second 546 third 819 432 486 512 648 576 682 768 second 546 third 819 first 729 Note that you can use any name you like after the asterisks or but the convention is to call them args and kwargs You can also use the argument gathering asterisks in the function call which basically expands the tuple or dictionary in line so that the function sees the tuple or dictionary s contents as normal positional arguments and keyword arguments gt gt gt args 486 648 432 512 576 682 768 gt gt gt kwargs first 729 second 546 third 819 gt gt gt collect args args kwargs 432 486 512 648 576 682 768 second 546 third 819 first 729 Notice that args in the function call does not contain the same values as args in the function You can use this technique to pass arguments through one function into another which can sometimes be useful Using docstrings to Document Your Function Once a function is fully coded and passes the tests you have set for it it is a good time to edit its docstring to fit with the expectations of Python s documentation too
84. push items onto the stack with list append and pop them back off with pop Note that there is no additional index argument so it will be the last item of the list that is popped gt gt gt inventory append ring gt gt gt inventory pop ring gt gt gt inventory shield wand scroll 10 gold coins emerald potion The other approach involves creating a first in first out FIFO structure called a queue This works more like a pipe where you push items in at one end and the first thing you put in the pipe pops out of the other end Again we can push items into the pipe using append and retrieve them using pop 0 this time with an index of 0 to indicate that the data items should be popped from the start of the list gt gt gt inventory append ring gt gt gt inventory pop 0 shield gt gt gt inventory wand scroll 10 gold coins emerald potion ring Note There is also another data type known as a deque pronounced deck which works very similarly to a list but has been designed with extra features to facilitate double ended stacking and queuing operations Sorting Lists You can sort lists either using a list method list sort which directly modifies the original list or using sorted list which returns a new sorted list and thus doesn t modify the original gt gt gt sorted inventory 10 gold coins emerald potio
85. representation and returns a single object that gives you access to that representation Exactly how the internals work isn t important but from your perspective the HTML has been turned into a hierarchy of objects each corresponding to a plain text string or a HTML element within the original document The hierarchy means that once you ve got the Beautiful Soup object corresponding to the lt body gt tag you can access tags like paragraphs lt p gt within it Similarly a paragraph object lets you access such tags within the paragraph as lt em gt or lt a gt Beautiful Soup also provides you with many simplifying methods some of these slice through the hierarchy meaning you don t have to always drill down explicitly into the depths of paragraphs to find lt a gt tags and maybe not find them if they re in lt 1i gt or lt div gt tags but could instead grab all the lt a gt tags in the document straight away Beautiful Soup is not part of Python core but is contributed software The module is only a single Python file as discussed previously single file modules are easy to install However at the time of this writing the module is still not available in a separate Python 3 version from its maintainer You can work out how to convert the Python 2 x module using documentation included with the download and it s very straightforward certainly on Linux Unix systems But to make life easier for you in the meantime Apress has prov
86. results in the error report This is usually called a traceback which we ll discuss in more detail later Some programming languages talk of throwing and catching exceptions rather than raising them the division operator throws an exception up to function B if function B doesn t catch that exception it carries on to function A This is just another metaphor for the same underlying process In this book we ll use Python s terminology instead Using the try Statement with an except Clause in Python If the exception is created deep in a program e g within a method it can be caught and dealt with in between the original error and the program s output Python s try statement is used to deal with exceptions and in its most basic form it can be used as shown in Listing 10 1 Listing 10 1 The Basic Structure of the try Statement try Something that might cause an exception r dangerous method a b c CHAPTER 10 EXCEPTIONS except SomeSpecificTypeOfError as e A particular class of errors we might expect r something else a b c except Catch all other errors regardless of their classes r something else again a b c To demonstrate this in action let s create two methods to do division for us one of these will be able to handle a ZeroDivisionError the other will not Run the code in Listing 10 2 you can paste it straight into the interactive prompt or save it to a file called basic handling py and r
87. says rewrite the character sheet output using string format This task is fairly straightforward except this time I have a lot of values to pass to the method which are all contained in the profile dictionary What I d like to do is pass the entire dictionary to string format and reference its keys by name No problem Listing 7 5 shows the code to do just that Listing 7 5 Character Sheet Formatter Output the character sheet Deleted section TODO rewrite the character sheet output using string format fancy line lt 4 44 7 7 gt print print fancy line print t profile Name print t profile Race profile Gender print t profile Desc print fancy line print print tMuscle profile Muscle tlife profile life print tBrainz profile Brainz tmagic profile magic CHAPTER 7 WORKING WITH TEXT print tSpeed profile Speed tprotection profile prot print tCharm profile Charm tgold profile gold print Hit rpcharacter sheet lt 0 ss H S e Ot 844 H gt Name s Race s Gender s Desc s lt 0 ss H FV 844 H Muscle Muscle lt 2 life life lt 3 Brainz Brainz lt 2 magic magic lt 3 Speed Speed lt 2 protection prot lt 3 Charm Charm lt 2 gold gold gt 7 format profile print rpcharacter sheet
88. sect soreotts tthe tetsu Aaaa araa aa aaa epa a a aeaa ap a 239 SUMMAN oeaan aa E E A 240 Chapter 11 Reusing Code with Modules and Packages sssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 241 Understanding Python Modules ccscssssssssssessessesssssesesseseeseesaesaesessaesaesaesaesaseaes 241 Everyday Module USAC sicecetc ccs succeeded cantscedcaedcautswedanedcautecedenedsbeesaebenudsaeiascnediienaneneess 244 Advanced Module Behavior ssssssssesssssnsnnrnnnsunnnrnnnnunnnnnnnnununnnnnnununnnnnnununnnnnnunnnnnnnnnnnnnne 249 Combining Modules into PaCkages csssssssssssessesesesesesseseessesaesaesaesaesaesaesaseass 252 The Universe Of Python PaCkagQes scsssssssssessssssssssessesessessessesaesaesassaesaseaseaseas 254 Jargon BUSUN ite ee ee ee ee ee 259 SUMMAN ge ee ee SEES SOE EE SERS epee 260 www allitebooks com viii CONTENTS Chapter 12 Simple Windowed Applications ENNEA E E 261 Using Tkinter sssesesssessesuensusnnnnnnusnnnnnnnununnnnnnununnnnnnununnnnnnunununnnnununnnnnnununannnnunnnnnnnnnnnnnn 261 Saying Hello with PyGTK s ssasessssssesusnsusnsnunnsunnnnnnnnunnonnnnnununnnnnnununnnnnnunnnnnnnnunnnnnnnnnnnnns 265 Using Glade and tepache to Build Interfaces sscscsssesssessseessesseeseeseeeseesenees 279 Jargon Busting ssssesesssssssnusnsusnenunnusunnnnunununnnnnnununnnnunununnntunununnnnnnununnnnnnunununnnnnnnnnnnnnnnn 282 gt U7 TE 111731 A ARPETTI ETTE rer repre nee npr ser
89. some other built in functions and methods I ll start by opening a plain text file in the same directory as I started the interpreter gt gt gt open story txt lt io TextIOWrapper object at 0xb7ba990c gt Oh look another Python object This one says it s an io Text 1OWrapper in plain language that s a file object If the file doesn t exist you ll get an 10Error JO stands for input output in other words reading and writing to files The file object now holds the contents of story txt but in order to do anything with it you need to learn some file methods 141 142 CHAPTER 7 WORKING WITH TEXT I hope you re getting used to the concept that everything in Python is some type of object and that any type of object will usually have several methods that you can use to access its values or edit it Before you can make use of the different file methods it is important to understand the different ways that Python can open a file to create a file object Modes and Buffers Opening the file by just passing the filename to open creates a read only file object If you want to be able to write to that file as well it is necessary to set the optional mode argument This argument can be a single character r read w write or a append any of which may be followed by b binary or read write If you don t provide a mode argument Python will assume your file is a text file and mode r will be assumed gt gt gt
90. something Python recognizes as a number using float Once I have assigned the four starting values as suitable types the calculation that follows is fairly straightforward as explained in the comments of Listing 3 2 Listing 3 2 curtains py curtains py Problem Calculate how much material to buy given the size of the windows Target Users My friend who wants to make some curtains Target System GNU Linux Interface Command line Functional Requirements Print out the required length of fabric in meters Print out the total price of the fabric User must be able to input the measurements of the window Testing Simple run test Maintainer maintainer website com 43 44 CHAPTER 3 M VARIABLES AND DATA TYPES version 0 1 To start with all the measurements will be in cm Assume that the roll of material is going to be 140cm wide and that the price per meter will be 5 units of currency roll width 140 price per metre 5 Prompt the user to input the window measurements in cm window height input Enter the height of the window cm window width input Enter the width of the window cm Add a bit for the hems First we must convert the string into a number otherwise we will get an error if we try to perform arithmetic on a text string curtain width float window width 0 75 20 curtain length float window height 15 Work out how many widths of cloth will be needed and figure out the tot
91. start keeping an eye out for nested parentheses The preceding code has an awful lot of brackets in it You could always evaluate each nested expression separately to make the code easier to read Readable code is a good thing 127 128 CHAPTER 7 WORKING WITH TEXT gt gt gt ab sep join str a str b gt gt gt sep join skidoo ab skidoo 1 2 37 You still need to make sure that each opening bracket is matched by a closing bracket and that the closing brackets for the inner nested expressions come before the outermost closing brackets Leaving out closing brackets can lead to a syntax error being reported on the next line because the interpreter is still waiting for closure of the previous statement Changing Case User input often contains unexpected capital letters Python has a variety of ways that you can neaten up the use of capital letters string capitalize returns a string with the first character capitalized and all the rest in lower case gt gt gt S3 eXamPLe tEXt gt gt gt s3 capitalize Example text If you want to capitalize every single word in the string leaving everything else as lowercase use string title gt gt gt s3 title Example Text The one little gotcha with this is that every letter following some space or punctuation gets capitalized gt gt gt s5 it s thirteen o clock gt gt gt s5 title It S Thirteen O Clock You can convert the whole str
92. string strip to clean up the results but you could also construct a smarter regular expression The pattern string consists of the characters that I m looking for plus a special character the period This period matches any single character except for newlines no more and no less Because it matches any character the period is known as a wildcard character The asterisk matches 0 or more characters of the preceding expression The plus sign matches one or more characters and the question mark matches one or zero characters of the preceding expression gt gt gt pattern gt gt gt re split pattern string 9 Yu The preceding greedy version of the pattern gobbles up everything up to the last chevrons which is not the sort of behavior I want to see We can mark an expression as nongreedy and match as few as possible this is indicated with an additional question mark and gt gt gt pattern gt gt gt re split pattern string aes Z ry D j V 5 Yu Adding in the question mark makes a better match It would also be possible to specify the number of repetitions to be matched following the pattern with a pair of numbers in curly braces m n which matches the pattern repeated between m and n times CHAPTER 7 WORKING WITH TEXT gt gt gt pattern 2 2 gt gt gt re split pattern string 5 796 vs D j V y Yu This example is s
93. target Calculate velocity of hit or miss return velocity def calc damage attacker target velocity Calculate the damage of the hit return damage Next I moved the code into the functions replacing each of the empty declarations with a call to the function and testing them one at a time Once the program ran without errors again I moved on to deal with the final alteration of adding more combatants For this slightly more complex change the idea I had was to multiply the list of players by itself to produce a matrix of attacker and target matches where the attacker and target are not the same character I started by creating a general function to create a tuple of matches out of more or less any sequence Notice how I have used empty slice notation in order to copy the given sequence to opponents this is to prevent the side effect of accidentally modifying a global list if one is sent to the function def ziply sides None opponents list sides opponents reverse matches actor target for actor in sides for target in opponents if target actor return tuple matches Again the main work is done using a comprehensive list and I made the function return the data in an immutable form just to be sure the data can t be accidentally modified by any other process Next comes the implementation of my new function I can replace this messy old code 113 114 CHAPTER 6 FUNCTIONS for attacker pla
94. that matches either the class or any of the superclasses gets to handle the exception So if we imagine Python s many exception classes as giving us information to distinguish between different errors then the class hierarchy lets us ignore the differences between what we might think of as similar classes of error Listing 10 9 gives examples of this You should see no output from this code Listing 10 9 Handling Exceptions by Referring to Their Superclasses dict try dict foo except KeyError pass dictionary specific lookup problem try dict bar except LookupError pass lookup problems in dictionaries tuples and lists try dict quux except Exception pass any problem of any sort except KeyError raise Exception Should have been handled by the first match Almost all exceptions you encounter could be handled by explicitly handling the Exception class similarly any dictionary or sequence lookups can be handled with LookupError rather than explicitly handling both KeyError and IndexError Note again in the last example in Listing 10 9 that they are handled by the first matching except clause found not the closest match Using finally to Clean Up After a Problem If you don t handle an exception locally that s generally a signal to the rest of your program that there might be some cleaning up to do However sometimes an exception can interrupt your program at such a point that there are other unfinished tas
95. the details about Animals and Mammals to get your Dog to bark you just need to know what commands your Dog understands Organizing your data into self contained units makes it easier to figure out problems using the same sort of logic that you are used to using in real world situations Many programmers find that object orientation allows their code to grow in a more natural way than constantly having to create new functions from scratch Empowering objects Objects are the primary means of representing data in Python All data in a Python program is represented by some type of object or by the relationship between objects A class is a template for an object that bundles up several related pieces of data and defines a bunch of functions for accessing and manipulating that data A class definition is similar to a function definition in that nothing happens until you tell Python to make an object based on the class template this is called creating an instance of that class Even then any methods that are part of the class won t do anything until they are specifically called just like functions Defining Classes The simplest way to understand how classes work is to create one Based on the first definition here is a simple version of a class in Python class Person def setName self name self name name title def getName self return self name end of class definition start of program personi Person create instance of c
96. the height of the window cm 100 Enter the width of the window cm 200 width height widths total price 170 0 115 1 21428571429 139 642857143 2 79 13 96 You need 2 79 metres of cloth for 13 96 And here are the results of another test Enter the height of the window cm 200 Enter the width of the window cm 100 width height widths total price 95 0 215 0 678571428571 145892857143 2 92 14 59 You need 2 92 metres of cloth for 14 59 And these are the results of our final test CHAPTER 4 E MAKING CHOICES Enter the height of the window cm 200 Enter the width of the window cm 300 width height widths total price 245 0 215 1 75 376 25 7 52 37 62 You need 7 52 metres of cloth for 37 62 From this sample of test data I can tell that I am getting closer to the right sort of results but if I went out and bought some material on the basis of these results I would find that I didn t have quite enough Somehow I ve managed to consistently calculate results that are too small Knowing what results to expect is the only way of really telling whether our program is working or not The difference between our expectations and the actual output can often help in finding the point where the calculation went wrong Before reading any further try to figure out why this version of the program is still returning the wrong length and most importantly where it is going wrong In order to refine this design furth
97. the window which you can do by clicking the Quit button Tkinter supports 15 core widgets which are explained in Table 12 1 263 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Table 12 1 Tkinter Widget Classes Widget Description Button A simple button used to execute a command or other operation Canvas This widget can be used to draw graphs and plots create graphics editors and to implement custom widgets Checkbutton Represent a variable that can have two distinct values Clicking the button toggles between the values Entry A text entry field Frame A container widget The frame can have a border and a background and is used to group other widgets when creating an application or dialog layout Label Displays a text or an image Listbox Displays a list of alternatives The list box can be configured to get radio button or checklist behavior Menu A menu pane Used to implement pull down and pop up menus Menubutton A menu button used to implement pull down menus Message Displays text similar to the label widget but can automatically wrap text to a given width or aspect ratio Radiobutton Represents one value of a variable that can have one of many values Clicking the button sets the variable to that value and clears all other radio buttons associated with the same variable Scale Allows you to set a numerical value by dragging a slider Scrollbar Standard scrollbars for use with canvas entry list box and text widget
98. up to you Do not attempt to use word processing software such as Word or OpenOffice for this task it will mess you up badly Be warned Choosing to Code with an Integrated Development Environment An integrated development environment IDE is a graphical interface with lots of useful features designed to make programming with Python even easier You still have to type the code in the same way but you can do all your coding using a single application and these programming environments can provide some useful shortcuts and reminders There are now several Python specific IDEs Popular applications include IDLE which is bundled with Python itself Eric you may notice a theme here nudge nudge wink wink say no more Geany DrPython and SPE Python is also supported by more general programming environments like Bluefish and a whole host of others This book doesn t cover the use of IDEs or any of the alternative distributions of Python because each of these approaches would require a chapter unto themselves at the very least However the examples contained in this book will still work if you do choose to explore these options This book will take the simplest approach using tools that come as standard with most operating systems that is a text editor and the Python Interactive Shell Getting Started with Programming The process of writing a new program usually starts with a problem Before you can code anything you need to have an
99. us import re Our custom exceptions class SecurityError Exception Standard error for house security pass class SecurityNotCertain SecurityError Exception raised when security can t be guaranteed pass class BuildingIntegrityError SecurityError Exception raised when a door is open 227 228 CHAPTER 10 EXCEPTIONS pass class MotionDetectedError SecurityError def def Exception raised when motion detected pass get report file report txt Get a report on the security status of the house Attempt to open the report file and read in the lines try f open file lines f readlines f close except I0Error raise SecurityError Cannot open status file 0 format file For each line trim whitespace split it at the colon and make a dict return dict re split s l strip 1 for 1 in lines check report report Check a house report dict that everything is secure to check door open front door open rear motion in living room motion in kitchen motion in attic for check in to check See if the value is what we expect for security try if report check no if check 0 9 door open raise BuildingIntegrityError Door open 0 format check elif check 0 9 motion in raise MotionDetectedError Motion detected 0 format check If we can t find the item to check security isn t certain except KeyError SecurityNotC
100. usr bin bin usr games This adds a directory called bin to the PATH that is searched for executable files by the system Note Other users will also need to add this directory to their PATH and the appropriate permissions need to be set in order for them to be able to make use of programs located in custom directories Also note that altering the PATH is temporary for the current session only You will need to edit the bash profile file to make changes reappear after logging in again If you re not familiar with Linux or Unix you should refer to an introductory text on this and related subjects You might want to take a look at Beginning Ubuntu Linux Fourth Edition by Keir Thomas Andy Channelle and Jaime Sicam Apress 2009 The next step is to create a link to the script from the bin directory In s full path to myscript py bin myscript Now I can call my new script from anywhere without giving an explicit path myscript Nice Rather than making a link you could write a short Bash script that calls the Python file and save that in bin The beauty of calling myscript py indirectly like this though is that it gives a lot more room if you want to rename or reorganize your code It doesn t make any difference to your users if myscript points to bin myscript py or usr local lib python3 0 myscript 1 0 stable py so long as the program still accepts the same arguments 163 164 CHAPTER 8 M EXECUTABLE FILES ORGANI
101. values from one type to another These are the most useful ones to start with e int x converts number x to an integer e loat x converts number x to a float e str object converts more or less anything into a printable string Functions have a very different syntax from operators the value to be acted upon goes inside the brackets after the function name CHAPTER 3 VARIABLES AND DATA TYPES gt gt gt Float 23 23 0 gt gt gt int 23 5 23 gt gt gt float int 23 5 23 There are a few gotchas when it comes to converting types notice that float int x in the preceding example loses its fractional part Not all conversions are reversible and the result of the conversion may not be equal to the input value any more Working with Base 8 Octal and Base 16 Hexadecimal It is possible to input and display integers in other formats such as base 16 hexadecimal or base 8 octal Integers can be entered in octal form by putting a 00 in front of the octal number that s the number zero followed by the letter o and in hexadecimal by putting 0x zero followed by x in front of the hexadecimal number In hexadecimal you use the letters A to F to represent the decimal numbers 10 to 15 gt gt gt octal number 0012 gt gt gt hexadecimal number OxFC6 gt gt gt print octal number 10 gt gt gt print hexadecimal number 4038 Let s break things down to see what s going on here In a decimal num
102. velocity lt vel min vel min velocity miss type int velocity vel max if miss type gt 7 miss type 7 if trace print t tMiss miss type print players attacker Name misses miss type players target Name continue Calculate damage inflicted by blow attack strength players attacker Muscle weapon damage players attacker weapon 1 attack damage attack strength weapon damage velocity target strength players target Muscle armour strength players target armour 1 target chance random randint 9 players target Brainz target defence target strength armour strength target chance potential damage attack damage target defence if potential damage lt 1 potential damage 2 damage random randint 1 potential damage if trace print print t tDamage damage if damage gt dam max dam max damage Print damage report 97 98 CHAPTER 5 M USING LISTS damage type int 7 damage dam max if damage type gt 7 damage type 7 if trace print t t tDamage damage type change type int 5 damage life left if change type gt 7 change type 7 if trace print t t t tChange change type print inflicting a damage report damage type and life changing change type Inflict damage on target players target life damage Check whether target is still alive or not if players target life lt 0 Print winner print
103. worth your time Next the velocity is calculated If the value is positive the attack counts as a hit a negative value is a miss The value is set with a rather arbitrary calculation designed to produce a value somewhere between 0 and 7 and then deliberately constrained if it is greater than 7 to prevent unnecessary errors This value is then looked up in the hits or misses tuple and a message is printed about the success or failure of the blow This result is fed into the equation to calculate the amount of damage which works in a similar manner to produce a weighted random value which is then constrained These underlying values are required merely to produce some plausible output which is hopefully fun to play Finally the damage score is subtracted from the target s life and the target s life score is checked If it is less than or equal to 0 the game ends with a break statement and a declaration of the winner This momentarily ejects the program s execution into the while loop which checks both players life stats As you already know one of them does not have a life score above 0 so the while loop terminates too and the program should exit cleanly Listing 5 2 rpcombat py usr bin env python rpcombat py Purpose Simple Role Playing Combat Game Target System GNU Linux Interface Command line Functional Requirements The user must be able to generate more than one character profile equip those characters with
104. write this as u a Z a D j Vu a u prefacing the string with u to indicate that you wanted to force Unicode encoding The main difference is in Python s internal representation of the string In Python 3 the string stays the way you typed it Python 2 has to turn everything into 7 bit ASCII representation to store it which involves lots of backslashes xc2 xaba xc2 xb9 xc2 xbb Z xc3 xb8 xc3 xab xc2 xaba xc2 xb2 xc2 xbb D xc3 xa9j xc3 xa0 V xc3 xb9 xc2 xaba xc2 xb3 xc2 xbb xc2 xbd xc2 xb5 The unicode version looks like this u xaba xb9 xbb Z xf8 xeb xaba xb2 xbb D xe9j xeO V xf9 xaba xb3 xbb xbd xb5 Fortunately both strings print correctly as at Z a D j V a3 By To go along with the string type changes the Python 2 x print statement has been replaced with a built in print function which takes keyword arguments to replace most of the special syntax of the former print statement To balance this the old raw input is replaced by input and you have to use eval input to get the old functionality of input Pll explain how eval works in Chapter 8 Splitting Strings As strings are immutable you will often want to split them up into lists in order to manipulate their contents my string split sep maxsplit returns a list of the words in string using sep as the separator or delimiter string Quick reminder a delimiter is a character or string of characters that are
105. you use many times throughout your code Sometimes you simply don t care about the error but other times you do how can your program handle errors intelligently in this scenario While any specific abnormal or exceptional event can be catered for by tedious extra coding by the programmer the ideas of exceptions and exception handling are implemented in many languages to make such procedures convenient safe and maintainable across all the programs you might care to write Python s exception handling which we explore in this chapter is intended to be sufficiently simple and fast that you should rarely if ever need to handle errors in any other way 221 222 CHAPTER 10 EXCEPTIONS Handling Simple Exceptions Let s begin with one of the most basic errors that a program can encounter trying to divide a number by zero You ll see how Python s own exception handling deals with the problem and how it can be used in quite subtle ways to turn a simple error into intelligent reporting and management In interactive mode type the following and press Enter to see the result gt gt 1 0 Traceback most recent call last File lt stdin gt line 1 in lt module gt ZeroDivisionError int division or modulo by zero You ve seen this sort of error report before while experimenting with Python In fact the report you re reading is the final stage in a procedure that Python has already gone through to try to handle the underly
106. you want to turn an already functioning program into a module That means of course that your module code must already be syntactically correct However there are a number of conventions that you should follow in your modules if you want to make the full use of Python s modular support From the perspective of legibility it helps if you can put any import statements at the top of the module so that many of the dependencies of that module on other code are immediately obvious Conditional import statements must of course go within the control structures that optionally execute them Any important variables either for exporting or internal use throughout the module should go next Most stylistic conventions for naming variables in Python are just that stylistic Python doesn t mind if you name your classes using camel case or lowercase with underscores One exception however is that Python treats any module variables named with a leading underscore slightly differently from other module variables Such variables are considered to be reserved for internal module use only and will not be imported by from modulename import However you can explicitly import the variable by specifying its name these variables are therefore called weakly internal Save the code in Listing 11 3 as internals py and run the interactive code after it Listing 11 3 Weak Internal Variables An internal value config True def return config Cat an i
107. 0 gold coins You can get the index position of the first item that corresponds to x using list index x gt gt gt inventory index scroll 2 You can insert an item x at position i with list insert i x gt gt gt inventory insert 2 wand gt gt gt inventory shield wax wand scroll 10 gold coins emerald pole potion You can also remove the first item that is equal to x in list s using list remove x gt gt gt inventory remove wax gt gt gt inventory shield wand scroll 10 gold coins emerald pole potion Sometimes you may want to remove and return a value from the list at the same time This can be done using list pop i This method takes an index position as an argument If the index is not specified it defaults to the last item in the list gt gt gt inventory pop 2 pole gt gt gt inventory shield wand scroll 10 gold coins emerald potion 81 82 CHAPTER 5 M USING LISTS Stacks and Queues Because lists are an ordered type of data it is possible to use them to store and retrieve data items ina particular order The two main models for doing this are described in traditional programming speak as stacks and queues A stack is a last in first out LIFO structure used rather like the discard pile in a card game You put cards on the top of the pile and take them back off the top You can
108. 1 on quit1 activate def on quit1 activate self widget args print on quit activate called with self s widget get name Window1 on quit1 activate Window1 on cut1 activate def on cut1 activate self widget args print on cut1 activate called with self s widget get name Window1 on cut1 activate Window1 on copy1 activate def on copy1 activate self widget args print on copy1 activate called with self s widget get name Window1 on copy1 activate Window1 on paste1 activate def on paste1 activate self widget args print on paste1 activate called with self s widget get name Window1 on paste1 activate Window1 on delete1 activate def on delete1 activate self widget args print on delete1 activate called with self s widget get name Window1 on delete1 activate Window1 on about1 activate def on about1 activate self widget args print on about1 activate called with self s widget get name Window1 on about1 activate Window1 on treeview1 cursor changed def on treeview1 cursor changed self widget args print on treeview1 cursor changed called with self s widget get name Window1 on treeview1 cursor changed Window1 on button1 clicked def on button1 clicked self widget args print on button1 clicked called with self s widget get name Window1 on button1 clicked main
109. 2to3 on file Iterate through lines in script Check that the script calls the correct version of the interpreter Check that the script declares Unicode encoding Turn comments into proper docstrings Check magic variables are correctly formatted if present Check for TODO amp FIXME lines Print out results Check for existence of AUTHORS ChangeLog NEWS README and TODO files in the same directory as file If no AUTHORS file write out author s to AUTHORS Check TODOs and Warnings If fixed append Changelog else append TODO HH He HR HE HE HE Note As well as importing the re module imported the time module using time strftime to give mea timestamp passed it time formatting code and no time so it will use the current time by default The time module is further discussed in Chapter 10 The script see Listing 7 4 sets up a bunch of regular expressions so I can check the content of the script against them later I have also set up a dictionary called spellbook to hold all the magic variables set in the script I used awhile loop that will loop around if the filename given does not have a py extension The script iterates through the lines in the script being examined and compares the lines against the regular expression objects I set up at the start Finally another function format info gathers all the information from spellbook and formats it nicely for output 148 CHAPTER 7 Listing 7 4 fix_st
110. 35 advanced formatting 129 133 avoiding concatenation 127 128 changing case 128 129 editing 134 finding 134 135 formatting methods 129 joining strings 127 128 293 294 INDEX splitting 126 127 terminology 159 text applications creating 35 37 text editors coding with 5 text fragments joining creating text applications 35 37 escaping sequences 34 immutability 35 nesting quotes 33 overview 31 32 using quotes 32 33 using special whitespace characters 34 35 text strings 15 TextView widget PyGTK 271 Text widget Tkinter 264 Tkinter 261 264 282 token defined 124 top level design 17 25 Toplevel widget Tkinter 264 trace defined 74 traceback 30 defined 240 handling exceptions 234 235 237 module 235 single stage 235 trace table 57 defined 74 trace variable 58 91 traversing defined 100 triple quotes 16 33 truth testing defined 47 try clauses defined 240 try statement defined 240 using with except clause 222 224 tupleQ function 157 tuples 75 79 80 changing values in 80 creating 79 80 TypeError 30 message 225 type function 184 U unbound method 192 unicode method defined 159 Unicode standard 125 unusual values critical values 57 use cases 13 use dict pop key default method 88 user requirements documents 13 users potential identifying 11 12 observing 13 talking to 12 13 user stories 13 user stories 13 25 UTF8 encoding 162 V
111. 59 formatting data 154 156 formatting strings 129 133 for statements 69 73 85 fractions 40 Frame widget Tkinter 264 functional requirements 14 function call 102 defined 123 function definition 102 defined 123 functions 19 24 101 124 accessing privileged information defining functions 101 102 overview 101 passing unknown number of values into function 103 104 sending invitations 102 103 using docstrings to document function 104 refactoring rpcombat py matrix refactored 113 123 overview 108 109 readable code 113 testing 109 111 terminology 123 124 variable scope 105 108 manipulating lists and dictionaries 108 overview 105 107 using global statement 107 INDEX G generalization 108 123 defined 123 generate method 186 get_args function 172 get_input function 172 get key default method 87 get_selection method 275 getvalue key default_value method 177 Glade 279 282 globals function 105 global statement variable scope and 107 global variables 105 123 graphical user interface GUI 261 greedy expression 136 greedy regular expressions defined 159 gtk Clipboard 270 GtkInfo class 269 H hashes 86 99 hash symbol 15 Hbox widget PyGTK 267 help files creating 6 helpQ function 22 23 hexadecimal base 16 defined 46 hex x function 42 home directory defined 179 html folder 173 l identifiers 18 24 selecting keywords 27 28 naming
112. 6 chaining defined 239 CHANGELOG file 114 characters 32 defined 45 character sets matching regular expressions using 138 char not in punctuation conditional construction 73 Checkbutton widget Tkinter 264 chmod command 162 class es 219 application of 200 219 creating simple 182 183 customizing attribute access 197 constructors 192 193 emulating existing types 194 196 emulating numeric types 197 199 output 193 194 overview 191 192 properties 196 defined 123 182 219 of exceptions accessing properties of raised exceptions 229 230 creating 225 227 229 hierarchy of 224 225 raising 225 227 id function 184 namespaces 185 overview 181 182 self parameter 183 terminology 219 220 type function 184 when to use inheritance 189 190 methods 190 191 overview 185 189 class definitions 181 close method 144 code defined 3 command line 4 command line interface CLI defined 6 commands module 268 comments 15 Common Gateway Interface CGI defined 179 scripts 173 comparing values 55 combining conditional expressions 53 manipulating Boolean values 52 53 operator precedence 54 55 overview 49 51 using assignment operators 53 54 comparison operators 50 defined 73 compiling defined 6 complex data types defined 99 concatenate defined 45 concatenation avoiding 127 128 operator 32 conditional code 61 64 conditional expressions combining 53 conditional statem
113. 6 dice 1 This would allow you to call rol1 with no arguments which would return a random number between 1 and 6 as if you had called rol1 6 1 Another option is to call the function using keyword arguments where we can specify which parameter gets which value Although this involves a little more typing the benefit is that the order no longer matters and it the function call is easier to understand gt gt gt roll dice 3 sides 33 35 Passing an Unknown Number of Values into the Function Sometimes you may want your function to accept an unknown number of arguments To do this you precede the name of the parameter with an asterisk This syntax collects all the remaining positional arguments into a tuple that it assigns to the parameter name following the asterisk gt gt gt def collect args args print args gt gt gt collect args one two three one two three You can also collect keyword arguments using a double asterisk and you will get a dictionary gt gt gt def collect kwargs kwargs print kwargs 103 104 CHAPTER 6 FUNCTIONS gt gt gt collect kwargs one 1 two 2 three 3 three 3 two 2 one 1 You can use these two concepts together in a single function but you must be careful to specify positional parameters before keyword arguments Also parameters that are to gather up any remaining arguments go at the end in the same order
114. 7 ss HL S 0 Ot 844 H gt name s race s gender s desc s lt 0 ss H Sa eet RV 844 H gt Strength strength lt 2 life life lt 3 Brainz brainz lt 2 magic magic lt 3 Speed speed lt 2 protection prot lt 3 Charm charm lt 2 gold gold gt 7 lt 0 ss HL Sao F 844 H gt Equipment o s format self strInv vars self return rpcharacter sheet 207 208 CHAPTER 9 def def def CLASSES len self Return the number of attributes return len vars self getitem self key Retrieve values by index names vars self item names key return item setitem self key value Set values by index self dict key value return def buy self purchase Buy item If the item is in the shop and the player has enough gold buy it Takes one argument purchase which is the name of the item you want to buy items item for item in shop inventory if issubclass type item Thing and item name purchase capitalize if items print We don t have a purchase for sale elif 0 lt items 0 gold lt self gold item items 0 msg fix gender self gender self name buys themself some equipment print msg print You buy a purchase for item gold gold pieces self gold item gold self inventory append item print You have a self strInv in your bag print Yo
115. 8 dictionaries 75 85 86 89 defining 86 88 deleting items from 88 89 manipulating 108 sorting 89 using 89 dict popitem method 88 Distutils defined 260 Distutils Python standard 255 dividing numbers 39 40 docstrings 104 using to document function 104 documentation 10 22 24 strings 6 defined 6 Don t Repeat Yourself principle 15 double asterisk 103 double equals sign 50 double quotes 33 double underscores 169 duck typing 184 dynamic typing 29 31 defined 45 E easier to ask for forgiveness than permission EAFP 238 239 style 238 Easy Install defined 260 Easy Install Python package manager 255 editing strings 134 285 286 INDEX Egg format defined 260 elif keyword 55 56 else clause defined 239 handling exceptions 232 234 else keyword 55 else suite 68 emulating existing types 194 196 numeric types 197 199 encapsulation 183 defined 220 Entry widget Tkinter 264 enumerate function 83 92 environment code block 185 programming 5 equals sign 53 equip method 186 error correction 21 22 Escaped Characters with special meaning in regular expressions 137 138 Escape Sequences 34 escaping 18 24 sequences 34 evalQ function 157 172 173 except clause 231 defined 240 using try statement with 222 224 Exception constructor 230 exception handling defined 239 exceptions 221 240 chaining 235 237 classes of accessing properties
116. 84 ziply0 function 11 295 You Need the Companion eBook Your purchase of this book entitles you to buy the companion PDF version eBook for only 10 Take the weightless companion with you anywhere W believe this Apress title will prove so indispensable that you ll want to carry it with you everywhere which is why we are offering the companion eBook in PDF format for 10 to customers who purchase this book now Convenient and fully searchable the PDF version of any content rich page heavy Apress book makes a valuable addition to your programming library You can easily find and copy code or perform examples by quickly toggling between instructions and the application Even simultaneously tackling a donut diet soda and complex code becomes simplified with hands free eBooks Once you purchase your book getting the 10 companion eBook is simple Visit www apress com promo tendollars Complete a basic registration form to receive a randomly generated question about this title Answer the question correctly in 60 seconds and you will receive a promotional code to redeem for the 10 00 eBook My Waer eBookshop Apress HE EXPERT S VOICE 233 Spring Street New York NY 10013 All Apress eBooks subject to copyright protection No part may be reproduced or transmitted in any form or by any means electronic or mechanical including photocopying recording or
117. 9 m What happened The conversion from centimeters to meters is off by a factor of ten as shown in the fourth column total which is fairly easy to fix I also have far more numbers after the decimal point than I really need This can be fixed using the built in round function which takes two arguments inside the brackets the number I want to remove the excess digits from and the number of decimal places I want to run to Two will probably do us fine right now The edited portion of the code looks like Listing 4 2 Caution Notice that we only round numbers when displaying them If you round the numbers you are actually calculating with that change will affect the results of your calculations and depending on the calculation the error introduced could be bigger than you expect 59 60 CHAPTER 4 MAKING CHOICES Listing 4 2 Rounding the Tesults to Two Decimal Figures Actually I have two curtains so I must double the amount of material and then divide by 100 to get the number of meters total length total length 2 100 print t t t t round total length 2 Finally I need to work out how much it will cost price total length price per metre print t t t t t round price 2 And print out the result print You need round total length 2 meters of cloth for round price 2 Now the design needs to be tested again with the same values Here is a different sample of the results Enter
118. ASSES Functional Requirements The user must be able to generate more than one character profile equip those characters with suitable weapons and model hand to hand combat between characters Testing methods trace table and play testing Expected results All statistics should be integers in the 1 99 range Apart from that this script needs play testing Limitations Too many to mention version 0 6 maintainer maintainer website com status Prototype date 16 12 2008 Import modules import random import time Preferences HH Boilerplate lt 0000 Cloud Cuckoo Land A simple role playing game version 0 s lt 0000 format version Set to True to trace variables trace on False Set amount of delay between combat strikes Higher numbers make combat slower drag 23 Create players list players Constants armour types set shield cuirass armour hits hits bashes smites whacks shreds stabs mutilates lunges at slashes lacerates carves up wipes the floor using misses misses nearly hits tickles fumbles fails to connect with swipes wildly at flails ineffectually at gets nowhere near hurts themself badly as a result of gross stupidity nearly decapitates themself instead of hits themself on the foot to the amusement of damage report a s
119. ATION AND PYTHON ON THE WEB Creating a Quick CGI Form Once you ve got your web server working correctly you can move on to getting user input as shown in Listing 8 4 Input is supplied to the CGI script by an HTML form You can retrieve the data from the form s fields using the FieldStorage class from the cgi module I ll explain how classes work fully in the next chapter For now all you need to know is that cgi FieldStorage returns a dictionary like interface which allows you to access the form fields by name The items in the FieldStorage dictionary contain more than just the value so you have to explicitly ask for the value There are several ways you could do this the best is to use the getvalue key default value method which is provided by the FieldStorage object Listing 8 4 form cgi usr bin env python coding UTF8 CGI form example import cgi import cgitb import time Enable CGI traceback cgitb enable Create datestamp now time strftime A d B Y Get the contents of the form form cgi FieldStorage name form getvalue name new user def print content print Content type text html print def print header print lt html gt lt head gt lt title gt Simple CGI form lt title gt lt head gt lt body gt def print form print nnn lt form action form cgi gt 177 178 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON
120. Byte A byte is the smallest unit of storage that can be accessed in a computer s memoty It holds exactly 8 bits e Case sensitive In case sensitive text uppercase letters are treated as completely different characters from their lowercase counterparts Treating uppercase and lowercase variants as the same character is known as case insensitive e Characters These are letters digits punctuation marks and spaces basically anything that can be typed in using a single key on the keyboard even if it doesn t cause anything to be printed on the screen e Concatenate When you create a string by joining together copies of two or more text strings without any spaces in between you concatenate the string e Decimal base 10 Decimal numbers are what you probably think of as normal numbers e Dynamic typing Python determines the type and checks the correct usage of variables of different types during execution of a program rather than during compilation Some other programming languages like C are statically typed the compiler will not allow the use of a variable or function unless that function or variable has already been initialized and declared to be of a certain type You don t need to bother with declaring the type of your variables in Python 45 CHAPTER 3 VARIABLES AND DATA TYPES Expression This refers to a section of code that can be worked out to produce a value Flag A flag is a Boolean variable used to record w
121. Certainly it could have been created on the fly via an input statement just consider the security issue first Using eval is just as easy and allows you to evaluate a statement Examine the following CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB gt gt gt t 3 gt gt gt result eval t gt 5 3 is not greater than 5 gt gt gt print result Note that we are evaluating a code string in this case t gt 5 which of course results in False Both of these functions provide the possibility of an alternative method of making choices and controlling the flow of program execution This approach is an interesting avenue to explore briefly and may be of some use before you come to building your own classes in the next chapter Caution Using exec and eval opens up whole new areas of security risks Take care to strictly control the content of any strings you send to either of these functions Putting Python on the Web If you run your own web site or have access to a server you may be interested in using Python to run web based services You will need FTP access to try this example Tip If you re running Linux and have root privileges or you re running another operating system that supports Apache you could install Apache and configure it to run as your own private local server to use as a web testing zone Refer to the official Apache documentation for information on how to set things up to ru
122. Check whether the first value is some text or not Print the first value Save it Every time you sit back from the screen and wonder What next you should save your work Get into this habit if you haven t already it will save so much brain ache and frustration You may also wish to copy your top level design along with the finished design back into the original text file so that you can refer to it when you have finished the coding stage 17 18 CHAPTER 2 DESIGNING SOFTWARE Coding the Design Now you are ready to start turning your design into proper Python code Great The data your software will be manipulating will take various forms these values are assigned to things called variables which you can think of as a box or a bucket that contains a single piece of information often a word or a number Some people find it useful to think of them as slates that can be written on with chalk The important thing is that the information in the container can be changed by putting a new value into it this automatically wipes out the old value and replaces it with a new one The fact that these values can be changed gives rise to the term variable There are two stages to creating a variable the first is to create the container and stick an identifying label on it this is called initialization The second is to put a value into it this is called assignment In Python both these things happen in the same statement Assignment an
123. ER 7 WORKING WITH TEXT Regular expressions are complex powerful and difficult to read Most of the time you can manage without them but they are particularly useful when dealing with complex structured pieces of text It s best to take regular expressions slowly learning a bit at a time Trying to learn the whole regular expression syntax all in one go could be quite overwhelming Regular expression matching operations are provided by the re module As re is not built in by default it is necessary to import the module first before it is possible to use it gt gt gt import re The module supports both 8 bit and Unicode strings so it should be possible to recognize any characters that you can type in from the keyboard or read from a file Next you need to construct a regular expression string to represent the pattern you want to catch Let s use the rather colorful string from earlier in the chapter again gt gt gt string at Z a D j Vu a y gt gt gt pattern a gt gt gt re split pattern string Z D j vu By Here I have used the re split method instead of string split to isolate the data from its strange labels This method takes a regular expression and a string as arguments The string is split according to the substrings that match the pattern these substrings are discarded returning the strings that remain as a list Obviously you could make use of
124. ER 9 M CLASSES command command format self target player trace Command command try exec command except print No can do return def enter self player Commands run when the player enters a location Takes a player object as an argument Adds Player to the location s inventory Provides a command line prompt until exit is called No return value command enter self inventory append player name print self print You enter the self name while command exit command input gt self interpret command player self exit player return def damage self attacker target velocity Calculate the damage of the hit Takes three arguments attacker and target are Player objects velocity is an integer representing the velocity of the strike Returns a tuple of two integers damage and potential damage attack strength int attacker strength weapon damage int attacker weapon strength attack damage attack strength weapon damage int velocity roll 172 target strength int target strength armour strength int target armour strength target chance roll int target brainz 3 target defence target strength armour strength target chance potential damage int attack damage target defence 0 3 if potential damage lt 1 potential damage 2 damage random randint 1 potential damage return int damage int potential damage def resolv
125. I would need 8 6 m of cloth to make a pair of curtains to fit a window measuring 2x3 m e However if the measurements were reversed 300x200 cm I could get away with only 9 45 m using the same method as the fourth case for the last case It would make the logic much simpler however to decide that horizontal seams are going to look ugly and go and buy 12 6 m of material to do this window This analysis brings up a couple of issues which I want to bear in mind as I refine the design First it doesn t look like measuring material along the width is going to provide many advantage unless the window is wider than it is deep and less than 1 25 m deep and unless the material has no pattern One of the programming decisions I now need to make is whether to bother pursuing an option to measure material along its width at all Second there is a minor issue of accuracy I actually need an answer rounded up to the nearest half meter This only really applies to the final output as the figures used in the calculations need to keep their fractional parts to maintain accuracy Now let s see how this compares to the trace produced by the existing calculations The simplest way to create a trace table is to insert a bunch of print statements into your code as you can see in Listing 4 1 Listing 4 1 Tracing Variables print headers for the basic trace table print print twidth theight twidths ttotal tprice I need to add a bit for the hem
126. ING SOFTWARE the letters after the last dot in the filename usually provides a clue as to what format a file is in though there are other ways of finding out this information The purpose of this exercise is to find out how this job is usually done and what information is needed before you begin An example calculation with realistic examples of the type and range of information you are likely to be dealing with is invaluable at this stage A written procedure could also be useful Watching Users Perform the Task at Hand If possible watch users carry out the task and note down any points of interest Again you don t need to go into a huge amount of detail at this stage and you need to cover only the information that is relevant to your software project and your potential users It can be useful to put yourself in the user s position and walk through the process yourself Another technique is to collect up all the pieces of paper that go into or come out of the process and use them to create a storyboard This stage of the design process becomes much more relevant in larger projects but it s worth getting into the habit of doing it now Compiling User Stories A user story consists of one or two lines in everyday language that specifies something that the user wants from the software The idea is that stories should be brief enough to fit on an index card ona sticky note or into one cell of a spreadsheet A user story should indicate
127. Linux people within that culture prefer v code to v program whereas outside it the reverse is normally true The noun program is defined as being e A magic spell cast over a computer allowing it to turn one s input into error messages e An exercise in experimental epistemology e A form of art ostensibly intended for the instruction of computers which is nevertheless almost inevitably a failure if other programmers can t understand it As the information contained in the Jargon File is more entertaining than strictly informative will be providing some simpler definitions at the end of each chapter CHAPTER 1 INTRODUCING PYTHON Learning While Having Fun Having fun is often underestimated in the realm of technical disciplines We learn best and are most productive when we re having fun Developing software using Python is often an engrossing and enjoyable experience partly because you can test out your changes as soon as you have made them without having to perform any intermediary steps Python also deals with many background tasks the household chores of programming so you don t have to bother with them as you might in languages like C This kind of immediate gratification makes it easier to stay in the creative flow and keep being inspired Python is also easy to read being closer to natural language than many other programming languages so you spend less time trying to dec
128. NS In the next chapter I will pull together the threads of your existing knowledge with such diverse topics as formatting strings and pattern matching debugging project organization style guides and dealing with version changes I will also be showing you how to make your programs executable as stand alone applications If you want to examine your code further remember you can start Python in interactive mode from the project directory and import the program as a module gt gt gt import rpcombat You will have to play through the game once Then you can use the following command gt gt gt help rpcombat to check out what happens to all those docstrings You could also use this command for a comprehensive overview of the program gt gt gt vars rpcombat Jargon Busting This chapter s jargon is defined here e Abstraction When you introduce abstraction you move a number of program statements into a function thereby reducing the level of detail and making the main program easier to read e Calling procedure A block of code containing a function call is a calling procedure e Class A class is a user defined compound type e Date stamp This string of numbers and digits represents the date of an action or the present moment e Exceptions An exception is an error that occurs when a program is run e Formal parameters These are the names used inside a function to refer to the values passed as arguments e Func
129. Python Programming Language Official Website gt gt gt Going up and down the tree soup div id search 0 fetchParents 0 name form urllib in the preceding example fetches a remote web page Automatically retrieving content over the Web is another common task these days and it s worth having urllib up your sleeve Think of it as a bonus fourth example for you A Package for Everything Eventually Python 3 is still a relatively new dialect of the Python language As such it doesn t necessarily have the same breadth and depth of third party package support as Python 2 x At the time of this writing many of the more popular modules have been ported or are being ported to Python 3 thanks to the effort of the core Python developers in providing tools to aid automatic migration to the new dialect of the Python programming language Ultimately it s likely that the majority of 2 x only code will gradually migrate to 3 x soon In the long term only underused or legacy packages will remain in 2 x only versions That means that eventually you should be able to find a Python package out there somewhere to solve most of your day to day problems PDF creation integration with other proprietary file formats image conversion and more Here s the trick whatever you find yourself about to build consider if other people might have been in a similar situation in the past If so they may well have packaged up their code a
130. RS selected 24 bytes Figure 8 1 My home directory Writing Stylish Code Python was designed to be readable on the grounds that code is read more often than it is written Python does not dictate how you should name things or use comments and docstrings These matters are considered to be part of your coding style so it is worth putting some thought towards the style you use You can create your own unique style if you want but it does pay to be consistent Consistency prevents you from getting confused yourself and when you want to share your work with other coders you will either need to be able to explain the conventions you have used or adopt a common style 165 166 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Becoming More Pythonic Every so often you will come across the word Pythonic in discussions about coding It refers to a combination of approaches to programming that are considered idiomatic to Python as opposed to the way you might approach programming in other languages there are a growing number of conventions involving methods and code constructions that are generally agreed to be more efficient or mutually understandable by the Python community These approaches are not dictated by Python you won t get an error message if you don t follow these guidelines so it is up to you how much of this you want to take on board However I would recommend following the generally accepted Pythonic styl
131. THE EXPERT S VOICE IN OPEN SOURCE All you will ever need to start programming Python www allitebooks com Python 3 for Absolute Beginners Tim Hall and J P Stacey Apress www allitebooks com Python 3 for Absolute Beginners Copyright 2009 by Tim Hall and J P Stacey All rights reserved No part of this work may be reproduced or transmitted in any form or by any means electronic or mechanical including photocopying recording or by any information storage or retrieval system without the prior written permission of the copyright owner and the publisher ISBN 13 pbk 978 1 4302 1632 2 ISBN 13 electronic 978 1 4302 1633 9 Printed and bound in the United States of America 987654321 Trademarked names may appear in this book Rather than use a trademark symbol with every occurrence of a trademarked name we use the names only in an editorial fashion and to the benefit of the trademark owner with no intention of infringement of the trademark Lead Editor Matthew Moodie Technical Reviewer Duncan Parkes Additional material Dr J Burton Browning Editorial Board Clay Andres Steve Anglin Mark Beckner Ewan Buckingham Tony Campbell Gary Cornell Jonathan Gennick Michelle Lowman Matthew Moodie Jeffrey Pepper Frank Pohlmann Ben Renow Clarke Dominic Shakeshaft Matt Wade Tom Welsh Project Managers Beth Christmas and Debra Kelly Copy Editor Heather Lang Compositor LaurelTech Indexer BIM Indexing and
132. WORKING WITH TEXT data performs int list and tuple conversions on some of the data types and assigns them all to their correct variable names individually But life would be much simpler if I could just load the entire dictionary back into place That would also cut down on potential errors as I would know that my data is exactly the same as the data the program used last time it was run The simplest answer to this conundrum is to use two built in functions repr and eval In some ways repr is to eval what str is to print In other words repr outputs variables in the format that eval expects to find and eval then turns those strings back into a legal Python expression and evaluates them It is not a perfect solution but it is simple workable and fixes this TODO item I created two new functions in rpcombat py write out and read in The first function write out players is shown in Listing 7 7 It opens data file in write mode and iterates through the players list writing out the repr form of each dictionary to a separate line in the file rpcharacters rpg I could have given it a txt extension but there is nothing to stop you creating your own file extensions if you want to The file will still be a plain text file but it is a way of indicating that the data contained within it is of a specific format Listing 7 7 Function to Write Data Out to File def write out players print Saving character sheets
133. ZATION AND PYTHON ON THE WEB Sooner or later you will encounter the major gotcha which is that any references to files using relative path names won t work Now is a great chance to test whether any parts of your code were dependent on being called from a specific directory The most likely fix will be to replace any relative paths with full paths that go right back to the root directory of your filesystem or that are relative to a path stored as a variable Organizing Your Project At the start of this book I suggested creating a separate folder for your programs It s up to you to organize your code how you want it but having some kind of plan is a really sensible idea I like to keep the programs that I am developing in a folder called Projects Once the application is stable and I want to make it available for use I copy a cleaned up version of the folder and its contents into 1ib python3 0 following my system s conventions prefix bin python This is the recommended location of the interpreter prefix lib python lt version gt This is the recommended location of the directories containing the standard modules Note that these modules are reusable individual application code might be in other directories prefix include python lt version gt This is the recommended location of the directories containing the include files needed for developing Python extensions and embedding the interpreter I have replaced prefi
134. a label on it and a label is a plain text string The stock argument is for using preformatted stock Gtk icons the list of available options is at http www pygtk org pygtk2reference gtk class reference html The use underline is a function that allows you to underline letters in the label as a mnemonic for key combinations To find the syntax for some methods you to trace the ancestry of some of these objects For example the origin of the connect method lies way back in the gobject GObject class which is the parent of all graphic objects used in Gtk gobject GObject connect detailed signal handler is the given syntax detailed signal is a text string identifying the signal to be acted upon The gtk Button widget can understand activate clicked enter leave pressed or released in this context in Listing 11 2 the value clicked is used The handler method that you have already specified is the command to be run when the signal is received The handler is the self callback method in this instance All subsequent arguments are passed on to the method 267 268 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS The show method is inherited from gtk Widget and mercifully takes no arguments Because of the way the class hierarchy is organized you can assume that all widgets have connect and show methods which work in exactly the same way The buttons are packed using the gtk Box pack start child expand fill padding method
135. a sentence separately which can prove to be a useful technique There are a few useful built in ways to convert sequences into different types of sequences First list seq turns any sequence into a list if you do this with a string you ll get a list of the individual characters gt gt gt list banana b nay n ai n a gt gt gt list vegetables carrots potatoes onions leeks celery If you want to turn a sequence into a tuple the same rules apply to tuple seq gt gt gt tuple vegetables carrots potatoes onions leeks celery Converting a list into a string will make a readable text representation of the list gt gt gt str vegetables carrots potatoes onions leeks celery If you want to know how many elements a sequence has use len seq gt gt gt len vegetables The number of items in a sequence that are equal to x can be counted up with sequence count x In other words it counts each item where x item is true CHAPTER 5 M USING LISTS gt gt gt vegetables count carrot 1 If you want to retrieve the greatest item in a sequence use max seq gt gt gt max vegetables potatoes The least item can be accessed similarly with min seq gt gt gt min vegetables carrots It may be worth taking a moment to consider the results of the last two examples Were they what you expected The results came out based on alphabe
136. a sequence is an instance of a data type that consists of more than a single item It does not refer to a series of statements to be executed one after another as in some other languages Statement This refers to a section of code that does something such as manipulate a piece of data perform a calculation or produce some output www allitebooks com CHAPTER 3 VARIABLES AND DATA TYPES e String literal This refers to words and sentences and text composed of these that is any form of literal text e Truth testing Evaluate whether a condition is True or False Summary In this chapter you have learned how to assign different types of values to variables how to manipulate simple text strings and perform basic mathematical operations using expressions and statements Now you know how to construct and run a simple Python script that can perform a useful calculation In the next chapter I will show you how to make decisions that can alter the way the program runs based on those calculations 47 CHAPTER 4 Making Choices So far you have learned how to enter commands and information and get some output returned using fundamental data types text strings numbers and Booleans You have learned how to use Python as a pocket calculator but most importantly you have learned how to start designing your own software Now it is time to begin the process of refining your designs As you learn more about Python and practice your log
137. ables you encountered throughout the book These are predefined names that allow you to override your custom type s behavior in respect to the various standard code constructions in Python The names used for these special methods are part of the programming language so you can t use arbitrary names 191 192 CHAPTER 9 M CLASSES Magic methods are easy to recognize having double underscores before and after the name Each magic method has a specific purpose and needs to be defined in an object s class definition in order for it to work For example you could manipulate your type with mathematical and comparison operators or make it imitate the behavior of an integer or list This is Python s approach to a technique called operator overloading which plays a major role in polymorphism If you want to emulate the methods of a built in type it is important that the methods make sense for the object being modeled For example some sequences might work well with retrieval of individual elements but extracting a slice might be meaningless Constructors The first special method to be used in most class definitions will be the object init self method This method is called immediately after the object has been created The one special constraint on constructors is that no value may be returned doing so will result in a TypeError The entire function of a constructor is to set up the new object so a return value would be meaningless anywa
138. aces rather than tabs remember Identifier This is a user defined name both function names and variable names are identifiers Identifiers must not conflict with keywords Initialization This process sets the initial value of a variable Initialization is done only once for each variable when that variable is created Module A module is a Python program that can be imported by another program Modules are often used to define additional functions and classes Nesting Put one piece of code inside another similar construct Nonfunctional requirements These are needs or limitations of the software that are not specifically about what the software will do Program implementation This is the actual realization of the program as opposed to the design Prompt This string of text or punctuation marks indicates that the user needs to input some information or commands CHAPTER 2 M DESIGNING SOFTWARE e Pseudocode This program design language is intended to be read by humans not performed by a computer e Return This refers to the process of transferring execution from a function back to the place from which that function was called in the main program The return statement can also pass a value back to the main program for use in further calculation e Statement This instruction to do something in a programming language manipulates a piece of data performs a calculation or produces some output e String This refers to a l
139. ack them into the box You need to call the show method on each widget in order for it to be displayed starting with the details like the buttons and moving out to the main window last Calling main invokes the Gtk main event processing loop which handles mouse keyboard and window events So finally a little Gtk window should open on your desktop like the one shown in Figure 12 2 Figure 12 2 A little Gtk window Catching Signals Gtk not only provides nice graphical building blocks but it also monitors user input and other events Each widget can respond to different signals such as mouse clicks and text input Gtk provides a huge number of widgets which are detailed at http www pygtk org pygtk2reference gtk class reference html Get used to understanding the contents of this reference Each page gives a brief description a list of the methods available to the widget object a list of its properties with explanations and a list of signals that it can receive The full syntax of each method is explained at the end of the page In order to understand the arguments given to the various gtk Button methods used previously you need to go to the gtk Button page First you should read the Constructor section You should also make a note of the signals it can receive such as click and right click You will see that gtk Button can take three optional arguments label stock and use underline Mostly you ll just want to stick
140. acting I noticed some interesting anomalies I had to set up the stock dictionary after the class definitions because it contained objects of some of those classes It struck me that a shop could be a kind of object one that the players could inhabit rather than something they d pick up and use Taking this thought process to its logical extreme I figured that all my custom objects could benefit from sharing a few attributes so I set about designing one class to rule them all I called this class GameObject and used it to set up all the properties that a game object might need Next I subclassed GameObject into Player Thing and Location types Most of the code that was previously in a function was moved For example generate rpc was moved into Player init allowing me to either generate a Player from scratch or load stored details by providing a dictionary of suitable values as an argument The old buy equipment function was largely moved into the Player object as Player buy item with some of the functionality moved into the Shop object Similarly calc velocity became Player strike target This final stage of abstraction moves as much of the action into self contained objects as is practical Where an outcome is the result of the interaction of two objects you must decide which object runs the code which object will be the actor and which will be the recipient If you have designed your objects well the flow of events may be
141. ading Inclusive Numerical Operators Magic Method Operation object iadd self other self other object isub self other self other object imul self other self other object idiv self other self other object itruediv self other self other object ifloordiv self other self other object imod self other self other object ipow self other modulo self other object ilshift self other self lt lt other object irshift self other self gt gt other object iand self other self amp other object ixor self other self other object ior self other self other These methods should attempt to do the operation in place modifying self and return the result which could be but does not have to be self If a specific method is not defined the augmented operation falls back to the normal methods Table 9 5 shows methods that overload some built in functions 199 200 CHAPTER 9 M CLASSES Table 9 5 Overloading Some Built in Functions Magic Method Operation object neg self self object pos self self object abs self abs self object invert self self object complex self complex self object int self int self object float self float self object index self operator index Must return an integer Application As I started looking through the code of my little role playing program to see what else needed abstr
142. administrator or consult log files to decide what to do next in order to get Python to run properly on your web server You will notice that the error messages you receive from the web server really aren t that informative The situation can be improved by using the cgitb module which will display some familiar looking traceback information in the browser window if an error is encountered This can be enabled by adding the following two lines to the start of your script import cgitb cgitb enable If this provides you with a useful Python error message see Figure 8 4 you re in luck the error was with your code which means you can do something about it File Edit View History Bookmarks Tools Help KE y A O http glastonburymusic org ukjcgi bin hello cgi gt j NameError A problem occurred in a Python script Here is the sequence of function calls leading up to the error in the order they occurred home virtual site21 3 fst var jwww cgi bin hello cqi 28 print_content 29 print_header 30 print lt hl gt It s s lt hl1 gt 31 lt p gt Your server is correctly set up to run Python programs lt p gt now then 32 print footer now Friday 14 November 2008 then undefined NameError name then is not defined args name then is not defined Done im Figure 8 4 A useful Python error message CHAPTER 8 M EXECUTABLE FILES ORGANIZ
143. age report 1 damage type int size damage self dam max if damage type gt size damage type size elif damage type lt 0 213 214 CHAPTER 9 M CLASSES damage type 0 size len life changing 1 change type int size damage life left if change type gt size change type size elif change type lt 0 change type 0 outstring join with their attacker weapon name lower causing damage report damage type and life changing change type output fix gender attacker gender outstring print output trace vel o hit 1 dam 2 3 type 4 change 5 format velocity strike msg damage potential damage damage type change type Inflict damage on target target life damage Pause slightly to stop this all scrolling past too fast gap drag velocity time sleep gap Check whether target is still alive or not if target life lt 0 Print loser print n target name collapses in a pool of blood n End this round of combat immediately return True def combat self Initialize combat sequence Takes no arguments Creates a list of matches iterating through them until one Player is victorious Returns False if no victory is achieved print n Let the combat begin n self vel max self dam max 23 self vel min self dam min 1 Loop while more than one player is still alive players item for
144. al length of material for each curtain in cm still widths curtain width roll width total length curtain length widths Actually there are two curtains so we must double the amount of material and then divide by 10 to get the number of meters total length total length 2 10 Finally work out how much it will cost price total length price per metre And print out the result print You need total length meters of cloth for price Any of you who have actually set about making curtains will know that this is a gross oversimplification of the process Those of you who can add numbers better than I can will realize that I have made a couple of dumb mathematical errors so this script will return some crazy values which won t be of much use Clearly this is going to need some work so let s examine the problems Unless the width of each curtain is less than the roll width you will end up buying much more fabric than you need However there is no way of working this out until you know what the initial window measurements are If the length of the curtains is less than the roll width you could turn the whole thing on its side and just use one width of fabric I m assuming you re using unpatterned fabric But if the curtains need to be both longer and wider than the roll width there is a problem if the extra material required is less than half the roll width you would need to buy an additional width of mat
145. ance Issues that are worth thinking about early on are how the software might need to change in future and how to keep track of issues You might also consider how the application is going to be kept in working condition while you re integrating new possibilities and responding to new challenges In many cases the application will be maintained by the person who wrote it but it is good practice to organize your project and write your code in a way that makes it easy for someone else to understand what you ve done and contribute fixes and new features One day you might want to pass on the responsibility of keeping it up to date to someone else To facilitate maintenance the software should include some means of contact such as the author s e mail address so that users can get in touch if there are any problems For now just note your e mail address Later on I ll show you how you can integrate this information into your code so you don t have CHAPTER 2 M DESIGNING SOFTWARE to retype this information every time you want to use it Don t Repeat Yourself is a very sensible principle of programming that states that every piece of knowledge must have a single unambiguous authoritative representation within a system No piece of information should ever have to be typed in twice if you find yourself doing this it s a sure sign that some part of your script needs redesigning Great You should now have a little text file that looks some
146. and printing out lots of different sorts of strings gt gt gt boilerplate T O e a O aaa O Egregious Response Generator Version 0 1 FiliBuster technologies inc vay OEE e gt gt gt print boilerplate Egregious Response Generator Version 0 1 FiliBuster technologies inc This can be very useful if you want to format a whole page or block of text In Chapter 7 I ll be looking at more sophisticated methods of string formatting 33 34 CHAPTER 3 M VARIABLES AND DATA TYPES Escaping Sequences There will be times when nesting different types of quotes will not be sufficient in those cases you can include further literal quotation marks by escaping them so or If you need to print a literal backslash you have to escape the backslash itself like this The input function which I used to get user input in hello word py stores the string with extra escapes where necessary so the string will print out exactly the way it was typed gt gt gt variable input Type something in Type something in String with lot s of bad punct uation in it gt gt gt variable String with lot s of bad punct uation in it gt gt gt print variable String with lot s of bad punct uation in it Using Special Whitespace Characters It is also possible to specify whitespace characters using a character sequence that begins with a backslash as shown in Table 3 1 n produ
147. ange 1 dice result random randint 1 sides return result def ziply seq None Create a matrix of matches from a sequence Takes one argument seq which should be a sequence of length gt 1 Returns a tuple of tuples matches opponents list seq opponents reverse matches actor target for target in opponents for actor in seq if target actor random shuffle matches return tuple matches def fix gender gender phrase Replace the word them with gender specific pronoun Takes two arguments gender a string which can be male female or something else phrase the string to be modified Returns a string with non gender specific pronouns replaced by gender specific ones if gender female phrase phrase replace them her phrase phrase replace their her phrase phrase replace themself herself elif gender male phrase phrase replace them him phrase phrase replace their his phrase phrase replace themself himself else phrase phrase replace them it phrase phrase replace their its phrase phrase replace themself itself return phrase def write out players Save Players Write Player stats out to file print Saving character sheets data file open rpcharacters rpg w lines for player in players lines append repr player n 217 218 CHAPTER 9 M CLASSES data file writelines lin
148. ar Expressions Character Meaning lt number gt Match the contents of the group of the same number A Match only at the start of a string Z Match only at the end of a string b Match an empty string but only at the start or end of a word B Match an empty string but not at the start or end of a word 137 138 CHAPTER 7 WORKING WITH TEXT Table 7 5 Escaped Characters with Special Meaning in Regular Expressions continued Character Meaning d Match any decimal digit D Match any nondigit character s Match any whitespace character S Match any nonwhitespace character w Match any alphanumeric character W Match anything that isn t an alphanumeric character Match a literal backslash The good news is that the application of backslashes can be automated to a certain extent using the re escape pattern method See the Letting the Characters Escape section Matching Character Sets and Alternatives If you want to match a specific range of characters you can put them in square brackets abc will match any one of the characters a b or c but not all of them together You can also specify ranges such as a zA Z0 9 which would include all uppercase and lowercase letters and digits Inserting a caret as the first character negates the set so a zA Z0 9 would match any single nonalphanumeric character The characters in square brackets don t have to be escaped in the same wa
149. aracter profile dictionary in place It returns a value which evaluates as either True or False intended to control the shopping loop Display shop stock list with prices print print lt SHOP gt for item in stock print t item stock item o print lt gt print print You have profile gold gold def def CHAPTER 6 M FUNCTIONS Prompt user to make a purchase purchase input Please choose an item or type done to quit If the item is in stock and the player has enough gold buy it if purchase in stock if stock purchase 0 lt profile gold test phrase profile Name buys themself some equipment print fix gender profile Gender test phrase print You buy a purchase for stock purchase 0 gold pieces profile gold stock purchase 0 profile inventory append purchase print You have a join with and profile inventory in your bag print You have profile gold left else print You don t have enough gold to buy that elif purchase done or purchase return profile inventory and profile gold gt 10 else print We don t have purchase in stock return purchase calc velocity attacker target Calculate velocity of hit or miss Takes two arguments attacker and target are integer pointers to the players list This function looks up values from the players l
150. arch to find out if an item is a techno CD I would use and gt gt gt is techno and is CD False Both conditions have to be true for and to return True If you want to be totally contrary and test whether the opposite of a condition holds true use not This will return True if the value you are testing is false otherwise it will return False gt gt gt not is techno CHAPTER 4 E MAKING CHOICES True gt gt gt not is techno and not is DVD True All these comparisons boil down to a single True or False answer which can then be used to tell the program in which direction to go next Boolean operators have a lower precedence than all the other mathematical operations not first then and and or last This means that in the preceding example the expression is actually evaluated as if it is bracketed like this not is techno and not is DVD Comparisons can be used to set flag variables A flag is used to record whether an event happened or a test was passed so that information can be used later on in the program gt gt gt is cool not is techno and not is country and not is emo Combining Conditional Expressions Conditional expressions can be combined to produce relatively complex conditions using the logical operators and and or For example we can check if two conditions are true var1 lt 6 and var2 gt 7 This will return True only if var1 is less than 6 and var2 is greater than 7 We can also use or
151. armlessly to the Screen def log error err Simple logging method writes error s details to standard output print LOG 0 1 format err class name err KeyError accessing a dictionary key that does not exist try d foo bar d quux except KeyError as e log error e 229 230 CHAPTER 10 EXCEPTIONS TypeError trying to perform an operation on incompatible operands try str 5 except TypeError as ex log error ex NameError referring to a variable that does not exist Note that neither e nor ex exist outside their except clauses above try print ex except NameError as exc log error exc Listing 10 7 prints the following message to the screen LOG KeyError quux LOG TypeError Can t convert int object to str implicitly LOG NameError name ex is not defined We have been able in the preceding code to interrogate each exception object and then log its type its class name and the message associated with it But what if we want to raise more information alongside the exception You will see later that Python attaches a lot of diagnostic messages to exceptions so that they can be traced through the code by the programmer But there might be extra information in the exception s original environment that for example helps us clean up after it or otherwise recover from it helps us track down the precise problem in a configuration file or helps us decide wheth
152. arr weightables lt function yarr at 0xb7c6b12c gt 0 20000000000000001 0 29999999999999999 You can also import from a module at any point in your code In fact you can import within conditional statements that way under certain conditions the import never happens at all However note that the wildcard version of the import syntax i e from modulename import cannot be used anywhere except in the main body of a program or module If you try to use it in a method Python will raise a syntax error Finally all of these invocations of the import keyword rely on the internal method import For example these two statements are equivalent i e their internal representations in Python are similar in structure gt gt gt import convert as foo gt gt gt foo import convert 245 246 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES More information about the arguments to import is available from the help built in method for advanced users You will almost never need to use import but one useful application is to import a module whose name is not known until the program is being run The module name convert is actually a string in the second example so it could be set to another variable or even set based on user input gt gt gt my module name convert gt gt gt foo import my module name Structuring Your Modules Broadly speaking you don t need to structure your code differently if
153. arts of objects but the term usually refers specifically to an instance of a class e Pointer This variable name doesn t hold any information itself but points to another variable e Positional arguments These are values passed to a function in order e Pure function A pure function has no side effects and can make changes to the calling program only through its return values e Refactoring When you refactor you break a process down into smaller procedures e Result This is the value that comes back after a function has run e Scope A scope is the area where a variable is available to your code such as global scope where a variable is available to all your code or function scope where a variable is available to just the function e Side effects A side effect is a change to a value caused during the execution of a function that isn t caused by reading the return value of the function e Source data A function performs its operations on source data e Symbol table The symbol table is a container for all the names created during the course of a procedure s execution e Token This part of a program is treated as a separate unit by the compiler It s analogous to a word in a human language such as English Summary In this chapter I covered how to abstract and generalize your code into functions Functions allow you to make your code more readable maintainable and efficient And it s not just for your main a
154. ash with names used in the other parts of the program This is why we have the concept of scopes When any program is run the interpreter keeps track of all the names that are created and used in the program These names are tracked in a table referred to as a symbol table and can be viewed as a dictionary using the built in vars function The variables created in the main program s symbol table are known as global variables because any part of the program can access these variables These can be viewed with the built in globals function You will notice that the result of running vars with no arguments inside the main program is the same as running globals So far the script examples in this book have only used global data Caution Do not attempt to modify the dictionary returned by vars locals or globals Doing so may not have the effect you want Any variable that is created in a function is stored in a symbol table unique to that function this data is known as local data and can be accessed within that function using the locals function The main body of the program global scope cannot access the variables set inside the function local scope The function however is still able to access names in the main body that is globals Hence you have two ways to get at a function to process some data The correct way is to take that data as a parameter for the function the function will process it and then return the resu
155. atements I will introduce you to the different exception objects and the way they are organized and created and show you how to access the properties of the raised exception and then log the output You will discover how to make proper use of tracebacks and how to create your own exception classes to cover the specific needs of your programs Later in this chapter you ll find out how to handle more complex errors and grok the art of exception chaining Finally I ll deal with cleaning up after the error so the program can continue to run smoothly When Something Goes Wrong Only the simplest programs are guaranteed to always behave exactly as expected The hello world py script in Chapter 2 when run will always echo the text that the user types in and the programmer can tell just by looking at the script that if it runs successfully once it will do so every time As programs get bigger and more complex their behavior becomes harder to predict For example a program might have a configuration file what happens if that file contains settings that are unsuitable or if the file doesn t exist at all Almost certainly programs will use input data that will change over time what if the data is corrupted or somehow unusable And what if another system resource for example a network connection needed to contact a remote web service to obtain additional data is unavailable Worse what if any number of these abnormal events occur in some method that
156. atements in our code Fortunately Python provides a variety of data types that can make handling groups of data much simpler The data types that are most used in Python are strings tuples lists and dictionaries These are collectively called data structures Strings are just pieces of text Tuples and lists are ordered groups of individual data items Dictionaries are groups of key value pairs Strings tuples and lists are examples of a particular data model called sequences The methods used for accessing the data in a sequence are same which you will see later in this chapter There is another way of looking at these data types according to whether they can be modified or not which is called mutability Strings and tuples are immutable which means that we cannot modify an existing string or tuple although we can use them to create new strings and tuples respectively Lists are mutable which means we can add or remove items from a list Accessing the items in a sequence We can fetch an individual item from a sequence using an index which is the position of the element The index is specified as an integer a whole number in square brackets immediately following the 75 76 CHAPTER 5 M USING LISTS variable name So s i will retrieve the item at position i of sequence s This allows you to access a single character in a string gt gt gt fruit apple gt gt gt fruit o a or an item in a list or tuple g
157. atements that are the core functionality The function can also return some data using a return statement The function s last line specifies which variables get returned to the main program If there is nothing to return you do not have to use a return statement and Python will assume None as the default return value Note The convention is to name functions in a similar way to variables using lowercase letters with underscores The main difference is that functions should have names that suggest actions verbs rather than things nouns Up to this point the block of code in our function has not yet been run it has simply been assigned to the function definition To get the code to run you need to call the function from the main body of the program This is called a function call Since we have given names to functions we can call those functions any number of times gt gt gt generate rpc New Character We haven t specified any parameters in this example hence the empty parentheses after the function name Sending Out Invitations Most functions work on some data that they have been given by the main program In order for the function to receive data it needs to set up some empty containers to hold the data These become variables unique to the function and are known as formal parameters and it s best if they don t use the same names as in the main program These formal parameters have to be specified in t
158. b W a z A Z W 4 lt b gt 2 lt b gt 3 text Underline words between underscores text re sub w lt u gt 1 lt u gt text Join up Header text and footer text join header text footer Write out web page output file open output filename w output file write text output file close print output filename written out CHAPTER 7 WORKING WITH TEXT Story Once upon a time it wasn t your time nor my time but a very long time ago there lived a gentleman and a lady in a very beautiful part of the country They only had one daughter who was very pretty and good and both her parents were very fond of her When the girl was about five years old her mother died The father was heart broken over the loss of his wife and left the little girl pretty much to herself She cried endlessly because she did not understand where her mother had gone that she did not come to her After a respectable time her father married a young widow who already had three daughters of her own They all thought proudly of themselves as fine ladies but they were also very plain and were jealous of the beauty of their step sister They resolved to banish her to the kitchen along with the servants but the step mother was afraid to do that for fear of her husband At last she devised a plan which she thought might in time cause her step daughter s death or at least rid her of her beauty The little
159. ber the positions of the digits represent units tens hundreds thousands and so on in reverse order In hexadecimal and octal numbers the positions represent exponents of the numerical base In octal these positions would represent units 8s and 64s for example because 8 is the numerical base of octal You can work out the decimal value by multiplying the digits according to the value of their position and adding the results together as shown in Tables 3 2 3 3 and 3 4 The first row in these tables is the value of the position in the number the second the multiplication sign to show that the value of the position is multiplied by the digit in that position the third the number in question and the fourth the results of the multiplication Table 3 2 shows a decimal to give you the idea Table 3 2 Decimal 2037 1000 100 10 1 2 0 3 7 2000 0 30 7 2037 decimal 41 42 CHAPTER 3 VARIABLES AND DATA TYPES Table 3 3 Octal 003765 512 64 8 1 3 7 6 5 1536 448 48 5 2037 decimal Table 3 4 Hexadecimal 0x7F5 256 16 1 7 F 15 5 1792 240 5 2037 decimal All three representations give 2037 as the resultant decimal value Octals are encountered in situations such as specifying file permissions and hexadecimals are used to specify colors so it s worth familiarizing yourself with this notation if you have not encountered it before gt gt gt permissions 00755 gt gt
160. bers and a minus sign on negative numbers Here s an example that formats a number and uses precision as well The part we are interested in is 1 0 gt 4 3 This right aligns the second input with 0 as padding and states that we want to use a sign for all numbers The field is to be a minimum of 4 wide and we want to have three decimal places 3 gt gt gt msgs soso ok you know terrible do I know you gt gt gt numbers 23 5 42 37 1 234567 98754 320999999996 gt gt gt letters v w x y z gt gt gt string Value 0 s 3 equals 1 0 gt 4 3 t message s lt 20 gt gt gt for i in range 0 5 print string format letters i float numbers i message msgs i Value v equals 23 5 lokjen eanes Value w equals 42 0 well you know Value x equals 37 0 not bad Value y equals 1 23 actually terrible Value z equals 9 88e 04 do I know you The next optional character which you have already seen in use in the preceding example is a decimal integer to specify the minimum field width in characters If this is not specified the field width will be determined by the content Zero padding can be enabled by starting the width value with a 0 131 132 CHAPTER 7 WORKING WITH TEXT This number may be followed by another decimal integer after a period This is the number of decimal places to
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162. cal variable that has not been bound an UnboundLocalError exception is raised When Should Classes Be Used You may frequently find yourself writing several functions to manipulate similar sets of global variables If the function is not used for any other data structures and the variables could be grouped together as aspects of a single object then creating a class is probably the way to go In the original version of rpcombat py first shown in Chapter 5 most of the action involved manipulating a dictionary containing a set of variables specific to each character It might simplify things to create a new type of object called Player which manages its own state that is it manipulates its own attributes and provides means for other objects to interact with its attributes as well The combination of attributes and methods that are available for use are known as the object s interface Many of the functions in rpcombat py could be further abstracted so they are encapsulated within the Player class This means the rest of the program only needs to worry about the methods available in the object s interface while the inner workings of the Player object remain invisible Another way of working out what classes you need is to read through the original statement of requirements for your application The nouns may suggest potential classes the verbs may suggest methods and the adjectives in the specification may suggest attributes The original spec
163. can be installed with the Easy Install Python package manager This is a whole separate infrastructure for installing Python packages including the ability to search the online Python Package Index PyPI by name Here are a few examples of the Easy Install syntax se 6 Install a package in the PyPI by just specifying a name easy install pylint Install a package from a zipfile or egg on the web easy install http example com NewPackage 1 2 3 tgz Install an egg that you ve already downloaded easy install Desktop Documents NewPackage egg SL SL SL SL se 3 Packages may also be distributed as installers for your specific operating system The developers might provide a Windows installer RedHat Linux package or Debian Ubuntu package These are also straightforward to install but you should follow the instructions for your operating system Note that installing eggs and running any setup py files may require you to have full administrator access on your machine you might have to discuss installing such packages with your system administrator if you re developing on shared web hosting as they can have knock on effects for all users of Python on the server As always you should only ever install software from sources that you trust Check that your downloaded software is coming from that source and you re not accidentally following a link to an untrustworthy site Ifin doubt confirm with the software developers Also consider us
164. ccessor methods 183 defined 219 alphanumeric defined 159 American Standard Code for Information Interchange ASCII 125 defined 159 append a mode 142 append method 81 application s 145 158 checking and correcting styles 147 154 converting text 145 147 defined 4 6 formatting data 154 156 storing data 156 158 arbitrary defined 99 arguments 19 24 assignment 18 24 28 assignment operator 53 54 defined 73 associative arrays 86 asterisk functions and 103 attribute 181 access 197 defined 219 B backslash 33 BaseException class 231 base 16 hexadecimal numbers 41 42 base 8 octal numbers 41 42 Bash prompt 161 Beautiful Soup 258 259 binary base 2 defined 45 binary b mode 142 binary numbers 37 bin directory 163 bits 37 defined 45 blocks defined 219 Boole George 38 Boolean 29 defined 45 Boolean values 37 38 manipulating 52 53 Bourne Again Shell Bash defined 179 break keyword 67 68 exiting loops 67 buffers files 141 142 built in defined 45 built in element defined 73 built in functions overloading 200 Button widget Tkinter 264 bytecode 251 defined 259 bytes 37 283 284 INDEX defined 45 C caching 249 calling procedure 105 defined 123 Canvas widget Tkinter 264 carriage return CR 34 case changing strings 128 129 case sensitive 28 alphabetical ordering 51 defined 45 cgi directory 173 cgitb module 17
165. ce decdescusensanice ssnsancassuntecaavadancesvandsceaysdusaucsesdscedeuianeesenatinens xi About the Technical Reviewer sssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnna xii Chapter 1 Introducing Python sssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 1 Running Python on Various Systems cccssssssseseeseseseeseesesseeseeseesessessessassaseassaseaseateas 1 Learning While Having Fun s esessssssesssnsusnerunnsunnnrnnnnunnnnnnnnunnnnnnnnununnnnnnunannnnnnunnnnnnnnnnnnnnennn 4 Introducing the Interactive Shell s ssssessssssssersersnnnnrnnnnnnnnnnnnsunnnnnnnnunnnnnnnnunnnnnnnnnnnnnnnnnnnne 4 Choosing to Code with a Text Eqitor eee ssssseeseesssessesesseseesseseeseeseesaesaesaesaesaeeaeeaees 5 Choosing to Code with an Integrated Development Environment cccsseeseseeeeeees 5 Getting Started with Programming cccssssessseesesesesesseeseesesseeseeseeseesesseesessaesansaeenees 5 Creating Your Own Help Files ccssessssssesesseeseseeseeseeseeseesessessesseesessessessessessensesseneates 6 dargon BUSTING crodeciccducededecscitcucedtecesuties soak casa sicelissdeguts cabal aeaa eaha aaan baaa aeaa 6 SALLALI 751 A E 7 Chapter 2 Designing Software ssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnn 9 Designing Your Own Software Why Bother s ssessssssssersnssnsnnnnnnnunnnnnnnnunnnnnnnnnnnrnnnnnnnne 9
166. ces a linefeed LF character this is subtly different from the r carriage return CR On a mechanical typewriter the linefeed would just give you a new line while the carriage return would properly start a new paragraph One of the instances where you might need to know the difference is translating text files from one operating system to another OS X uses the CR character at the end of lines whereas Unix and Linux use the LF character and Windows uses both The meaning and usage of some of these sequences is somewhat lost in the mists of time Mostly you ll want to use the n escape The other most useful one is t which produces a tab character that can be useful for quick and dirty indentation of text Most of the other whitespace characters are only likely to be useful in very specialized situations see Table 3 1 Table 3 1 Escape Sequences Sequence Meaning n Newline LF r Carriage return CR t Tab v Vertical tab VT e Escape character ESC f Formfeed FF b Backspace a Bell BEL CHAPTER 3 VARIABLES AND DATA TYPES The following example allows you to nicely format your output for the screen gt gt gt print Example Heading n nFollowed by a line nor two of text n tName n tRace n tGender nDon t forget to escape Example Heading Followed by a line or two of text Name Race Gender Don t forget to escape The More Strings Change the More They Stay the Same The
167. ch interaction do you expect them to have with the application e Clicking a single button e Using a text interface e Answering a complex form How computer literate do you expect them to be e Will they need to learn the ropes e Should the application be self explanatory What System Will Users Be Running It On It may make a difference whether your program will be running on a Windows OS X GNU Linux or some other operating system It may be running on one kind of operating system and be accessed by clients using a variety of other systems like with most Internet applications It may use a standard computer keyboard and mouse for input or have some custom designed interface What other software will users need to have installed in order to be able to use your script All these questions may have a bearing on choices you make later on in the development process Answer them now and you will be better prepared for the challenges to come At this stage keep your answers simple and to the point you want to produce a concise overview that neatly summarizes what the software is intended for to begin with If you can t answer some of the questions yet that s fine too Later in this chapter you will break this down into detailed steps but before getting on to that it s your turn to start asking questions Creating Your Wish List In this first stage of defining what you want the software to do it can be useful to create a wish list
168. ch key has to have a unique identifier there cannot be duplicate values for the same key Therefore we can use the key to store the items of input data leaving the value part to store the results of our calculations For example suppose I wanted to find out how many times each letter turns up in a sentence like The quick brown fox jumps over the lazy dog I could iterate through the characters of the sentence and assign each one to a key in a dictionary gt gt gt sentence The quick brown fox jumps over the lazy dog gt gt gt characters gt gt gt for character in sentence characters character characters get character 0 1 gt gt gt characters en Big ie Tes Wy Naren Dg HENS Ah MID ad OM A De TBE Sead APRA i 1 h 2 k 4 j AE A 2 Dye Use Dee EE A Simple Role Playing Combat Game For this chapter s special guest application I will introduce a really simple role playing combat game to demonstrate dictionaries and sequences in action This is an expansion of chargen py the role playing character generation script that I started to design in Chapter 3 Starting with the top level design there will be three main sections to this program e Generate character statistics e Buy some equipment e Fight Each step needs to be refined separately 89 90 CHAPTER 5 M USING LISTS 1 We need a container for our character statistics a dictionary would be perfect for this We want th
169. contains a word or sequence of characters within it this is called a substring You can test for simple membership using the in operator gt gt gt bop in bopshoowopshoowop True However it may be important to know where in the string this substring occurs For this you need string find sub start end This returns an index number which is the number of characters into the string where the substring is found starting at zero of course Note that find returns the position of the first substring it finds or 1 if it doesn t find anything You can include further arguments to specify the start and end of the search these arguments work the same way as slice notation including start and up to but not including end CHAPTER 7 WORKING WITH TEXT gt gt gt s find a 8 16 10 If you want to find the position of the last occurrence of sub you could use string rfind sub start end instead which searches from the end of the string backward gt gt gt s rfind a 24 If you specifically want an error to be raised if the string is not found see Chapter 11 use string index sub start end instead of find This returns an integer index just like string find but raises a ValueError if the substring is not found gt gt gt s index a 12 25 gt gt gt s index a 26 Traceback most recent call last File lt stdin gt line 1 in lt module gt ValueError substring not found
170. conventions 28 overview 27 id function 184 if keyword 55 immutability 35 immutable 35 defined 46 immutable data type 86 immutable value defined 99 implementation 24 287 288 INDEX imp module 250 importing 260 _import_ method 246 import statement 170 inclusive numerical operators overloading 199 indentation 17 24 index 75 defined 99 inheritance 189 190 defined 220 initialization 18 24 28 192 defined 220 in len function 83 input function 20 34 43 installing Python on miscellaneous systems 2 on Windows 2 instances defined 220 Integer Format Specifiers 132 integers 38 defined 46 integrated development environment IDE 5 coding with 5 interactive mode 4 interactive shell 4 interface defined 220 object 185 international characters 126 interpreter defined 6 int function 62 157 int x function 40 invitations sending 102 103 IOError 141 items method 87 iterable defined 99 iteration defined 74 iterator objects 83 iterators defined 99 J joining strings 127 128 joinQ method 78 K keys method 87 keyword arguments 103 defined 123 keywords 27 28 L Label widget Tkinter 264 lambda expression defined 124 last in first out LIFO structure 82 len0 function 51 len seq 78 linefeed LF character 34 list append method 82 Listbox widget Tkinter 264 list comprehensions 84 85 list function 157 list pop i method 81 li
171. cope and you execute any other contents of the module when doing so e Module This Python file contains methods classes or other variables that can be reused e Package This collection of Python modules is conveniently stored in a directory hierarchy so modules of similar function are stored in the same directories Each directoryhasan init py so Python knows it s part of the package and each init py can contain configuration code and variables for that particular subpackage e PyPI This is the abbreviation for the Python Package Index an online searchable repository for Python software Easy Install can retrieve software from PyPI by specifying the package name e PYTHONPATH This environment variable configured by your operating system but modifiable at a command prompt contains a list of directories Python first checks these directories when an import statement causes it to look for a suitable candidate module before checking in the contributed and standard libraries e Standard library This is the set of packages with which Python is distributed they are always available to your programs e Weak internal use indicator When a variable in a module is named with a leading underscore Python treats it as being for internal use only this weak variable will not be imported unless explicitly specified Summary Python comes with a lot of useful functionality but it can t do everything To make up for this it giv
172. ctions to something that can make decisions on the fly based on the information that it receives It would be useful to be able to tell the program to perform different actions according to whether certain conditions are satisfied You could write this out in pseudocode like this if one condition is true then perform one set of actions if another condition is true then perform another set of actions Each if then pair is a conditional statement Before we get into that let s look at how to specify conditions 49 50 CHAPTER 4 MAKING CHOICES Values can be compared using the following comparison operators lt Less than e lt Less than or equal e gt Greater than e gt Greater than or equal e Equal e Not equal These affect the fundamental data types in different ways each time giving us an answer of True or False Operands are the bits of data on the left and right of the operator in an expression The operands on either side of the comparison operator must be of types that can be compared The operands and the comparison operator together are a conditional expression Python might not throw an error if you compare illogical types so it is a good idea to test your conditional expressions thoroughly and check the data that goes into them The results obtained from comparing numbers using conditional expressions in interactive mode are fairly self explanatory gt gt gt 2 lt 5 True gt gt
173. ctured data from your program Listing 11 6 contains an example CSV spreadsheet representing a number of purchases The first column is item name the second column is the number of items and the third column is the per item price Save this to a file called purchases csv Listing 11 6 Example CSV Spreadsheet for Parsing with Listing 11 7 Gromit 5 0 57 Widget 2 1 20 Splatchet 10 0 27 Snorklet 3 0 99 You can manipulate this CSV file using the code in Listing 11 7 Save this file to csv demo py and run it from the command line you should see the output shown following Listing 11 7 Listing 11 7 The csv Module import csv Set up some store variables total 0 priciest 0 0 0 Loop over CSV rows r csv reader open purchases csv for row in r Work out the total cost of this row and add it to running total cost float row 1 float row 2 total cost If this is the priciest row so far replace the priciest if cost gt priciest 3 priciest row cost CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Report on the total and the priciest purchase print You spent 0 format total print Your priciest purchase was 0 1 o 0 s at a total cost of o 3 format priciest Let s try it out python csv demo py You spent 10 92 Your priciest purchase was 3 Snorklets at a total cost of 2 97 The csv module can also be used to write CSV files and you can use either the online
174. cussion at http mail python org pipermail tutor 2009 February 067443 html Using Tkinter Before you can use Tkinter you need to have the python tk package installed Once the tkinter module has been imported it provides all the necessary building blocks to make text areas menus buttons and so on and to hook them up to the commands you want to run These graphical building blocks 261 262 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS connected to program functions are known as widgets presumably because calling them watchamacallits or dubreywotsis would have sounded silly The most obvious example of a widget is a button but in fact every part of what you see is some kind of widget or other Does this sound familiar Even the containing box into which all the other widgets are packed usually in a top bottom and then left right fashion is a widget This approach to widgets is a little ungainly but it makes for a fairly quick way to get graphic applications up and running as you can see in Listing 12 1 Listing 12 1 hellotk py usr bin env python HelloTk Example windowed application Usage HelloTk py Target System GNU Linux Interface tkinter Functional Requirements Display a GUI window containing two buttons which write a message to stdout when pressed version 0 1 maintainer maintainer website com status Prototype date 12 01 2009 Import modules import Tkinter class HelloTk def
175. d CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Let s run this python convert py Hello world Hello world me hearties python convert py example txt Mister Pudey the very real problem is one of money me hearties Yarr i m afraid that the Ministry of Silly Walks is no longer getting the kind of support it needs Yarr you see there s Defence Social Security Health Housing Education Silly Walks They re all supposed to get the same python convert py lt example txt gt example converted txt We ll discuss how modules are run in Python and how it therefore uses the name variables to keep track of the context in which code is being executed later on in the Python Module Internals section Advanced Module Behavior Now that you ve got the basics of modules let s look at some slightly more advanced module topics In particular we ll cover how to reload modules so that you can test them effectively and will have a look at how Python handles modules internally Reloading Module Changes Dynamically When you re in the middle of developing a module and you want to keep testing changes to its functionality at the interactive prompt one of the most frequent problems you ll encounter is that only the first import statement visits the module later import statements just take a copy of the module stored or cached internally in Python s memory allocation and provides you with that instead It doe
176. d It should return a negative integer if self lt other zero if self other and a positive integer if self gt other object operation bool it should return False or True When this method is not defined bool self This is called to implement truth value testing and the built in len is called if it is defined and True is returned when the length is not zero If a class doesn t define either len or bool all its instances are considered true Table 9 3 shows methods that implement numerical operators Table 9 3 Methods Implementing Numerical Operators Magic Method Operation object add self other self other object sub self other self other object mul self other self other object div self other self other object floordiv self other self other object mod self other self other object divmod self other self other divmod self other object pow self other modulo self pow self other modulo other object Ishift self other self lt lt other object xrshift self other self gt gt other object and self other self amp other object xor self other self other object or self other self other 198 CHAPTER 9 E CLASSES If one of those methods does not support the operation with the supplied arguments it should return NotImplemented Table 9 4 shows methods that overload inclusive numerical operators Table 9 4 Overlo
177. d how to get started using them Of course there could be a whole book on each of these kits so this chapter is in no way an exhaustive treatise on the subject of GUI tool kits Most of the programs you use on your computer have a GUI when you click an icon a new window opens on your screen which can display data in a graphic format and provide buttons and menus for the various commands that you might want to run on that data This relationship of data and commands strongly suggests a class structure what we need is some means of easily creating graphic containers for displaying data and other graphic objects representing commands Python has interfaces available for several graphical toolkits including Tkinter PyGTK Glade and tepache GIMP on which PyGTK is based is an open source image manipulation program similar to Photoshop We ll mainly focus on PyGTK and Tkinter with a little on Glade and tepache at the end of this chapter First let s look at Tkinter as it is the simpler of the two major toolkits One of the more interesting aspects of building a GUI interface for a Python program is that the resulting program would be a cross platform GUI application a truly powerful aspect of Python Note The examples in this chapter use Python 2 5 rather than Python 3 because Python 3 support for graphical interfaces wasn t complete at the time of this writing Some of the issues relating to Python 3 and GUI code can be found in the dis
178. d results in your scripts is to use parentheses to dictate which order the subexpressions should be evaluated gt gt gt 2 3 64 4 2 4 3 25 0 gt gt gt 2 3 64 4 2 3 10 0 gt gt gt 2 3 6 4 2 3 20 0 The interpreter will evaluate the expression in the innermost set of brackets first Otherwise operators will be evaluated in the order shown in Table 4 1 which includes some additional operators that will be discussed later in this book The important thing to note here is that arithmetical operators have a higher precedence than comparison operators so the sums are done first before they are compared and only then are the effects of not and and or evaluated Expressions that are equal in precedence are evaluated from left to right except for comparisons which are chained 2 3 4 evaluates as 2 3 4 but2 lt 3 lt 4 evaluates as 2 lt 3 and 3 lt 4 Table 4 1 Operator Precedence CHAPTER 4 E MAKING CHOICES Operator Description gt L and gt s i s i j s attr and f x y x x and x x y x y and x y x y and x y X lt y X lt y X gt y X gt y X y and x y x is yandx is not y x in sandx not in s not x x and y x or y Creates a tuple list and dictionary and converts a string Indexes and slices assigns attributes and calls functions Elevates to the power of Performs unary operations Performs multiplication division and mo
179. d s strength s speed s format vars self return stats def len self return len vars self 209 210 CHAPTER 9 M CLASSES def getitem self key names vars self try item names key except item 0 return item class Weapon Thing Weapon subclass of Thing def repr self stats Weapon name r gold s strength s speed s format vars self return stats class Armour Thing Armour subclass of Thing def repr self stats Armour name r gold s strength s Speed s format vars self return stats class Location GameObject Game Location Takes two optional arguments either Location name which creates a new location called name Location dict which loads a stored location from a dictionary Locations provide the environment in which game play can occur Locations have several public methods Location enter player adds player to location and provides prommpt Location interpret command player executes in game commands Location combat initializes comabt sequence Location resolve conflict attacker target resolves comabt rounds Location exit player removes player from location def init self name Somewhere store if store self name self desc self setInv self commands look print 0 s attack self combat say print me name says o
180. d that has not been commented out delimited by quotation marks or escaped in some other way as an identifier of some kind An identifier is just a name label so it could refer to more or less anything including commands so it helps to keep things readable and understandable if you choose sensible names You need to be careful to avoid choosing names that are already being used in your current Python session to identify your new variables Choosing the same name as something else can make the original item with that name inaccessible This could be particularly bad if the name you choose is an essential part of the Python language but luckily Python does not allow you to name variables after any essential parts of the language Therefore the next section contains an overview of the most important words used in Python so you can avoid this problem this is the territory that you will be exploring and learning to work with over the course of this book Python Keywords The following words are the keywords which form the basis of the Python language You are not allowed to use these words to name your variables because these are the core commands of Python They must 27 28 CHAPTER 3 M VARIABLES AND DATA TYPES be spelled exactly as written here for the interpreter to recognize them as keywords The words True False and None have a special meaning which will be covered later e False e elif e lambda e None e else e nonlocal e True e
181. d therefore initialization is performed using the sign like this variable value One of the specific features of Python is that once a value has been assigned to a variable the interpreter will then decide what sort of value it is i e a number some text or some other relevant piece of information Each variable is referred to by a name known as an identifier which is rather like a name tag that identifies the variable to the rest of the program It s a good idea to choose names that give an idea of what sort of values they represent Python will regard any word that has not been commented out delimited or escaped in some other way as either a command or an identifier for this reason you need to be careful to avoid choosing words that are already being used as part of the Python language to identify your new variables Details of which words you cannot use will be covered in the next chapter Turning the Design into Executable Code Now you have to come up with a strategy for solving the problems you have agreed to tackle First you want to print out a message and you want the user to be able to input some text OK so you re expecting your message and users text to be strings of text It would be a very good idea to add this information to the Testing section of your docstring at the start of the file Text strings are represented by enclosing the text in quotes Now you just need to know the function that prints things out t
182. d they re accepted Support options will depend strongly on what license the software is made available under The most simple contributed package is a single module file When downloaded it can be placed anywhere on Python s sys path even in your program s own directory if you don t need to use it anywhere else or don t mind keeping multiple copies and used immediately with import statements The accepted location for storing downloaded modules is in one of the site packages directories on sys path Large complex packages will often be distributed with installation instructions The most basic distribution is as some type of compressed file like zip GZip or BZip These will typically unpack to provide a package directory hierarchy with init py files in each subdirectory You can sometimes just put the top level directory somewhere on the sys path and the package will be discovered by import Python packages also often use the egg format and these can usually just be dropped into a directory on the sys path as with unzipped packages Sometimes ZIP files will contain a setup py file or eggs will require special installation check any README file included with the package In the first instance the package has been prepared to the Distutils Python standard and you will generally want to run the setup py file ensuring you re using the right version of Python cd unpacked zipfile directory python setup py install Eggs
183. dint 3 33 random randint 3 33 random randint 3 33 random randint 3 33 random randint 3 33 random randint 3 33 profile Charm random randint 3 33 random randint 3 33 random randint 3 33 profile Speed Work out combat stats life magic prot gold life profile Muscle profile Speed 2 random randint 9 49 2 magic profile Brainz profile Charm 2 random randint 9 49 2 CHAPTER 5 prot profile Speed profile Brainz 2 random randint 9 49 2 gold random randint 9 49 random randint 9 49 random randint 9 49 Validate stats if life gt 0 and life lt 100 profile life life else life random randint 9 99 if magic gt O and magic lt 100 profile magic magic else magic random randint 9 99 if prot gt O and prot lt 100 profile prot prot else prot random randint 9 99 if gold gt 0 profile gold gold else gold random randint 9 99 Output the character sheet fancy line lt 4 4 gt print print fancy line print t profile Name print t profile Race profile Gender print t profile Desc print fancy line print print tMuscle profile Muscle tlife profile life print tBrainz profile Brainz tmagic profile magic print tSpeed profile Speed tprotection profile prot
184. documentation or help csv in the interactive prompt to give you more information on how csv can parse different variations of the CSV format datetime Handling dates and times is always a difficult problem in any language and it s tempting for the new programmer to develop all sorts of string munging libraries to do quite specific tasks datetime exists in Python s core libraries so that you can resist that temptation should it ever arise The datetime module contains a lot of functionality that we won t cover explicitly here However Listing 11 8 demonstrates its core features parsing creating and manipulating date and time objects Save this code to datetime py and run it from the command line Listing 11 8 The datetime Module import datetime Date parsing Original data in a fairly human readable format guess 19 Feb 2001 08 23 Parsing into a Python object time obj datetime datetime strptime guess d b Y H M Printing the object generates a slightly different readable format print Python thinks that o is 1 format guess time obj Date manipulation Define a time interval of five days interval datetime timedelta 5 day datetime date today for i in range 1 10 Count back in intervals of five days day interval print 0 days ago was 1 format i 5 day In Listing 11 8 we have to give strptime a format mask which it uses to try to parse the guess string into the re
185. dulo operations Performs addition and subtraction Makes comparisons Ascertains identity Ascertains membership Performs Boolean negation Returns True if both x and y are True Returns False otherwise Returns True if either x or y are True Returns False if both x and y are False Taking Control of the Process Sometimes you need to make a choice about what happens next during the execution of the program using what is known as a control flow statement In your scripts you can now use the results of comparisons to create conditional statements They are a form of control flow statement that allow different actions to be performed based on whether certain conditions hold true In Python conditional statements are constructed using the if elif and else keywords If you are used to other languages you will notice that Python doesn t use the keyword then The syntax for conditional statements is very specific so pay close attention to the punctuation and layout if condition Perform some actions print Condition is True elif condition True Perform some other actions print Condition is not True else Perform default or fall through actions print Anomaly Condition is neither True nor False 55 56 CHAPTER 4 MAKING CHOICES Here the first line begins with the keyword if which must be followed with a conditional expression that gives a True or False answer and a colon The colon could be consider
186. during the process of learning the art of programming Technical terms and jargon words that need further explanation will be highlighted in bold the first time you encounter them Any terms not immediately explained in the chapter will be covered in the Jargon Busting section at the end of each chapter You may also wish to study one of the most famous resources on the Internet the Jargon File which is part of the New Hacker s Dictionary available at http catb org esr jargon As an example here s the definition of the noun code from the Jargon File The stuff that software writers write either in source form or after translation by a compiler or assembler Often used in opposition to data which is the stuff that code operates on Among hackers this is a mass noun as in How much code does it take to do a bubble sort or The code is loaded at the high end of RAM Among scientific programmers it is sometimes a count noun equivalent to program thus they may speak of codes in the plural Anyone referring to software as the software codes is probably a newbie or a suit For comparison s sake here s the definition of the verb code To write code In this sense always refers to source code rather than compiled I coded an Emacs clone in two hours This verb is a bit of a cultural marker associated with the Unix and minicomputer traditions and lately
187. e a SomeError perilous CHAPTER 10 EXCEPTIONS If it doesn t then this handler method is called handle no problem except SomeError If it does then this handler method is called handle some error finally Whatever happens this clean up method is called clean up On closer inspection the code in Listing 10 11 won t suffice If we execute handle no problem in the try clause depending on its internals it could raise a SomeError exception which would then prompt the exception handling to call handle some error before clean up The two handle methods might between them perform some action twice so they would have to be coded in a way to make sure that never happened In essence by just putting that line at the end of the try clause we ve actually complicated our code at the next layer down because these lower level methods now have to take into account how each other behaves to recover from an internal exception Although you might generally be able to work around this in your own coding when you re using other people s code doing so might be quite difficult If we liken our try except statement to an if statement then we are able to execute code if an exception is raised but we also need a way of executing code if no exception is raised we want to have the equivalent of if else functionality So to complete our error handling functionality we add one last clause an else clause This is executed a
188. e conflict self attacker target Conflict Resolution 212 CHAPTER 9 E CLASSES Takes two Player objects as arguments relating to the attacker and target Calculates velocity hit or miss calculates and inflicts appropriate damage Prints out a commentary on the action to the world Returns True if the blow resulted in fatality False if the blow misses life left target life Calculate velocity of blow velocity attacker strike target if velocity gt 0 size len hits 1 Print sutable Hit message if velocity gt self vel max self vel max velocity hit type int size velocity self vel max if hit type gt size hit type size strike msg join str hit type print attacker name hits hit type target name end else size len misses 1 Print suitable Miss message if velocity lt self vel min self vel min velocity miss type int size velocity self vel max if miss type gt size miss type roll 7 if miss type lt 0 miss type roll 7 1 strike msg join str miss type print attacker name fix gender attacker gender misses miss type target name End player turn return False Calculate damage inflicted by blow damage potential damage self damage attacker target velocity if damage gt self dam max self dam max damage elif damage lt self dam min self dam min damage Print damage report size len dam
189. e correct number of characters of the correct type and format that numbers are within a specified range and that the information is consistent with other records www allitebooks com CHAPTER 4 E MAKING CHOICES Testing and validation are such important parts of programming that I ll be presenting several ways of doing it at various levels of sophistication over the course of this book Dealing with Logical Errors As your applications become more complex you will need more formal methods of testing your designs One of the ways of doing this is to construct a trace table like the one shown in Table 4 2 I m going to use this method to get some clues as to what is wrong with curtains py To find out what is up with the script I am going to trace the values of all the variables and conditional expressions over the course of the program s execution The variable names and test conditions form the headers of the table columns and each step is given a new line Table 4 2 Trace Table Template var1 var2 condition condition2 step2 x step4 X A trace should be performed with as many different sets of data as is necessary to make sure that all the possible alternatives get tested Most errors in programming don t occur if the values lie within some expected range but they often occur for unusual values also called critical values Critical values are values that lie outside the tolerances of the program such a number t
190. e fractional part of the calculation and used the modulo operator to calculate how much would be left over The final script is shown in Listing 4 3 Listing 4 3 curtains 0 2 py usr bin env python Calculate how much material to buy given the size of the windows Target Users My friend who wants to make some curtains Target System GNU Linux Interface Command line Functional Requirements Print out the required length of fabric in metres Print out the total price of the fabric User must be able to input the measurements of the window Testing Trace table Test values 100x100 100x200 200x100 200x200 200x300 and 300x200 Expected results 1 9 3 4 4 3 6 45 9 8 9 45 version 0 2 maintainer maintainer website com status Prototype To start with all the measurements will be in cm I will assume that the roll of material is going to be 140cm and that the price per metre will be 5 units of currency roll width 140 price per metre 5 Prompt the user to input the window measurements in cm window height input Enter the height of the window cm window width input Enter the width of the window cm Print headers for the rather basic trace table print print twidth theight twidths ttotal tprice tshorter twider I need to add a bit for the hems First I must convert the string into a number Otherwise I will get an error if I try to perform arithmetic on a text string
191. e go There are many more methods available but this is enough to get you started Methods corresponding to operations that are not supported by the particular kind of number implemented should be left undefined Table 9 2 Overloading Rich Comparison Operators Magic Method Operation object lt self other self lt other object le self other self lt other object eq self other self other object ne self other self other object gt self other self gt other object ge self other self gt other These are called rich comparison methods and are called for comparison operators in preference to cmp Arich comparison method may return the singleton NotImplemented if the method does not implement the operation for a given pair of arguments By convention False and True are returned for a successful comparison However these methods can return any value so if the comparison operator is used in a Boolean context for example in the condition of an if statement Python will call bool on the value to determine if the result is true or false 197 CHAPTER 9 M CLASSES Note There are no implied relationships among the comparison operators The truth of x y does not imply that x y is False Therefore you need to define all of these methods so that all forms of comparison work correctly object cmp self other This method is called by comparison operations if rich comparison is not define
192. e results CHAPTER 10 EXCEPTIONS Explicit chain Exception Explicitly raised global name foo is not defined Implicit chain KeyError bar global name foo is not defined Traceback most recent call last File chaining py line 4 in return explicit chain try foo NameError global name foo is not defined The above exception was the direct cause of the following exception Traceback most recent call last File chaining py line 35 in lt module gt raise ex ch File chaining py line 6 in return explicit chain raise Exception Explicitly raised from e Exception Explicitly raised Now comment out the next to last line to see the traceback from an implicit chain Explicit chain Exception Explicitly raised global name foo is not defined Implicit chain KeyError bar global name foo is not defined Traceback most recent call last File chaining py line 14 in return implicit chain try foo NameError global name foo is not defined During handling of the above exception another exception occurred Traceback most recent call last File chaining py line 34 in lt module gt raise im ch Uncomment this to see the implicit chain File chaining py line 16 in return implicit chain bar KeyError bar As you can see more complex tracebacks will distinguish between explicit and implicit chaining by leaving a newline and then referring to the next except
193. e some data for the combat system to use and I need to be able to keep a record of how much money each character has I m sure you ll be itching to learn how to automate your accounts and database your stock records but please try to temper your enthusiasm while I take a brief excursion in order to introduce you to the inhabitants of Cloud Cuckoo Land Joining Up Text Fragments In hello world py you learned how to get some basic output by using the print function You can use it to print out a literal string of characters or the value of a variable Normally each print statement starts on a new line but it is possible to print several values on a single line by separating them with commas print will then concatenate them into a single line separated by spaces gt gt gt Race Elf gt gt gt Gender Female gt gt gt print Gender Race Female Elf There are many ways to combine separate pieces of information into a single line some more efficient than others Two adjacent strings not separated by commas will automatically be concatenated but this doesn t work for variables gt gt gt print Male Gnome will give the output MaleGnome 31 32 CHAPTER 3 M VARIABLES AND DATA TYPES The following line gt gt gt print Male Race results in this File lt stdin gt line 1 print Male Race A SyntaxError invalid syntax This approach doesn t work for variables because writing two adjacen
194. e step Natural language This is language that humans speak as opposed to a programming language Operating system This is the software component of a computer system that is responsible for the management and coordination of activities and the sharing of the resources of the computer The operating system communicates between the applications that are run on the machine and the machine s hardware components Prompt This visual cue tells the user to input something ym Script and application The terms script application and program are used somewhat interchangeably in this book though there are some subtle differences in meaning e Script This short but complete piece of code is intended to be run using an interpreter A script will exist as a text file that you can read e Application This refers to any applied use of a programming language It usually means a complete packaged up and usable piece of software It is the preferred term among Linux Unix users www allitebooks com CHAPTER 1 INTRODUCING PYTHON e Program This refers to the sequence of instructions involved in a script or application it is also used as a general term to describe an executable computer file e Software development cycle In this structured approach to developing software each stage can be fed back into previous stages See Chapter 2 e Version number This number corresponds to a new development in a piece of sof
195. e to start with and adjusting it to your taste when you have more coding experience Listing 8 1 shows a somewhat formal way of laying out code It is a legal Python script although the program itself doesn t actually do very much Up till now I have been using docstrings in a slightly haphazard way to keep a record of the initial specification of requirements for the program Once you have decided that the application is ready to release into the wild the docstrings will need to be updated to provide some information that is more appropriate for your users The docstrings should all be self explanatory Also in the example script in Listing 8 1 each statement should be on a separate line with consistent indentation The first line tells the system where to find Python the second tells it to use Unicode encoding Tip Older versions of Python don t require the encoding to be declared You should use ASCII or Latin 1 instead of UTF 8 if you include this line The first line of Python code following these declarations should be the docstring This is followed by the version information for this application I am using magic variables with double underscores before and after the name to record this information so that it can be used by helper applications like pydoc The pydoc module imports a Python module and uses the contents to generate help text at runtime pydoc outputs docstrings for any objects that have them Additionally all of
196. e to use for GUI programming but this chapter will give you enough to look into it when Python 3 is ready for GUIs As for the book as a whole you know Python You will probably be more aware of what you still don t know about programming than when you started reading this book Good Over the coming months you will want to familiarize yourself with the parts of Python s standard library that look like they might be useful Just take one module at a time and construct an application that does what you want it to do As we ve done in this book s examples start with a pseudocode design or flowchart and improve the design until you have ticked all your user requirements boxes Once you have done that move on to the next thing You now know how to go about designing applications with command line windowed and web based interfaces from a basic set of requirements As you progress you will develop your own methods through experience and come into contact with more sophisticated programming ideas which you may want to integrate I hope this book provides a first step on the path of programming that you can always come back to when you want to remind yourself of the basics Happy programming Index A abstraction 109 defined 123 accessing privileged information defining functions 101 102 overview 101 passing unknown number of values into function 103 104 sending invitations 102 103 using docstrings to document function 104 a
197. e user to define the Name Desc Gender and Race fields The main statistics Muscle Brainz Speed Charm and gold need to be randomly generated The combat statistics life magic and prot protection need to be derived from the main statistics For generating random numbers I will have to use the random module As I don t cover modules until Chapter 9 I should explain that modules are collections of extra functions that you can call on to do specific jobs They need to be imported at the start of the script and then the contained functions can be used with a similar syntax to the object methods we have been using to manipulate strings and lists so far 2 Next I want the characters to be able to buy some equipment with their gold This means I ll have to set up shop I can do this by creating a dictionary called stock where the names of the stock items are keys and the corresponding values are the prices 3 Finally we get to the combat phase Each character must choose a weapon and take turns attacking opponents Rather than having some boring numerical score I want to translate the damage results into descriptive text For this I m going to create some more tuples containing suitable phrases I need to work out how to calculate whether each blow meets its mark and if so how much damage it does To help this calculation I want to add two more items of data to my already existing stock list to repres
198. ed no further The error in divide by on the other hand is raised by the division operator and then by the divide by method itself rising into the main body of the program then the program as a whole finally gives up and reports to the user It s also important to realize that discarding the exception and returning a result that doesn t reflect the inputs is not generally advisable most programmers using your method would expect it not to return a standard big number when the divisor is zero in fact they would expect an exception So we ll now expand on this simple exception handling to show you how it can help you while still letting your methods behave the way that they ought Classes of Exceptions Python has many different built in classes of exception This means that when an exception is raised you can tell a lot about what has gone wrong from the exception s class alone Exception objects and the hierarchy of classes Although there are a lot of different exception classes they re actually all ultimately subclasses or subclasses of subclasses etc of the BaseException class In fact all exceptions you might ever use in Python must be based on this class Let s raise some example exceptions and use the bases attribute oftheir class attribute to investigate what classes they inherit from In Listing 10 3 we raise a number of exceptions and handle them by storing them in an array We then look recursively at their sup
199. ed to mean then It is not usually considered good style to explicitly state condition True if whatever condition is gives a value of True or False you can simply write if condition or elif not condition This advice particularly applies to Boolean variables but don t forget you can perform comparisons on anything because anything in Python evaluates to either True or False Note Variables assigned values such as 0 or None will return False if you test them as shown previously Any number is True The statements that follow must start on a new line and be indented The number of spaces doesn t strictly matter so long as all the instructions after the colon are indented by the same amount though it s good practice to use the same number of spaces to indicate control flow throughout your code The statements following after the colon are known as a suite You can include further conditional sections using the elif keyword an abbreviation of else if which is not a Python keyword statements following elif will be evaluated only if the previous test fails i e the conditional expression is False You can also include a final else statement which will catch any value that did not satisfy any of the conditions it doesn t take any conditional expression at all This can be used to specify a default set of actions to perform In our previous example things would have to go very wrong for us to ever see the final anomaly wa
200. ed to vary according to the range of values that need to be returned This gives me a clue what the input parameters should be and I decided to call them sides and dice In return I want to get a random integer back that falls within the specified range Now I can construct my new function def roll sides dice 1 Dice rolling simulator sides Number of sides the die has dice number of dice to be rolled defaults to 1 CHAPTER 6 FUNCTIONS Returns a random number between dice and dice sides weighted towards the average result 0 for rolls in range 1 dice result random randint 1 sides return result Remember that this code will not get executed by the program yet so the next thing to do is to go through the script and replace all the calls to random randint with calls to the new roll1 function Tip The Find or Search and Replace functions in your text editor can be very useful in helping to make sure you don t miss any lines and keeping your changes consistent Once you have made the changes they need to be tested before anything else happens In fact it s a good idea to test each case before you move on to the next Otherwise you could give yourself headaches trying to trace any problems that arise from the changes In order for the new function to pass the output must be exactly the same as before Whatever the new function does should be invisible to the user Cutting Out the Waffle
201. elf if 0 pass else self self lt self life lt 100 pass self life int roll 33 3 magic int self brainz self charm 2 roll 49 1 2 lt self magic lt 100 pass self magic int roll 33 3 prot int self speed self brainz 2 roll 49 1 2 lt self prot lt 100 self prot int roll1 33 3 gold int roll 40 4 setInv CHAPTER 9 E CLASSES Equip player if the inventory is empty shopping self inventory if shopping Display shop stock list with prices in gold print You have self gold gold shop enter self handbag join with and self inventory print You own a handbag Choose a weapon print self name prepare for mortal combat See if player has any weapons available weapons item for item in self inventory if issubclass type item Weapon available weapons append Weapon Fist 0 20 50 self weapon available weapons 0 See if player has any armor available armour item for item in self inventory if issubclass type item Armour available armour append Armour None 0 0 50 self armour available armour 0 else Load character from stored value self dict store print self name is now ready for battle def repr self Output class constructor string stats Player 0 s format vars self return stats def str self Output the character sheet rpcharacter sheet lt
202. ent 49 55 defined 73 nesting 69 connect method 267 constants 91 defined 99 constrain defined 99 constraint on constructors 192 constructors 192 193 continue keyword 67 68 exiting loops 67 contributed modules defined 259 control flow statement 55 57 conversion field 130 defined 159 converting numbers 40 41 converting text 145 147 convert module 244 critical values defined 73 D data defined 3 formatting 154 156 storing 156 158 date stamp 114 defined 123 decimal base 10 defined 45 def statement 101 del method 88 deleting items from dictionaries 88 89 delimited text 15 delimiters 24 126 del keyword 81 deque 82 design document 23 designing software 9 25 devising solutions coding design 18 defining acceptable results 14 documentation 22 24 executable code 18 19 functional requirements 14 indentation 17 maintenance 14 15 nonfunctional requirements 14 notes of possible improvements 22 organizing task into steps 16 17 overview 13 refining 17 19 20 subgroups 17 testing 20 21 user requirements documents 13 goals overview 12 potential users 12 13 INDEX identifying problems to solve operating systems 12 overview 10 11 potential users 11 12 purpose of proposed software 11 overview 9 10 reasons for questions to ask 10 using Python 10 terminology 24 25 dict clear method 89 dict fromkeys iterable value None method 88 dict function 8
203. ent call last File lt stdin gt line 1 in lt module gt NameError name strings is not defined Note that as with our discussion of Python execution and Python internals any content in the init py script is executed by Python when it imports the package You can see this in the example in the propagation of the variable result of import statements from init py into the resultant code But try adding print foo into your init py and consider the following output gt gt gt from pirate import convert foo gt gt gt Importing the root of the package now does nothing import pirate gt gt gt So let s reload it explicitly import imp gt gt gt imp reload pirate foo lt module pirate from pirate init py gt 253 254 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES gt gt gt Re importing a sub module does nothing from pirate import convert gt gt gt Nor does reloading it imp reload convert lt module convert from convert py gt gt gt gt This is consistent with other package and module behavior only on the first import which needs to consult init py is init py actually executed to execute it again you have to reload pirate explicitly As with module files there s nothing special about init py files Python executes them like ordinary code So they can contain class or method definitions or package level variables or even control flow suchas if el
204. ent weapon damage and weapon speed In order to do this each value in the stock dictionary will be represented by a tuple of three values price damage and speed These calculations can all be set up at the start of the script because I don t want these values to change Finally I want the combat section to loop until one or other combatant is defeated Next I sketched the whole thing out in pseudocode as you can see in Listing 5 1 Listing 5 1 The Role Play Game s Pseudocode rpcombat py Purpose Simple Role Playing Combat Game Target System GNU Linux Interface Command line Functional Requirements The user must be able to generate more than one character profile equip those characters with suitable weapons and model hand to hand combat between characters Test values Ez Tall Pixie female Inkon Small Troll male Expected results All statistics should be integers in the 1 99 range Limitations Too many to mention set up constant data Set up shop CHAPTER 5 M USING LISTS 1 loop while players lt 2 1 1 Create profile dictionary 1 2 Prompt user for user defined information Name Desc Gender Race 1 3 Validate user input 1 4 Generate stats Muscle Brainz Speed Charm 1 5 Work out combat stats life magic prot gold And modify according to user defined info 1 6 Validate stats 1 7 Output the character sheet 1 8 Prompt user to buy some equipment
205. er I will need to know about the relationship of the curtain height and curtain width to the roll width Using Conditional Code in the Application Now you can apply your knowledge of conditional statements to allow for different ways of measuring up the material If the length of the curtains is less than the roll width it is possible to turn the whole thing on its side and just use one width of fabric However if the curtains need to be both longer and wider than the roll width there is a further problem If the extra material required is less than half the roll width I would need to buy an additional width of material at the same length If it is more than half I would need to buy two additional widths Somehow I need to come up with some code that reflects these conditions The first step is to translate the preceding statements into pseudocode if curtain width lt roll width total length curtain width else total length curtain length if curtain width gt roll width and curtain length gt roll width if extra material lt roll width 2 width 1 if extra material gt roll width 2 width 2 I will need to work out how many whole widths there are in the curtain width and how much extra material will be needed to make up the whole width widths int curtain width roll width extra material curtain width roll width 61 62 CHAPTER 4 MAKING CHOICES I have used the built in int function to drop th
206. er in this instance the script checks the value of the user input number While it is positive i e gt 0 the loop processes as normal but as soon as a negative number is entered the loop is broken and the script goes on to calculate the average An example is shown in Listing 4 5 Listing 4 5 averages py counter 0 total 0 number 0 while number gt 0 number int input Enter a positive number nor a negative to exit total number counter 1 average total counter print average CHAPTER 4 E MAKING CHOICES Because I don t know what sort of value the user is going to input the value returned by input is immediately converted to an integer If you try to enter text here the result will be a value error and the program will abort I will show you how to catch and use errors in Chapter 10 Listing 4 5 has a serious flaw in that the negative number which has to be input last gets included in the calculation and therefore messes up the final result See if you can figure out how to avoid this problem before moving on to the next section There are usually many ways of arriving at a good design but sometimes messing around with the details can lead to greater complication and create more problems In these cases it may be better to go back to the top level design and start again Fortunately there are still a few more weapons in our armory Now Get Out of That There are several methods of getting out of
207. er or not to ignore the error Along with the message you can store extra parameters in the exception object by including them as arguments to the Exception constructor The parameters are stored in the args attribute of the object In Listing 10 8 we store an extra parameter on the exception and access it later on during exception handling Listing 10 8 Storing Extra State Information in an Exception for ignore in True False try Pass a second parameter into the exception constructor raise Exception Ignore is 0 format ignore ignore except Exception as e Based on that second parameter work out if the error can be ignored or not if e args 1 print I can ignore this error else print I canNOT ignore this error The preceding script produces the following results CHAPTER 10 EXCEPTIONS I can ignore this error I canNOT ignore this error Implementing Complex Error Handling As you saw in the previous section Python s exception classes form a hierarchy where the BaseException class is the superclass of every other exception class All Python s core exceptions are then created as subclasses of Exception or Warning or are nonerror like conditions like KeyboardInterrupt These primitive exception classes are in turn all subclasses themselves of BaseException Why is this relevant Well an except clause that claims to handle an exception class will also handle any of its subclasses The first except
208. erclasses until we reach the object class which has no superclasses Run the code in this listing and examine the output Listing 10 3 Provoking and Handling Different Exceptions and Investigating Their Class Hierarchies store Create some exceptions and handle them try foo except KeyError as e store append e try 1 0 except ZeroDivisionError as e store append e try bar except AttributeError as e store append e Loop over the store of errors and print out their class hierarchy for exception object in store ec exception object class print ec name indent while ec bases Assign ec s superclass to itself and increase ec ec bases 0 print indent ec name indent indent CHAPTER 10 EXCEPTIONS The script in Listing 10 3 will return results that look like this KeyError LookupError Exception BaseException object ZeroDivisionError ArithmeticError Exception BaseException object AttributeError Exception BaseException object Here we have traced each individual exception s class hierarchy to the shared root of BaseException and object The full hierarchy of all built in exception classes is part of Python s online documentation at http docs python org 3 0 library exceptions html but typically you won t ever need this exhaustive list of classes either you will want to take care to catch a small number of classes treating each one di
209. erent classes of custom exceptions are useful because later on you might decide that some security problems are more important than others For example if you bought a cat and kept it in the kitchen while you were out you wouldn t want motion detection in the kitchen to raise an exception and motion anywhere else in the house might just mean that your cat has got out of the kitchen At that point because you ve already used different exception classes you can quickly rewrite your code to handle some security problems differently from others Accessing Properties of the Raised Exception Sometimes you will want to access the details of the original exception object and not just handle them based on their class The object will contain at a minimum the readable message you see when an exception reaches the interactive prompt but it may also contain extra information depending on how it was first raised Even with the minimum you might find it useful if you re doing a lot of otherwise silent discarding of exceptions to log those messages somewhere That way you can check your log files if something is going wrong with your program and perhaps notice patterns in the exceptions that are occurring The script in Listing 10 7 accesses the original exception s human readable message Either save it to a file or run it from an interactive prompt to see how this works in practice Listing 10 7 Handling an Exception by Logging Its Message H
210. erial at the same length if it is more than half you would need to buy two additional widths taking into account the extra material needed for the joins Still with me Good The script needs to take into CHAPTER 3 VARIABLES AND DATA TYPES account that fabric is sold by the meter or half meters in whole widths so I will need to round up the widths to the nearest whole number and the final length to the nearest meter To deal with this using Python it will be necessary to compare values and then execute different calculations based on those conditions This incidentally is what the next chapter is all about Jargon Busting You encountered a lot of new terms in this chapter so here are some more useful definitions e Binary base 2 Binary arithmetic uses the digits 0 and 1 This corresponds to the electric current in a wire which can only be on value 1 or off value 0 e Bit A bit is a digit in the binary number system It can have two values 0 or 1 The word is derived from binary digit e Boolean Variables of this type can take only one of two values True and False These correspond to 1 and 0 This is the most appropriate return type for a function that uses its return value to report whether some condition holds or not e Built in Anything built in is innate part of the programming language as opposed to something that has to be imported from a module A built in element is part of Python s standard library e
211. erns and this can be done in several ways If you want to find all occurrences of a pattern in a string re findall pattern string flags will return a list of all nonoverlapping matches including empty matches gt gt gt re findall a string atn a a Regular expressions have other useful methods available which allow more fine grained control over groups of matches They have a powerful language all of their own as shown in Table 7 6 which needs its own book to fully explain The methods I have described here will get you started and offer you a lot of power to manipulate text Table 7 6 Characters with Special Meaning in Regular Expressions Character Meaning Match any character except a newline i Match the start of a string Match the character at the end of a string or the one just before the newline at the end of a string Match zero or more repetitions of the preceding regular expression This character creates a greedy expression Match one or more repetitions of the preceding regular expression This character creates a greedy expression Match zero or one of the characters in the preceding regular expression This character creates a greedy expression CHAPTER 7 WORKING WITH TEXT Table 7 6 Characters with Special Meaning in Regular Expressions continued Character Meaning and Nongreedy versions of the previous three special charact
212. errors include missing quotation marks or spaces in the wrong places In this case you will need to go back to the coding stage and correct these errors before you test the code again The testing stage involves making sure that the design does what it is supposed to and that it matches the specification that you started with Compare the actual output from running the program to the output you said you were expecting in your original specification Are they the same If not why not What could be done better Apart from the initial design stage you will probably spend most of your time cycling around this testing stage Don t be tempted to view error messages as annoyances or irritations pay great attention to what they tell you as they can be one of the most useful aids on the path of learning Python On some CHAPTER 2 M DESIGNING SOFTWARE occasions you will want to deliberately produce error messages to prove that something is working Try it now Find out what you have to mess up to get an error message Take note of what sort of message you get and fix your script so that it runs cleanly again Detecting and correcting coding errors Let s take a brief tour of a few common errors you re likely to see For example the following error indicates that you ve left off the quotation marks python hello world py File hello world py line 16 print Hello World SyntaxError EOL while scanning string literal Turning on the l
213. ers m n Matches from m to n repetitions of the preceding regular expression m n This is the nongreedy version of m n Use this to either escape special characters or signal a special sequence Use this to indicate a set of characters A caret as the first character indicates a complementing set This denotes or For example A B creates a regular expression that will match either AorB G8 Match the regular expression inside the parentheses The contents can be retrieved or matched later in the string using the lt number gt escape Ris 2 5 Ignore this group Using Files So far data has either been written into the program itself or received via the input function and printed out using the print function Once the program has finished its execution the data is lost In order for an application to have any practical value it is necessary to be able to store that information and retrieve it the next time the program is run The vast majority of computer information is stored in files on a hard drive or similar storage medium or can be transmitted via some file like object File like objects share some similar properties with files and can often be treated with the same methods streams are an important kind of file like object which I will come to a little later Opening Files File objects can be created with the built in open filename mode buffering function File objects are also returned by
214. ertain raise SecurityNotCertain Can t check security for 0 format check Main program in real life this would be scheduled to run e g every minute report get report check report report The program begins by checking for a security report in the file report txt and when it can t find this file it raises the SecurityError you may have already seen This file is where you can tell the program whether or not the front or back door is open and whether there s any movement in any of the rooms Listing 10 6 is a complete report txt indicating a secure house save it to the current directory and run the program Listing 10 6 A Complete Security Report for Listing 10 5 to Parse door open front no door open rear no CHAPTER 10 EXCEPTIONS motion in living room no motion in kitchen no motion in attic no With this report in place the program should run and quit silently python home security py This is what you want a burglar alarm to do if everything is OK In the top of Listing 10 5 we define a SecurityError exception as a subclass of Exception However we then go on to define even more specific exceptions and use these to deal with different security problems that arise Try changing a no to a yes and running the program again or try removing a line from the report file altogether You should see different classes of exception being raised depending on what you do in the report file The diff
215. es data file close return def read in Ht Qpen Players Read in Player stats from file print Reading in data data file open rpcharacters rpg for line in data file player eval line players append player data file close trace Data players return The main body of the program starts here Ht if name main print Boilerplate Prompt to set number of players reply input How many players or 2 max players int reply Set up locations arena Location Main Arena arena desc Welcome to Cloud Cuckoo Land this is where all the action takes place You can type look here if you want to look around say stuff if you want or just type attack if you want to fight and exit when you want to quit arena inventory shop Shop The Emporium shop desc Welcome to your friendly local equipment store You can buy something if you want or type look to see the Stock List or look me to check your stats and exit when you want to quit shop inventory Armour shield 15 25 50 Weapon sword 60 60 50 Weapon dagger 25 40 60 Weapon halberd 80 75 40 Weapon club 15 40 40 Weapon flail 50 60 55 CHAPTER 9 E CLASSES Weapon hammer 99 100 40 Armour cuirass 30 50 20 Armour armour 101 100 0 Thing lantern 10 5 30 Thing pole 10 5 50 Thing rope 10 5 70 Thing box 5 1 90 Set up players read
216. es you the ability to add whatever functionality you want in the form of modules This allows you to organize and reuse your code and to use modular code provided by other Python developers We had a good look at Python s module system and built modules of our own to see how easy it is This allowed us to talk about the different ways to import and use modules another flexible aspect of Python modules We also covered some of the more advanced aspects of modules such as reloading them and how they work internally The power of Python modules can be seen when you consider the wealth of material provided by the Python community It s fair to say that almost any piece of functionality is out there in the form of a Python module and if it s not then get to work on it and get it out there CHAPTER 12 Simple Windowed Applications Now that you have a working understanding of the whole Python language you have the knowledge required to start creating graphical applications As you have seen rather than trying to code everything from scratch many tasks are covered by modules in Python s standard library which provides a more or less friendly programming interface for you to use in your own programs There are quite a few different tool kits available to help you create a graphical user interface GUI interface that interacts with the program code behind the scenes Therefore this chapter serves as a survey of some of the different kits an
217. ese same steps again to create a second button self button2 gtk Button Quit This time delete event is called and the window exits self button2 connect clicked self delete event the Quit button self box1 pack start self button2 True True 0 The order in which the buttons are shown is not really important but it is recommended to show the window last so that everything displays at once self button2 show self box1 show self window show def main gtk main if name main hello HelloGtk main This script is longer than the equivalent using Tkinter partly because Gtk provides more options Essentially it does the same thing When the HelloGtk class is instantiated a new Gtk window is created 266 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS with the argument gtk WINDOW TOPLEVEL This argument makes a window that is under the control of your window manager with the usual decorations and buttons You can set the title of the window using the gtk Window set title method and attach commands to the window using gtk Window connect The gtk Hbox widget works similarly to Tkinter Frame in that it creates a container for widgets which are packed horizontally To get more complex layouts you can nest several gtk Vbox widgets which pack their contents vertically inside a gtk Hbox widget or vice versa Then you create the buttons connect them to the appropriate callback methods and p
218. esee rrr any see TELTET TETTETETT 282 i eee 283 www allitebooks com About the Author Tim Hall currently provides front line support for 64 Studio He has also written newbie tutorials for Linux User and Developer magazine in between more mundane system admin and web authoring jobs Tim has released albums and performed as a musician and songwriter both solo and in collaboration with other artists He has been further honored as the holder of the Bardic chair of Glastonbury between 2005 and 2007 Tim uses Python as his main programming language primarily as a means for creative ends because it is easy to read and fun to learn J P Stacey has been a senior developer at Torchbox Ltd since 2005 building and maintaining among other things Python Django and Drupal applications He organizes the Oxford Geek Nights and gets involved in tuition and seminars at Torchbox In his spare time he reads and writes fiction and blogs buys too much music and tries not to startle Cotswold lions on his bicycle About the Technical Reviewer Duncan Parkes has been coding in Python both for work and for fun for roughly a decade He drifted into programming largely by accident after initially taking a doctorate in Combinatorial Group Theory a branch of Pure Mathematics As an ex mathematician he loves the clarity and simplicity of Python and needs a bit of persuading to code in anything else After completing a technical review of this book he
219. et it to say something different quickly becomes rather tedious Wouldn t it be great if the program were interactive In that case you would need to find a way to get some user input The quick and dirty method for doing this is to use the built in input function This function takes one argument a text string that is printed to the screen to prompt the user for input The function then returns the user s input All requests for input need a message to tell the user that input is required this is known as a prompt I assign this input to a variable called some text Then I can use the print function to print out the user s input Notice that this time some text isn t in quotes because I want the value contained in the variable called some text rather than the literal text string some text Variable names aren t allowed to contain spaces so you ll notice that I ve replaced the space with an underscore It s worth remembering that variable names can t contain dashes either because Python will assume that the dash is a minus sign Details of how to name your variables will be covered fully in Chapter 3 Most of the actual work in programs is performed by functions You can spot functions quite easily as they usually have parentheses immediately following the function name Any information that the function needs to work on is placed inside the parentheses these pieces of information called arguments have to be separated by com
220. etter e to indicate the exponent E This exponent notation specifier is the same as e except it uses an uppercase E as the separator character f This fixed point specifier displays the number as a fixed point number F This specifier is the same as f g This general format specifier prints the number as a fixed point number unless the number is too large in which case it switches to e exponent notation G This general format specifier is the same as g except it switches to E if the number gets too large n The number specifier works the same as g except that it uses the current locale setting to insert the appropriate number separator characters The percentage specifier multiplies the number by 100 and displays in fixed f format followed by a percent sign None This specifier is the same as g Here s an example that formats the same number as an exponent a floating point and a percentage gt gt gt 0 gt 8E format numbers 4 9 875432E 04 gt gt gt O gt 8F format numbers 4 98754 321000 gt gt gt 0 gt 8 format numbers 4 9875432 100000 A format specifier field can also include nested replacement fields within it Each nested replacement field can contain only a field name conversion flags and format specifications are not allowed The replacement fields within the format specifier are substituted before the format specifier string is interpreted This allows the formatti
221. except e not e and e finally e or e as e for e pass e assert e from e raise e break e global e return e class e if e try e continue e import e while e def e in e with e del e is e yield Following the Naming Rules So let s talk about what you are allowed to call your variables Variable names must begin with either a letter or an underscore Although they can contain numbers they must not start with one If the interpreter encounters a bunch of characters starting with a numeral rather than a letter or a quotation mark it will assume that it is a number You should not use anything other than letters numbers or underscores to identify your variables Also you should be aware that Python is generally case sensitive which means that lowercase and uppercase letters are treated as being different characters therefore true and True are interpreted as completely different entities as are myvariable MYVARIABLE and MyVariable It is also a good idea to keep your own record of names you have already used I recommend keeping a table of variables at the start of the program file so you can easily find your list when you want to look something up Creating Variables and Assigning Values In many programming languages there are two stages to creating a variable The first is to create the container and stick an identifying label on it this is called initialization The second is to put a value into it this is called assignment Init
222. f there s a problem log it but then re raise the error Don t handle it locally except ZeroDivisionError f write Error tried to divide by zero n raise Whatever happens close the file finally f close log data 12 5 29 1 log data 23 0 84 3 log data 66 4 55 9 log data 58 2 0 You will notice that the ZeroDivisionError is not entirely handled the logging method partially handles it with a matching except but then it reraises it with raise That means the method can write a warning message to the log and then let the surrounding program decide what needs to be done with the exception Nonetheless the file is always closed even when the exception occurs the finally clause is always run Putting Everything Together with else So far we ve used exception handling to try out some code handle a number of exception classes and tidy up after both handled and unhandled exceptions But what about handling the situation where there is no exception In that case the program executes all of the try clause and then goes straight to finally with no special treatment for the no exception scenario At first sight this scenario presents no real problem After all if there s no exception our program should just carry on so we could append our code to the try clause Consider the pseudocode in Listing 10 11 which demonstrates this idea Listing 10 11 An Example of Handling a No Exception Situation try This method could rais
223. fferent versions of Python without having to keep checking which version of the interpreter you are using when you want to run them coding UTF8 This tells the interpreter what encoding you are using UTF8 is Unicode and you want to use this for Python 3 scripts In fact Python 3 defaults to UTF8 so adding this line is just extra insurance Next you must set the file permissions on the script This allows you to execute it by itself without having to call it as an argument to python chmod 751 myscript py or chmod u x myscript py In the first version of this command chmod is given a numeric mode This is made up of up to four octal integers any omitted digits are assumed to be leading zeros so I have omitted the first digit in this example The second digit sets permissions for the user who owns the file the third sets permissions for users in the file s group and the fourth sets permissions for all other users The octal permissions value can be calculated by adding up the following values read 4 write 2 execute 1 The 7 in the first place of chmod s argument means that you the owner of the file can run and edit the file The 5 in the next place allows members of the same group as the file to read and run the program and the final 1 allows other users of the system to run the file but not read or write The alternative version of the command simply gives executable privileges to the owner of the file
224. fferently or you will want to play safe and catch all exceptions with a nonspecific except clause All exception classes in Python must belong somewhere on this hierarchy This includes any custom exception classes that you create yourself as explained in the next section new exception classes must inherit from an existing class We ll discuss the consequences of this hierarchy later in the Handling Complex Errors section Creating and Raising an Exception Object Typically you can rely on Python s internal methods to raise exceptions whenever something goes wrong in their own internals But in order to make full use of exceptions you will sometimes want to raise them yourself in the depths of your program so that the higher levels of the program will have more detail about what has just gone wrong and will know how to deal with it For this we use the raise statement This keyword can be followed by either an exception class for example TypeError or an instance of that class for example TypeError this is an error message It can also be invoked with no arguments inside an except clause to reraise the exception currently being handled Examples of the three main syntaxes of the raise statement follow gt gt gt raise KeyError Traceback most recent call last File lt stdin gt line 1 in lt module gt KeyError gt gt gt raise KeyError foo Traceback most recent call last File lt stdin gt line 1
225. fic item in the sequence Iterable This refers to a code object that can be iterated Iterate When you loop through items in a sequence one item at a time you iterate it Iterator This construct is designed to allow looping Mapping This refers to a sequence that maps hashable values to arbitrary objects Matrix A matrix is a multidimensional sequence Method A method is a function specifically attached to an object or class of objects Mutable A mutable value can be changed Operation This action is performed on two variables operands usually of a mathematical or logical nature Queue A queue is a first in first out FIFO structure You push things in at one end and pop values out of the other Resultant This value is returned as the result of a process Separator This text string is used to distinguish between items of data Sequence A sequence is the simplest sort of complex data type in Python Lists strings and tuples are the types of sequences Sequence packing Sequence packing is the action of assigning a sequence of comma separated values to a variable Slice This refers to a smaller segment of a sequence 99 100 CHAPTER 5 M USING LISTS e Stack This is a last in first out LIFO structure used rather like the discard pile in acard game e Traverse When you go through the items in a sequence in order you traverse them Summary You have taken on a lot of new information in this c
226. file inventory if item in armour types if armour stats profile armour armour stats 0 else profile armour 0 0 50 This worked fine but I don t know if it really constitutes much of an improvement It doesn t fix any particular problem but it does reduce one level of indentation Sections that have lots of nested indented blocks are always likely to be good candidates for refactoring and one of the aims of this exercise is to reduce the levels of indentation in the code This must however be balanced with an attitude of if it ain t broke don t fix it You can make the coding process much more fun if you stay focused on making the changes that give the most gratifying results and then move on as soon as those changes satisfy the design requirements So again I won t dwell on this section Keeping Your Code Readable I also wanted to improve a couple of output messages I wanted the inventory list to have an and separating the last two entries rather than a comma and I wanted to be gender specific when reporting self inflicted actions I started by creating an empty structure def join with and sequence Join up a list with commas and and between last two items 111 CHAPTER 6 FUNCTIONS print join with and sequence was called return Next I inserted a dummy call in the main body handbag join with and profile inventory Having an empty structure in place allows me to te
227. fter the try clause but before the finally clause only ifno exception is raised Confused That s understandable Let s take a step back and look at a listing that should illustrate as simply as possible everything that you can do with Python exception handling Examine and run Listing 10 12 and look at its output Listing 10 12 Python s Complete Exception Handling Using try except else and finally def divide by complex x y Division of two numbers with complex exception handling try print TRYING beginning division of 0 1 format x y result x y except ZeroDivisionError print HANDLED division by zero else print SUCCESS result is 0 g format result finally print FINALLY cleaning up This surrounding try except block helps clarify when exceptions escape unhandled from the divide by complex method try Normal behaviour divide by complex 2 1 print Internally handled exception divide by complex 2 0 233 234 CHAPTER 10 EXCEPTIONS print Raised exception divide by complex 2 None except Exception as e print RAISING exception 0 not handled rising up format e class name TRYING beginning division of 2 1 SUCCESS result is 2 FINALLY cleaning up TRYING beginning division of 2 0 HANDLED division by zero FINALLY cleaning up TRYING beginning division of 2 None FINALLY cleaning up RAISING exception TypeError not handled r
228. g char space flagged False else if not space flagged output string space flagged True print output string print print print CHAPTER 4 E MAKING CHOICES The variable punctuation is a string containing all the punctuation marks that I want to remove including spaces I will use this to contain all the values that I want to be replaced by spaces I need to initialize output string and space flagged at the start so the interpreter knows what they are when it gets to them Next the program prompts the user for input and loops through the string checking for punctuation and spaces The heavy lifting here is done by the conditional construction char not in punctuation This conditional expression checks to see if the character char matches up with any of the individual contents of the punctuation string printing out a space if it does or adding the string to the output string if it doesn t This is a rather long winded way of analyzing a sentence Python has various methods of making this sort of task a lot simpler In the next chapter I ll show you how to refine this script to make use of Python s list manipulation facilities Even using the limited commands that you have learned so far it would be possible to refine the design in Listing 4 7 further or customize it to deal with other special punctuation cases in different ways Jargon Busting The terms presented in this chapter follow e Assignment operato
229. g handled and another is raised either by accident or through an unconnected raise statement the first is setas the context attribute of the second If however you want to raise a new exception directly as a result of the one being handled you can use the raise from syntax to show the connection the original exception is setas the cause attribute of the new The combination of cause and context lead to complex but useful tracebacks Save Listing 10 14 as chaining py and run it Listing 10 14 Demonstrating Exception Chaining def return explicit chain try Catch a NameError and raise a new error from it try foo except Exception as e raise Exception Explicitly raised from e Re catch the new exception except Exception as e2 return e2 def return implicit chain try Catch a NameError but accidentally raise a KeyError try foo except Exception as e bar Re catch the new exception except Exception as e2 return e2 The explicitly raised exception and its cause ex ch return explicit chain print Explicit chain print ex ch repr print ex ch cause The implicitly raised error and its context print Implicit chain im ch return implicit chain print im ch repr print im ch context Re raise to see the corresponding traceback raise im ch Uncomment this to see the implicit chain raise ex ch When you run the preceding code you ll get thes
230. gather all the bones and bring them to her She gathered all the bones except for one shank bone Figure 7 1 HTML output from my original text file displayed in a browser Checking and Correcting Styles In order to help upgrade my existing programs to Python 3 I wanted to be able to go through my Python scripts and pull out any comments that contained the words TODO or FIXME and the magic variables set at the beginning of the script the version and maintainer variables I wanted to be able to check that they were formatted consistently and then update or create the appropriate text documentation Identifying these items is to help me stick to the coding style I have set for myself you might want to do things differently The initial pseudocode design in Listing 7 3 made this task look pretty simple Listing 7 3 Pseudocode for a Style Checker Script Purpose Python script style amp version checker Target System 147 CHAPTER 7 WORKING WITH TEXT GNU Linux Interface Command line Functional Requirements Automatically go through all lines in a Python file pulling out TODO comments and appending TODO file check amp update AUTHORS file check for existence of README NEWS and ChangeLog So all documentation can be auto created from main python files Print warnings and prompt before any changes are made Input get Python filename from user Open the file for reading Test if file is valid python 3 0 if not suggest running
231. haining To chain exceptions you attach the exception currently being handled by a try statement to any new exception which whether deliberately or accidentally occurs during the execution of any of the clauses in the statement If the new exception is raised using the raise from syntax the chaining is said to be explicit Otherwise if the exception is raised on its own or occurs because of a programming error then the chaining is said to be implicit Easier to ask for forgiveness than permission EAFP philosophy In this programming style the program always attempts to execute code that might raise exceptions and then handles any of those exceptions afterward This style implicitly treats exception handling like any other type of flow control e g for loops or if else statements and as such is very much suited to languages like Python else clauses This part of a try statement block is executed if there are no exceptions raised by the code in the try clause but before the code in the finally clause is run The else clause is analogous to except clause but is run in the situation where there is no exception Exception An exception is a Python object of the BaseException class or any subclass of BaseException that Python s core exception framework creates when some part of the program encounters an exceptional circumstance that can t be dealt with there and then Exception handling This refers to manipulating exception
232. hapter culminating in developing the ideas from previous chapters to create a much more complex program that processes several related pieces of information and returns a rather verbose human readable response This level of sophistication was made possible using Python s data structures strings tuples and lists which are all kinds of sequence and dictionaries a mapping You have also learned that some of these structures i e strings and tuples are immutable whereas others like dictionaries and lists are mutable which means they can be modified You know how to access items of a sequence using indexes and slices you can test for membership of a structure using the in keyword or iterate through an entire sequence using the for keyword or a list comprehension You have encountered several different uses of lists as stacks queues and matrices of information and you know how to create dictionaries out of key value pairs You also now have at your disposal a number of methods you can use to manipulate your data to produce useful output Combined with looping techniques these data structures provide the logical building blocks with which you can process your data In fact using what you have already learned you could probably write a program that will do most things you can think of However that code will be very long winded possibly difficult to read and not really reusable So in the next chapter I will introduce the next stage of prog
233. hat the application is not equipped to deal with Critical values should be worked out early on in the design process so that the program can be properly tested against them In my curtain calculation the value that most needs taking into account is that of the roll width which has been set at 140 cm Allowing 20 cm for hems means that the maximum width of curtain I can make out ofa single length is 120 cm before I have to start thinking about sewing widths together Each curtain has to be three fourths of the width of the window so the maximum width of window I can cover with two single widths of material one for each curtain is 160 cm If the window is 320 cm wide I will need four widths to cover the window and if I were really clever I could probably do a 240 cm wide window with three widths If the window is much wider than it is deep it might be possible to turn the cloth the other way Allowing for a 15 cm hem it would be possible to do this if the window height was less than 125 cm To represent these different conditions I could use a set of arbitrary values for the window size such as 100x100 100x200 200x100 200x200 200x300 and 300x200 I also need to work out some expected results for these values by hand so I can find out whether they tally up with the script s output 57 58 CHAPTER 4 MAKING CHOICES e The first case 100x100 cm could be measured either way against the material width each curtain needs a piece
234. have already seen it is possible to split up a string according to the occurrences of a pattern using re split pattern string maxsplit This method returns a list containing the matching substrings There is an optional argument maxsplit that allows you to specify the maximum number of splits allowed The re module can also perform replacement operations with the re sub pattern repl string count method This will return the string obtained by substituting the leftmost nonoverlapping 139 140 CHAPTER 7 WORKING WITH TEXT occurrences of the pattern in string with the replacement rep1 You can limit the number of substitutions made by setting the optional argument count The replacement rep can be either a string or a callable i e a method or function if it s a callable it must accept the match object as the only argument and must return a string gt gt gt re sub a data string data Z data D j Vu data 4y gt gt gt re sub a data string 2 data Z data D j Vu a p There is an alternative method re subn pattern repl string count that does the same thing as re sub but also returns the number of substitutions made in a tuple new string number where number is the number of substitutions that were made gt gt gt re subn a data string data Z data D j V data u 3 The re module is primarily used for matching patt
235. he alignment operators shown in Table 7 1 Table 7 1 Alignment Format Specifiers Option Description lt Align left default gt Align right s Center Place the padding after the sign if any but before the digits This alignment option is only valid for numeric types Here s an example that formats three strings The first string 0 s is aligned to the center given a minimum field width of 3 and padded with which fill any whitespace The second string 1 s is aligned to the right gt given a minimum field width of 4 and padded with 0 The third string message s is aligned to the left lt given a minimum field width of 42 and padded with CHAPTER 7 WORKING WITH TEXT gt gt gt astring Value 0 s 3 equals 1 s 0 gt 4 message s lt 42 gt gt gt astring format y 42 message not bad Value y equals 0042 not bad J 1 PIPLIIILIIIIIIIIIIII Note Unless a minimum field width is defined the field width will always be the same size as the data to fill it so the alignment option has no meaning in this case Next the sign option is only valid for number types and can be one of the values in Table 7 2 Table 7 2 Sign Format Specifiers Option Description A sign should be used for both positive as well as negative numbers A sign should be used only for negative numbers default Space A leading space should be used on positive num
236. he parentheses after the function name on the first line of the function definition import random def roll sides dice result 0 for rolls in range 0 dice result random randint 1 sides return result CHAPTER 6 FUNCTIONS This function might be called from the main program in the following way muscle roll 33 3 The values in parentheses in the function call are known as arguments they correspond to the formal parameters in the function definition In this case the first argument 33 is bound to the parameter sides and the second argument 3 is bound to the parameter dice This effectively creates two variables sides and dice to be used inside the function If you just send the function values like this there must be the same number of arguments as parameters and they have to be in the right order so these are known as positional arguments You could substitute the actual values in the function call with variables if you prefer Just remember that the function is going to refer to that value by the name of the parameter that receives it and the original variable will remain unaffected only the value gets passed on Note The variables sides and dice only exist inside the function ro11 If you try to use them outside the function you ll get a name error See the Understanding Scope section later in this chapter You can set default values for your parameters in the function definition def roll sides
237. hether or not something has happened Float Float is short for floating point and is a fundamental type used to define numbers with fractional parts Hexadecimal base 16 Hexadecimal is base 16 arithmetic where each digit is a value from 0 to 15 rather than the 0 to 9 of base 10 The decimal numbers from 10 to 15 are represented by the letters A to F Hexadecimal is a very convenient way of showing binary numbers as every four binary digits can be shown as one hexadecimal digit Integer An integer is a fundamental i e built in type used to define numeric variables holding whole numbers Immutable An immutable value cannot be modified Logical operator These commands perform basic manipulations on Boolean values Mapping In Python a mapping is a data type that relates a set of keys to a set of values It has nothing to do with planning your car journey Octal base 8 In octal arithmetic each digit has a value of 0 to 7 Operator These are commands often represented by mathematical symbols that perform simple manipulations on data known as operands Expressions take the following form operandi operation operand2 Operator precedence This is the order in which operators are assigned their arguments when Python evaluates an expression Where there is ambiguity the operator with the higher precedence is assigned as arguments the smallest expressions on either side of it that make sense Sequence In Python
238. his is what happens when you first assign a value to a variable using the operator or when you import a module The value is said to be bound to a particular name and each name can only refer to one object although it is perfectly legal to bind several different names to an object Scope defines the visibility of a name within a code block Each block has its own namespace Namespaces are organized like a family tree or directory structure a name will be valid in any code blocks contained within the defining one but not in any outer ones Like rooms within a room when you enter into a space you can see everything inside it but you are no longer aware of anything outside The outermost scope is therefore global names defined in the outermost scope will be recognized anywhere in the program s world as long as those names don t become masked by being assigned to different values within any of the subroutines The exception to this rule is that names defined inside a class definition don t extend to any of the methods contained in the class you have to refer to them via self inside the class definition The set of all scopes visible to a code block is called the block s environment Class attribute references are first looked up within the immediate scope of the class When the attribute name is not found there the attribute search continues in the base classes If a name is not found at all aNameError exception is raised If the name refers to a lo
239. hod which will show me whether the signal is being caught and allow me to fill in the details later def delete event self widget event data None Quits the application print You called a event return False Bye Now I can begin to fill in the details on the canvas provided by the main window Gtk organizes the graphical components of the interface by packing them into boxes which are aligned either vertically or horizontally and can be packed from left to right top to bottom right to left or bottom to top This type of organization means careful planning I started by dividing the space vertically inserting an invisible vertical box in which the main areas of the interface can be stacked Note The vbox has to be added to the window so that Gtk knows where to place it Later in this stanza it is necessary to explicitly call self vbox1 show to tell Gtk to actually display the widget even though it s invisible The first item in the window is the menubar which is constructed in a fashion similar to stacking Russian nested dolls First the headers for the three menus are constructed and submenus are set for each header After that the individual menu entries are constructed as separate widgets As with the delete event command I constructed stubs for these methods so I could get on with building the GUI first and worry about the functionality later Note Each menu still has to be specifically appended
240. hod calling that one and so on until the exception is dealt with using exception handling Traceback A traceback is the human readable report that an unhandled exception creates when it is raised up to either the command prompt or the Python interactive prompt try statements try clauses and except clauses These compound statements are the core of Python s exception handling A try statement consists of a try clause and usually one or more except clauses Execution of the contents of the try clause is attempted and if an exceptional circumstance causes an exception object to be created Python looks for an except clause that matches the class of the exception object or one of its superclasses Summary You can now include sophisticated error handling techniques in your Python repertoire You have seen how the try except else finally statement is used to handle exceptions and you now know how to create exception classes of your own based on the existing exception class The next chapter looks at some of the most useful modules in Python s standard library CHAPTER 11 Reusing Code with Modules and Packages The primary motivation for humans to offload work to computers is that for a human the work is boring and repetitive So it would be a shame if every time you wanted a new program and you were building to include functionality you d built before you had to cut and paste every method and class definition from the earlier p
241. ialization and assignment are performed with a single command in Python using the sign So you would assign a value to a variable by typing the following variable value CHAPTER 3 VARIABLES AND DATA TYPES Note Every variable has a value there s no such thing as an empty variable in Python A section of code that does something such as an assignment is known as a statement The part of the code that can be evaluated to produce a value is known as an expression The right hand side of an assignment can be an expression like the assignment to total price in the following list of simple assignment statements number 0 roll width 1 4 price per metre 5 filename data txt trace False sentence this is a whole lot of nothing total price roll width price per metre Each statement should have its own line If this looks like a shopping list to you or a list of materials or ingredients then you re on the right track A recipe usually begins with a list of ingredients eggs 2 butter 0 50z salt pinch pepper pinch The recipe might specify a list of tools that you will need knife fork bowl frying pan and then follow on with the method often in numbered steps The same happens in a Python program you define your variables and then carry out tasks on them Recognizing Different Types of Variables Python recognizes several different types of variables string literals words numbers
242. ic juggling skills you are likely to find simpler and more effective ways of coding your designs and you will want to update your old scripts This is a normal and natural part of the process of program design and a good discipline to develop early on Following this train of thought in this chapter we re going to see some techniques to improve our previous work I ll explain how to compare values in your code so that you can make decisions and take appropriate actions based on the comparison This approach to controlling the flow of your application will allow your applications to be more flexible and able to deal with changing input With this flexibility however comes complexity so I ll go over how to manage increasingly complex code The final part of the chapter covers loops which are another way to alter the flow of your application With loops you can iterate over a large set of data and perform the same operations on each member of the set Comparing Things The curtain making exercise at the end of the previous chapter needs some refining It is too simplistic to provide accurate answers and contains some logical errors that need fixing In order to generate more accurate answers you need to know how to compare values and specify different courses of action based on the results The code construction that allows us to do this is called a conditional statement Conditional statements transform the script from being just a list of instru
243. ical box and the remaining items are displayed with the main window coming up last Once the basic framework for the GUI has been constructed it is necessary to fill it with some information GtkInfo create treelist creates a gtk ListStore to structure the information that is to be displayed in treeview1 the left hand options panel as shown in Figure 12 4 Again a nest of items is created started with the columns and these are populated with individual cells Next the icons are put in place by the make pixbuf method and then packed into the columns The liststore is associated with treeview1 using the treeview1 set model model self liststore method File Edit Help _ Categories ay Asound Kernel Advanced Linux Sound Architecture CPU Driver Version 1 0 16 Compiled on Apr 9 2008 for kernel RAM 2 6 21 1 multimedia 486 SMP 0 CMI8738 CMI8738 C Media Sound Modules CMI8738 C Media CMI8738 model 37 at Oxe000 irq 9 0 0 control 1 sequencer 4 0 0 hardware dependent 8 0 0 raw midi 16 0 0 digital audio playback 17 0 1 digital audio playback 18 0 2 digital audio playback 24 0 0 digital audio capture a ee ee Sound Devices Quit Figure 12 4 The panel on the left is a treeview 271 272 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Making the Commands Do Something Useful The next step is to turn the stub methods into commands that actually d
244. ided the Python 3 compatible module on the Source Code Download page for this book at WWW apress com Once you ve downloaded the file from the Apress web site you can extract the file BeautifulSoup py and store it anywhere on Python s sys path and it will be available for use A quick example follows to show you how easy it is to start obtaining useful information from HTML Note though that the Python home page has quite probably changed since the time of this writing so this might no longer work as expected But BeautifulSoup is a flexible and usable module so you should be able to easily adapt this code to your own requirements gt gt gt import urllib request BeautifulSoup gt gt gt Connect to the remote site and read the content Y urllib request urlopen http python org gt gt gt text r read gt gt gt Parse into BeautifulSoup soup BeautifulSoup BeautifulSoup text gt gt gt Stepping around the tree soup contents 0 DOCTYPE html PUBLIC W3C DTD XHTML 1 0 Transitional EN http www w3 org TR xhtml1 DTD xhtmli transitional dtd gt gt gt soup contents 2 name CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES html gt gt gt soup contents 2 contents 1 name head gt gt gt How many links does the document have len soup a 91 gt gt gt Finding content by element name and class soup h1 class pageheading 0 renderContents b
245. idual item from a dictionary use del gt gt gt profile Name Adam Gender Race Charm 0 Muscle O Speed 0 Desc gt gt gt del profile Gender gt gt gt profile Name Adam Race Charm 0 Muscle 0 Speed 0 Desc If you want to fetch the value and remove it from the dictionary you can use dict pop key default Alternatively dict popitem will remove a random item from the dictionary and returns the key value pair as a tuple gt gt gt profile popitem Brainz 0 gt gt gt profile Name Adam Gender Desc Race Charm 0 Muscle 0 Speed 0 CHAPTER 5 M USING LISTS If you want a clean slate dict clear removes all items from the dictionary gt gt gt profile clear gt gt gt profile Sorting Dictionaries You can sort the output from any of the dictionary methods using the built in sorted function gt gt gt sorted profile items Brainz 82 Charm 64 Desc Tall Gender female Muscle 43 Name Ez Race Pixie Speed 76 gold 109 life 45 magic 73 prot 63 Using Dictionaries Dictionaries are used anywhere we want to store attributes and values that describe some concept or entity For example we can use a dictionary to count instances of particular objects or states Because ea
246. ifferent things even if they look the same to you In your text editor s Preferences window set it to insert four spaces instead of using tabs and you ll avoid so much future grief by doing this It is always best to keep your designs clear and readable use blank lines to separate the different sections of your pseudocode and use indentation to show how statements are grouped together As long as each line is a comment preceded by a you are free to format it how you like Caution The use of indentation in your actual code follows very specific rules This is one aspect of Python that takes a little bit of getting used to will explain the rules fully in the chapters on control flow statements and functions Refining Each Line of Your Pseudocode Until a Solution Is Reached Each line is then further refined by breaking it down into steps in the same way until each step comprises a single action and you have a complete design Subsections should be indented by four spaces as shown in Listing 2 4 While you are working in pseudocode your groupings and indentation can be quite arbitrary all you are doing is providing a framework for your code to live in As you learn more about Python your pseudocode will naturally adopt more Pythonic structures The process of refining your designs will be covered in greater detail from Chapter 4 onward Listing 2 4 Indentation of Subsections Do the first thing Set the first value
247. ification for 185 CHAPTER 9 M CLASSES rpcombat py was The user must be able to generate more than one character profile equip those characters with suitable weapons and model hand to hand combat between characters Table 9 1 breaks down the rpcombat py specification though note there are no pertinent adjectives in this case Table 9 1 Breakdown of rpcombat py Specification Nouns Verbs Adjectives User generate Character profile equip combat Weapon 186 This analysis makes it very clear what needs to be done I m going to merge User and Character into Player so each Player needs a generate method and an equip method plus it needs to be able to fight see Listing 9 1 In the specification combat occurs between players so each player will need something like attack and defend methods Our original profile from Chapter 5 was a dictionary containing Name Desc Gender Race Muscle 0 Brainz 0 Speed o Charm 0 life 0 magic 0 prot 0 gold 0 inventory so these will become the Player attributes User stories or use cases can also be helpful for refining your design Go through each case step by step and make sure all the requirements are covered in your new class specification Listing 9 1 Creating the Player Class class Player def setName self name self name name title def getName self return self name def setDesc self desc self desc
248. ilt in len function A matrix like the one in the preceding example incidentally would require two nested for loops one for each dimension of the matrix The positional index and value can be both retrieved at once using enumerate another built in function gt gt gt for i value in enumerate fruits print i value 0 avocados 1 bananas 2 oranges 3 grapes 4 mangos Let s do the same for our matrix example gt gt gt for row in matrix for i value in enumerate row print i value print 011 1 21 2 31 83 84 CHAPTER 5 M USING LISTS O 21 1 22 2 32 0 31 1 32 2 33 To loop through two or more sequences at a time entries can be paired with zip gt gt gt for fru veg in zip fruits vegetables if fru lt veg print fru are better than veg else print veg are better than fru avocados are better than carrots bananas are better than potatoes onions are better than oranges grapes are better than leeks celery are better than mangos List Comprehensions The last nifty tricks to cover in this list of things you can do with lists are list comprehensions List comprehensions evaluate an expression for each item in a list and return a list of the results Think of this as a shortcut to apply an operation to every element in a list and get back a new list without the pain of creating a temporary list List comprehensions usually result in clearer code than using built in functions like map or fi
249. image processing and already have a convert module That user wouldn t appreciate having to change all the references to those modules just to be able to use your pirate speak code More precisely for a collection of files to be a Python package you need the following e A top level directory plus any arrangement of subdirectories e Your py files somewhere in that directory hierarchy e A potentially empty file called init py in each directory in the package The init py file tells Python that the directory is part of a package and can contain configuration information and methods Building a Pirate Speak Package In my modules create a subdirectory called pirate Put strings py and convert py in there and add an empty init py Then change your current directory to my modules and run an interactive prompt with the following gt gt gt import pirate gt gt gt pirate lt module pirate from pirate init py gt gt gt gt Now import a submodule explicitly from pirate import convert gt gt gt convert lt module pirate convert from pirate convert py gt gt gt gt This submodule also gets added to pirate pirate convert lt module pirate convert from pirate convert py gt gt gt gt But the strings submodule isn t there yet dir pirate builtins doc file name package path convert gt gt gt If we import that explicitly too then it appears in p
250. ime A d B Y def print content print Content type text html print def print header print lt html gt lt head gt lt title gt Simple CGI script lt title gt lt head gt lt body gt CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB def print footer print lt body gt lt html gt print content print header print lt hi gt It s 0 s lt h1 gt lt p gt Your server is correctly set up to run Python programs lt p gt format now print footer Once the file is uploaded you should be able to test it out by pointing your browser to an address like http www mywebsite com cgi bin hello cgi If your server has already been set up correctly to run Python scripts you will see a rather plain web page bearing today s date and a positive message like the one shown in Figure 8 3 File Edit View History Bookmarks Tools Help F it D http glastonburymusic org uk cgi bin hello cgi Iz G Google It s Friday 14 November 2008 Your server is correctly set up to run Python programs Done ba Figure 8 3 Confirmation that we can run Python on our web server 175 176 CHAPTER 8 EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB If you see some other message displayed you may not have been so lucky as to have a correctly configured web server You will probably need to ask your Internet service provider ISP or your system
251. in Chapter 8 which tells your operating system where to find executable files Also you might not have the PYTHONPATH environment variable set on your system that doesn t matter as Python adds its own default values to the path so it can find files such as its own standard modules Try running the following experiments at the command line Note that we run Python below with the c option This tells Python to execute the following string as if it were a Python program It s a handy shortcut for very short programs that you don t want to either save to a file or run from the interactive prompt echo PYTHONPATH If it s not set this just gives an empty newline python c import sys print sys path opt python3 0 lib python30 zip opt python3 0 lib python3 0 opt python3 0 lib python3 0 plat linux2 opt python3 0 1ib python3 0 lib dynload opt python3 0 lib python3 0 site packages export PYTHONPATH home jp home jp bin CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES echo PYTHONPATH home jp home jp bin python c import sys print sys path home jp home jp bin opt python3 0 lib python30 zip opt python3 0 lib python3 0 opt python3 0 1ib python3 0 plat linux2 opt python3 0 lib python3 0 lib dynload opt python3 0 1ib python3 0 site packages When Python encounters an import statement in the code it checks each of the entries in sys path in list order i
252. ine numbers in the preferences of your text editor will help locate the relevant line Many editors also allow you to search for the specific line number The fix for this is easy go back and add the matching pair of quotation marks The little arrowhead shows you the place where the interpreter realized all was not well That won t always be exactly the same place as you made the error but it will probably have some logical relationship The following error means you misspelled some text the first time you mentioned it python hello world py Hello World Type in some words Some Text Traceback most recent call last File hello world py line 22 in lt module gt print some text NameError name some text is not defined The program runs fine up to that point but the misspelled word is considered to be an error in your logic Go back to the Python file and make sure your logic is sane and that you have spelled the names of your variables consistently In the following script you forgot to put quotes around the argument to input your prompt string python hello world py File hello world py line 19 some text input Type in some words SyntaxError invalid syntax Notice that the interpreter doesn t mind the space between the function name and the parentheses but it does mind the spaces in between the words in the argument because you didn t enclose them with quotes Syntax errors mean that you got the punctuat
253. ine of text or other characters intended to be displayed or processed as a single item e Top level design This is the first stage of a design and it provides a summary or general description of the actions that the software is intended to perform e User story A user story consists of one or two lines in the everyday language of the user that specifies something desired from the software e Validation This process tests whether a value is what the programmer expects it to be e Variables Use variables as a means of referring to a specific item of data that you wish to keep track of in a program It points to a memory location which can contain numbers text or more complicated types of data Summary Congratulations You have completed your first turn around the software design cycle and produced your first Python program In the process you have discovered how to identify and analyze problems and have created a simple framework by breaking the problem down into simple steps You wrote your first lines of Python code and learned how to assign values to variables You also obtained user input using your first function and then tested it out and you learned how to use error messages to help detect and correct coding errors Finally you learned about the importance of documenting your intentions expectations and results This chapter has concentrated mainly on preparing ideas in order to turn them into effective applications In the res
254. ing output when trace on is set to True The main body of the program is now wrapped upinaif name main condition to prevent the main body being executed when rpcombat is imported as a module The main body only deals with an overview of events First the user is prompted for a number of players and then the locations are set up by creating two Location objects arena and shop which are then stocked with values Next the role playing characters are generated and last of all the game begins with arena enter players 0 At this point all the code loaded into memory comes into play and because you are no longer limited by the order statements come in the script the action of the game can proceed in any order well in theory at least The limitation now is simply the commands that are recognized by the game Location s mini command line Listing 9 2 rpcombat 0 6 py usr bin env python coding UTF8 RPCombat Simple Role Playing Combat Game Usage rpcombat py First you are prompted to enter the number of players you want and then you are taken through character generation where you get the chance to equip your character with a suitable weapon at our fabulous Emporium Available commands look buy lt item gt The characters then engage in hand to hand combat in the arena until one is victorious Available commands look attack say Target System GNU Linux Interface Command line 201 CHAPTER 9 M CL
255. ing a new function are what kind of input it needs and what information the function will return Of close importance are the type and structure of the data that will be fed in and retrieved Data supplied to a function are called parameters and the final information that is returned is known as results or output Our initial specification for the function design should also include a general description of the function s specific purpose Note An important effect of using functions is that their internal working can be changed without affecting the other parts of the program Defining Functions Functions are defined using a def statement The word def is followed by the name of the function with an optional list of parameters in parentheses and the line ends with a colon which indicates that the next lines must be indented as a block or suite of instructions Let s start with a function that takes no parameters 101 102 CHAPTER 6 FUNCTIONS gt gt gt def generate rpc bes Role Playing Character generator profile print New Character return profile This block of instructions proceeds in exactly the same way as a complete script So in essence we give aname to each piece of functionality and they are called functions If you want to provide some notes on what the function does you can do so in the docstring which must be the first thing in the function The docstring is then followed by a series of st
256. ing checksum software to ensure that your downloads are as the original developers expected Many open source projects publish the checksums of their packages and their developer communities will have documentation explaining how this works 255 256 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Three Examples from the Python Universe To give you a flavor of the sort of solutions that already exist out there we ll discuss three straightforward but particularly neat modules below These were chosen not so much because they re the most impressive modules to be found but because they re freely available easy to use and solve common and to some extent quite dull problems which you might otherwise be tempted to spend a lot of time fixing on your own CSV This module is built to handle the comma separated variable CSV file format Most spreadsheet data can be exported to CSV which produces text only files However there s no recognized standard for CSV so along with other operations this module lets you worry less about whether or not your Python file is dealing with some particular variant A number of other modules do exist which can deal with more complex types of spreadsheet files but the csv module is part of the core Python distribution Given that it ll be immediately available to you and that much of the data in a spreadsheet can be exported to CSV format csv is a useful tool for importing or exporting stru
257. ing problem Let s look at that procedure in more detail As in written arithmetic the division operator takes two arguments a dividend to the left and a divisor to the right The operation is quite simple but the operator has a number of checks built into it for example both of its arguments must be numeric and the divisor must be nonzero Usually these conditions are expected to be satisfied However the operator itself has no way of recovering when one of these checks fails The operator instead falls back on Python s standard method of dealing with exceptional circumstances exceptions If the divisor is zero the operator causes an exception object of class ZeroDivisionError to be created We then say that this exception is raised by the operator or by the attempted operation This can be taken to mean raising in the sense of discussing a problem at work with your boss and more widely in the sense of a problem passing up through a chain of command When function A calls function B and function B includes a division by zero the division operator raises the exception within function B And if function B doesn t know how to handle the exception it raises the exception with function A Ultimately in our example the exception is raised with the interactive mode which doesn t know how to handle the circumstances and must therefore raise the exception with you the programmer This needs to be done in a human readable way which
258. ing to uppercase using string upper gt gt gt 3 upper EXAMPLE TEXT Or lowercase using string lower gt gt gt s3 lower example text It is very useful to convert strings to lowercase for many internal operations like storing and searching I d recommend lowercasing most user input as soon as it is received unless the use of capital letters conveys important information You can always put the capitals back using string capitalize or string title when you want to print out the string You can also get funky with string swapcase This returns a copy of string with the uppercase characters converted to lowercase and vice versa CHAPTER 7 WORKING WITH TEXT gt gt gt s3 Swapcase ExAMp1E TexT I m not quite sure why you d want to do this but it sure is great to know you have the power should you choose to wield it Simple Methods of Formatting You have various choices for controlling the way strings are printed out Basic formatting is provided through methods such as mystring center width fillchar which returns a string centered in a string of width characters long Padding is done using the specified fill character the default is a space So the following returns a 37 character string with mystring in the middle and as padding gt gt gt x 3 lower gt gt gt x center 37 1 HAAAAAKKKKKEK EX AMD LE text kkk kk kk There are related methods to align text within a st
259. ion or grammar wrong The fix for this is usually to go and look up the correct form for this function in the official documentation It might have been acceptable to have a one word argument if that word was the name of a variable containing a text string If no variable of that name existed you would be accused of calling an undefined identifier 21 22 CHAPTER 2 DESIGNING SOFTWARE The final example is slightly more obscure This error was actually caused by leaving off the comment marker at the beginning of the line and leaving a space python hello world py File hello world py line 18 2 Input some text IndentationError unexpected indent The interpreter wasn t expecting to hit an indented line here it was expecting another statement at the same level of indentation told you Python was fussy about indentation If the space hadn t been left you would simply receive a syntax error because 2 isn t a command that Python recognizes Noting Modifications Extensions and Enhancements As you read through your code make notes of any improvements that could be made I like to use comments in the script for this It is a useful convention to include the word TODO or FIXME in the line so you can search the source file later for things that you have decided need doing You could create a separate TODO file in the same directory if you want to keep track of multiple files Documenting the Solution Documentation is
260. ion that was raised en route to the eventual traceback 237 238 CHAPTER 10 EXCEPTIONS A Final Note on Pythonic Exception Handling Some languages try to make exception handling a last resort the syntax can often be cumbersome and the functionality basic and the performance of the language interpreter might suffer during its equivalent of try statements But as you have seen Python actively encourages error handling through simple syntax with a fully developed flow control based on exception handling Also in most Python distributions exception handling is no slower than simple if else flow control When a language tries to discourage you from using exception handling it is following a philosophy known as look before you leap LBYL This means that you should ensure that something is allowed first and do it only if it s allowed In contrast the Pythonic philosophy of trying something first and then using exception handling to deal with unusual consequences is often referred to as easier to ask forgiveness than permission EAFP In Python you should always use the EAFP style rather than LBYL wherever possible Listing 10 15 compares simple examples of the LBYL and EAFP programming styles Listing 10 15 LBYL Programming vs EAFP Programming LBYL not Pythonic if hasattr some object result return some object result else return None EAFP Pythonic try return some object result except
261. ions more or less likely It still needs to be used at the interactive prompt but we ll expand on this module later to make it more useful Everyday Module Usage When you use modules you ll commonly want some flexibility over how you import them into your applications Luckily Python gives you a few options which we ll look at in the next section You ll also need to organize your modules so they don t get out of control so we ll look at that too Flexible Importing Once you ve put your code into a module you have a number of flexible ways of using the import keyword to access the contents of the module That means you need import only what you require for a particular project To automatically import everything from a module you can use one of two Python constructs import modulename creates an object called modulename in the current scope with all the module contents as attributes of that object Alternatively from modulename import puts all methods classes or variables from the module into the current scope as separate variables not quite all as you will see later CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES gt gt gt Import the module convert with methods referenced inside it import convert gt gt gt convert lt module convert from convert py gt gt gt gt convert convert string lt function convert at 0xb7c1b82c gt gt gt gt convert weightings 0 20000000000000001 0
262. iple lines of text line breaks and other formatting can be enclosed in triple quotes or even triple double quotes Single and double quoted lines can be continued on to the next line by placing a backslash at the end of the line CHAPTER 3 VARIABLES AND DATA TYPES Note The backslash escapes the new line character which means it tells the interpreter to ignore any special meaning that the next character might have All the unescaped characters after that are treated normally A value created using text in quotes will be given the type str string Nesting Quotes Sometimes you will want to include literal quotation marks within your text It is possible to nest quotes That is have one set of quotation marks inside another so long as you use a different sort of quotation mark like this gt gt gt text She quoted the rules at him In this instance the interpreter will assume that it has reached the end of the text when it hits the second double quote so the substring the rules is considered to be part of the main string TA the single quotes This way you can only have one level of nested quotes Inside triple quotes or it is possible to use both normal single and double quotes without confusing things the interpreter will wait until the second set of triple quotes before it decides that the string has come to an end The best way to understand how they work is to experiment with assigning
263. irate Note we can use the dot notation to import it directly import pirate strings CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES gt gt gt dir pirate builtins doc file name package path convert strings Importing from packages already has a lot of flexibility For example using dot notation you can import pirate strings and not worry about what s in pirate convert But with an entirely empty init py importing submodules is hard work they won t be imported implicitly and from pirate import does nothing There are two main ways that you can make importing easier by running import statements in your init py and by defining alist variablein init pycalled all which is interpreted as matching the import wildcard form These are demonstrated by the sample content for init pyin Listing 11 5 and the examples at the interactive prompt that follow it Listing 11 5 Sample __init__ py Content Pirate speak package Import both modules when this package is imported import convert strings Just import convert with the wildcard form all convert Let s try it out gt gt gt import pirate gt gt gt dir pirate all builtins doc file name package path convert strings gt gt gt from pirate import gt gt gt convert lt module convert from convert py gt gt gt gt strings Traceback most rec
264. ironments 5 prompt 19 24 defined 6 properties 196 of raised exceptions accessing 229 230 pseudocode 17 25 defined 10 style checker script 147 text conversion script 145 punctuation 139 variable 73 pure function 108 defined 124 pydoc module 166 PyGTK 282 commands 272 279 complex interfaces 268 269 GUI 269 271 overview 265 267 signal response 267 268 Python 1 7 coding with IDEs 5 with text editors 5 help files creating 6 interactive shell 4 interpreter 4 learning while having fun 4 overview 1 running on miscellaneous systems 2 overview 1 2 on Windows 2 software development cycle 5 terminology 6 7 versions of 2 3 pythonic defined 239 Python Package Index PyPI 255 defined 260 PYTHONPATH 250 environment 260 Q question mark 2 136 questionnaires 12 queue 82 defined 99 INDEX quit function 4 quotation marks 15 quotes joining text fragments with 32 33 nesting 33 R Radiobutton widget Tkinter 264 raised exceptions 222 defined 240 random module 92 random randint function 108 random randint start end method 92 raw_input function 20 readable code 111 113 read_in function 157 reading from files 142 143 readline method 143 read r mode 142 re compile pattern flags mmethod 139 recursion 13 re escape pattern method 139 refactoring defined 124 refactoring rpcombat py to reduce repetition matrix refactored 113 123
265. ising up Here s a summary of the full try statements behavior in more or less plain English 1 Try some potentially dangerous code 2 Ifan exception has been raised look for an except clause that matches a The exception s class b Some superclass of the exception 3 Ifno exception is raised run the code in the else clause gt Finally always run any clean up code 5 Ifno except clauses have handled the exception raise it even further in the program until it s handled or it reaches the user The fifth step is core Python behavior for unhandled exceptions inside or outside a try clause that s why it s in italics It corresponds to the very last line of the output for Listing 10 12 With these four clauses try except else and finally plus the concept of unhandled exceptions rising up through the program Python s exception handling provides complex flexible and above all useful methods of flow control Along with handling errors the try statement is also similar to the standard if else flow control This means you can use exception handling in Python with as much confidence as any other flow control methods in fact you are actively encouraged to do so for reasons which we ll discuss at the end of this chapter Using the Traceback If All Else Fails As you ve already seen unhandled exceptions generate a report when they reach you or another user This report is called a traceback and it gives detai
266. ist and returns a weighted semi random integer representing the velocity of the strike attack speed layers attacker Speed weapon speed layers attacker weapon 2 attack chance roll players attacker Brainz attack velocity attack speed weapon speed attack chance target prot players target prot armour speed players target armour 2 target velocity target prot armour speed velocity attack velocity target velocity 2 return velocity TU calc damage attacker target velocity Calculate the damage of the hit Takes three arguments attacker and target are integer pointers to the players list velocity is an integer representing the velocity of the strike Returns a tuple of two integers damage and potential damage players attacker Muscle players attacker weapon 1 attack strength weapon damage velocity weapon damage attack strength attack damage 119 120 CHAPTER 6 FUNCTIONS target strength players target Muscle armour strength players target armour 1 target chance roll players target Brainz target defence target strength armour strength target chance potential damage attack damage target defence if potential damage lt 1 potential damage 2 damage random randint 1 potential damage return damage potential damage The main body of the program starts here So this is where the flow of execution begins
267. joined Apress as an editor in the Open Source group He currently splits his time between editing books and coding for mySociety a charitable organization that runs most of the UK s best known democracy web sites Duncan lives in Oxford England with his wife Ruth When away from his computer Duncan enjoys playing the guitar very badly cycling long distances on a Brompton folding bicycle and fiddling with old cars His friends and colleagues have been known to run sweepstakes on how far these vehicles can get without the assistance of a breakdown service xi CHAPTER 1 Introducing Python So you want to learn programming Welcome to one of the great adventures of the twenty first century Programming requires little in the way of specialized equipment the software tools can all be downloaded for free off the Internet and it can be practiced in the safety and comfort of your own home without having to ask anyone s permission This chapter will ease you in gently by introducing you to the software you will need to create your programs a command line interface which allows you to use Python in interactive mode and a text editor for writing scripts nothing more complicated than that I will also show you where to go to find help and documentation so you can decode the sometimes impenetrable jargon that seems to surround this the geekiest of all technical disciplines To begin with you will need to make sure that you have a decent
268. ks that are far more easily cleaned up locally even if the primary exception needs to be handled at a higher level An example is closing a file that you ve opened in the current method Your current method might want to write a log to a file If an exception is raised while the file is still open only that method has direct access to the file object in order to close it Leaving the file open could cause problems much later in your program should it try to access that potentially locked file It s best to be certain that the file object will be closed whether or not an exception is raised and whether or not it is handled 231 232 CHAPTER 10 EXCEPTIONS To ensure that such operations are always carried out you can use the finally clause If an exception is raised but not handled and a finally clause exists in the try statement the exception is put to one side temporarily the finally clause is executed and the exception is raised by Python as usual If there s no exception or the exception is handled the finally clause is still executed You can see an example of this in Listing 10 10 Run this code in the interactive prompt or from a file Listing 10 10 Doing Basic Clean Up with finally Without Handling the Error Locally def log data x y Logs some incoming data plus their division Try logging some data to an open file try f open finally log txt a f write O g 1 g 2 g n format x y x y I
269. l cover Python s module system so that you can apply it to your own programs We ll start with a simple module before seeing typical module usage This will lead us onto packages which are collections of modules and some standard Python modules that you can make use of Understanding Python Modules Before we begin you should understand that any Python file can be a module A module is just another Python file It can include methods classes references to other modules and even executing code When you bring the module into your program the methods classes and modules are made available in your current namespace 241 242 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Whether or not Python can find modules and packages depends on their relative location to your current file as you ll see later So let s start this chapter s tutorials by creating a separate directory for all the content call it my modules for now Creating Your First Module Let s create a fairly simple module with a number of straightforward methods Often one of programmers first modules will contain utility methods that they find themselves writing again and again to perform tasks like advanced string conversion so we ll build a module of string methods Imagine that you wanted to alter some text the content of your outgoing e mails perhaps or your next blog post so that it sounded like it was written by a pirate This might sound odd but
270. lag parts of the code that need attention These comments will only ever be seen by people who are actually reading the code If you are writing software as part of a team comments are a great way of communicating your thoughts and intentions to the other programmers Good code can often contain more comments than actual lines of code don t be tempted to remove them once you ve coded the design In a month s time you are likely to have completely forgotten what this script was intended to do and you will be thankful for these little reminders Text strings Text strings are delimited by quotation marks Delimited means that the text is enclosed between a matching pair of the specified characters Python uses a variety of forms of quotation marks principally single and double quotation marks There is a subtle difference between the two forms which I ll cover in detail in the section on text strings in Chapter 3 The main reason for the different forms is to allow nesting of quotes Nesting means putting one inside the other like this What on earth does nested delimiters mean she asked Like comments quotation marks only work if the text is all on one line Fortunately there are a variety of ways to get the interpreter to ignore line breaks In this example I shall use the 15 16 CHAPTER 2 DESIGNING SOFTWARE technique of triple quoting Triple quotes can be either single or double and they allow
271. lass personi setName Mr Smith set name print person1 getName get name Now that program doesn t do too much but it does explain via program code what we outlined at the start of the chapter To move along try the next example of a Player class which does a bit more If you are curious about self hang on as that will be explained shortly class Player def setName self name self name name title def getName self return self name def setDesc self desc self desc desc capitalize def getDesc self return self desc CHAPTER 9 CLASSES This code just creates the possibility of a type of object called Player You need to create a Player object and assign some values to it in order to use it player1 Player player1 setName inkon player1 setDesc short stocky and mean character sheet Name 0 s Desc 1 s format player1 getName player1 getDesc print character sheet Tip recommend using short singular capitalized nouns to name your classes This code produces the following results Name Inkon Desc Short stocky and mean First player1 is created as an instance of Player Notice the empty parentheses following the word Player This causes the class definition to produce an object that you can use I have given player1a name using the setName method I defined earlier and created a description string using the setDesc method The values ca
272. le called expected gains py you will need to edit it shortly and run it Listing 10 4 Raising an Exception During Conversion Where the Input Cannot Be Converted Cut and pasted from a statistics program def is odds on percentage Determine whether a percentage probability is likely or not return percentage gt 50 Cut and pasted from a financial program def expected gains amount bet 10 amount to win 10 frac 0 5 For a given fractional chance of winning and an amount bet return the most likely gains return amount to win frac amount bet Conversion methods in both directions def prob to perc pr Convert a probability to a percentage return pr 100 def perc to prob pc Convert a percentage to a probability but raise exception if too big if pc gt 100 raise ValueError Percentage 0 g does not give a valid probability format pc return pc 100 Execute this if the program is run directly see Chapter 8 if name main Some input data data CHAPTER 10 EXCEPTIONS Percentage chance of winning amount required to bet amount you ll win 23 52 274 75 43 60 48 118 71 111 144 159 Misprint in the percentage data Work out how much we d win if we only placed likely bets total 0 for bet in data if is odds on bet o total expected gains bet 1 bet 2 perc to prob bet 0 print Total gains 0 g format total
273. levant fields for day month year and so on If strptime can t parse the date using the format specified it raises a value error exception Chapter 10 explained how to use exceptions to your advantage in this case you could specify a different date format and try again 257 258 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES The datetime module should do much of your heavy lifting if you need to work with time related input or output Date and time objects can be created manipulated and time shifted with timedelta objects Methods like strptime can parse many different formats of timestamps If you want even more advanced date and time handling you might want to look at the contributed mxDateTime module available on the Web http www egenix com products python mxBase mxDateTime Beautiful Soup and urllib Fetching and understanding HTML is a common problem for web enabled programs Sometimes you ll be lucky and be able to retrieve data in a structured format like rich document format RDF XML or from spreadsheets or even a database but often data will be spread over multiple web pages some of them with poor or invalid markup Beautiful Soup is an intelligent HTML parser which can take markup of quite variable quality and turn it into an object oriented Python interface When provided with some HTML not necessarily completely valid HTML or even a full HTML document Beautiful Soup turns it into an internal Python
274. lf scrolledwindow2 show self hbox1 pack start self scrolledwindow2 True True 0 self hbox1 show self vbox1 pack start self hbox1 True True 0 Create QUIT button self button2 gtk Button Quit self button2 connect clicked self delete event the Quit button self vbox1 pack start self button2 False False 0 self button2 show CHAPTER 12 SIMPLE WINDOWED APPLICATIONS self vbox1 show self window show def create treelist self create treelist Create list of options Add some messages to the window Omitted for clarity return def make pixbuf self tvcolumn cell model tier make pixbuf Create icons for TreeView menu Omitted for clarity return The get selection method catches selections in the options list and displays the system information that I was after it is called by on treeview1 cursor changed The selection itself is retrieved by treeview1 get selection This selection is then dropped into a command string which is evaluated using eval Finally the output is prepared for sending to the textbuffer in much the same way as before def get selection self get selection Creates text appropriate to choices get selection from listview self choice self treeview1 get selection self choice set mode gtk SELECTION SINGLE self model self row reference self choice get selected self choice self liststore get value self row reference 0
275. ll users be running it on What Do You Want the Software to Do Open a text editor and write out what the problem is that you want to solve and very briefly what you actually want the software to do A simple text editor will do for this task see Chapter 1 It s best to start with some simple jobs that you already know computers can do like printing out a daft message adding up some numbers or updating the prices of items in a catalog Classically half the battle is identifying the problem and once you have done so the solution will begin to become clear Save the file as plain text with the txt extension and give it the title of your program to be The first program you re going to write later in this chapter will be called Hello World It will be designed to solve the problem of getting your computer to print out a friendly affirmative or daft message when you ask it to so you may as well save this as hello world txt Tip You might want to create a new directory to store all your Python experiments Who Will Be Using It Next you need to specify who will be using the application even if you re just doing it for your own personal gratification and no one is ever going to see the output Write this down What other people will be using the program e Friends e Relatives e Work mates e Registered users e Anonymous clients that you re never likely to meet 11 12 CHAPTER 2 DESIGNING SOFTWARE How mu
276. lphabetical ordering If you want to compare strings according to their lengths you would need to use the built in len function 51 52 CHAPTER 4 MAKING CHOICES gt gt gt len alpha lt len omega False gt gt gt len case len Case True gt gt gt len net lt len network True gt gt gt len Same len Same False Manipulating Boolean Values Before moving on to conditional structures no discussion of fundamental data types would be complete without covering the manipulation of True and False values recall from Chapter 3 that these are called Boolean values This used to be the subject with which computer students began their education and as it is essential to understanding the way computers work I won t skip the details While I m sorting things I may as well organize my audio and video collection I started off by creating a bunch of Boolean values to represent various bits of information about my collection Here I set the values of a particular CD gt gt gt is emo True gt gt gt is country False gt gt gt is techno False gt gt gt is CD True gt gt gt is DVD False This is a task in which I may need to make several comparisons at once I might want to be notified if the album is a particular genre such as emo or country gt gt gt is emo or is country True The or operator will return True if either variable is true If I wanted to narrow my se
277. lr name capitalize It looks like a building site nothing to see else self dict store CHAPTER 9 E CLASSES def str self rpcs item name for item in self inv if issubclass type item Player stuff str item for item in self inv if issubclass type item Thing view lt name s desc s Contents o s Players ils lt 0 format join with and stuff join with and rpcs vars self return view def repr self Output class constructor string stats Location 0 s format vars self return stats def contains self item in checks contents of inventory Can match against strings or Things ie GameObjects objects str i for i in self inv if item in self inv return True elif item in objects return True else return False def interpret self command player Game Command interpreter Takes two arguments command the command string player a player object Executes the command string by retrieving string from self commands formatting it and sending to exec It returns no value Note There are probably more secure ways of doing this here self me player command list command split if command and command list 0 in self commands command self commands command list o if len command list gt 1 command command format join command list 1 target player else 211 CHAPT
278. ls Docstrings are expected to follow convention The first line of a docstring should be a short description of the function that makes sense by itself and fits on only one line usually a maximum of 79 characters including any indentation The next line should be left blank After that the body of the docstring should contain a description of the function s parameters a longer description of the function notes about the algorithm used and an example of how to use the function that includes details of optional arguments keyword arguments and its return values You might also want to include information about possible side effects exceptions and restrictions on when the function can be called these matters will be covered later in this book In short all the information that another programmer would need to be able to use the function It is very important to update your docstrings and comments every time the code changes CHAPTER 6 FUNCTIONS Working with Variable Scope One way of conceptualizing functions is to think of them as black boxes that take in source data process it and pass it back The code in the main program that sends the source data and receives the result is known as the calling procedure It doesn t need to know what is inside the black box so long as the source data and results are clearly identified Understanding Scope When you write a function you should not have to worry about using names that will cl
279. ls of the files and the locations within them where an exception even if it s been partially handled has in some way led to the program coming to a halt CHAPTER 10 EXCEPTIONS Look back at our very first code example in this chapter involving a simple division by zero at the interactive prompt This is a single stage traceback It begins by announcing itself as a traceback and then gives a location in line 1 of lt stdin gt i e your typing or standard input It also specifies any method that the exception is raised in in this case it was raised outside of any method and so is considered to have been raised in lt module gt For now treat this as a special signifier for not being inside any method Let s generate a more complicated traceback Save the code in Listing 10 13 to a file called traceback py and run it Listing 10 13 Demonstrating a Complicated Traceback def f1 2 def 2 3 def 3 foo This will raise a NameError deep in f3 f1 Calling this simple function will produce the following traceback Traceback most recent call last File traceback py line 6 in lt module gt f1 File traceback py line 1 in f1 def f1 f2 File traceback py line 2 in f2 def f2 f3 File traceback py line 3 in f3 def f3 foo NameError global name foo is not defined As you can see the traceback provides you with a complete explanation of the stack of methods calli
280. lt which will be collected by the main program The other way is to let the function directly access and process the global data but this second way is discouraged because the function works with only particular variable names and cannot be reused anywhere else Listing 6 1 shows a simple example of this behavior Listing 6 1 dave py usr bin env python Dave the cardboard box A simple scope example fred 1 pete 2 def cardboard box dave fred pete return vars 105 106 CHAPTER 6 FUNCTIONS print Outside the box print vars print Inside the box print cardboard box This program gives the following output Outside the box pete 2 builtins lt module builtin built in gt file test py fred 1 cardboard box lt function cardboard box at oxb7ddf224 gt name main doc Dave the cardboard box n nA simple scope example n Inside the box dave 3 You can see that although fred and pete are global citizens and dave can borrow freely from them dave is the only local and does not exist in the global scope If I try to access dave outside the cardboard box function I get NameError name dave is not defined If dave was defined alongside fred and pete in the outer scope the opposite problem could occur If I tried to access dave from inside the cardboard box I would only be able to access the value set inside the cardboard b
281. lter but can also lead to the construction of unwieldy one liners if you include too many additional for loops and if statements The most basic construction is expression for var in list for if This means that you can have multiple for and if statements after the initial construction If the expression part is a tuple this is one occasion where it must be in parentheses gt gt gt fruit upper for fruit in fruits AVOCADOS BANANAS ORANGES GRAPES MANGOS Let s look at two for statements Here we extract each integer and pair it with each letter gt gt gt int list 0 1 gt gt gt char list a b c gt gt gt an int a char for an int in int list for a char in char list 0 a 0 b 0 c 1 a 4 b 4 c Note that an int a char is the pairing expression that operates on the results of the two for expressions List comprehensions are cool and very powerful The expression part can be any expression you can code and you can also make the evaluation of that expression conditional If the matrix were modified to include different values it would be possible to retrieve only the changed values using a list comprehension Note the way data is assigned to the cells of a nested list CHAPTER 5 M USING LISTS gt gt gt matrix data data data data data data a 3 data data data gt gt gt DE 1 2 o
282. ltiple superclasses Note that self is not included in the arguments to init CHAPTER 9 E CLASSES class Weapon Thing def init self price strength speed super init price strength speed self damage strength Now I can make Weapons with a single instruction too axe Weapon 75 60 50 Customizing Output In a similar way it is possible to automate a class s output format The str object and print object functions both look for a special method called object str self to compute the informal string representation of an object This method must return a string object Thing printValues self could easily be converted for this purpose def str self stats tPrice tDamage tSpeed price s t damage s t speed s t format vars self return stats I am using the vars function to send the Thing object s internal dictionary of values to the string format method The official string representation of an object is created by object repr self which is called by the built in repr function This should look like a valid Python expression that could be used to re create an object with the same value if this is not possible a string containing some useful value should be returned The return value in either case must be a string object Ifno str method is provided by the class repr may also be used for the informal representation def repr self stats Weapon price s strength s
283. lues self we print Price 0 s Damage 1 s Speed 2 s format self price self damage self speed gt gt gt axe Weapon gt gt gt axe setValues 75 60 50 gt gt gt axe printValues Price 75 Damage 60 Speed 50 gt gt gt box Thing gt gt gt box setValues 0 0 0 gt gt gt box printValues Price 0 Strength 0 Speed 0 This is a very rough example The principle is that all Things have setValues and printValues methods It doesn t matter what subclass of Thing it is but if we want to know if a Thing is some kind of weapon we have a simple test does it have a damage attribute gt gt gt box damage Traceback most recent call last File lt stdin gt line 1 in lt module gt AttributeError Thing object has no attribute damage gt gt gt axe damage 60 Clearly the axe can be used as a weapon and the box cannot This is similar to the way strings lists and dictionaries share some common methods but not others If you want to know if you re dealing with a list or not for example try to append something to it I could subclass Player in a similar way to allow for different professions such as Warrior Magician Priest or Hoodie they might share some methods but vary in their methods of combat and the weapons they can use Customizing Classes Classes also support some special methods known as magic methods which are similar to the magic vari
284. ly recent version of Python installed on your machine or follow the steps later in this chapter to install it see Choosing the Right Python Version for a definition of decently recent This chapter explains how to make sure that you have everything set up correctly and that you have suitable references at hand before you start your journey Python is an excellent language with which to learn programming There are many reasons for this but the simple explanation is that it s easy to read and fast to write it doesn t take long to come up with working code that does something meaningful Python has a very human friendly syntax which makes writing elegant code easy The basic language is fairly simple and therefore easy to remember and then it has an extensive library of predefined functions that you can use to facilitate most common computer tasks Writing effective applications in Python can be as simple as playing with conceptual building blocks It works really well for writing a little two line application to perform some routine system administration task or to provide interactive functions on a web page but it has enough power and flexibility to comfortably create much larger and more complex applications with graphic interfaces indistinguishable from the programs you are used to running from your computer s main menu If you follow the suggestions laid out in this book about writing self explanatory code in several months even years
285. m and AMD machines are available from http www python org download You should save this file which will have a msi extension somewhere you ll be able to find again easily You can then double click this file to start the Python installation wizard which will take you through the installation It s fine to accept the default settings if you re not sure of any answer Installing on Other Systems You may choose to install Python on other systems particularly if you want to take advantage of newer versions For Linux and other Unix like systems the installation instructions are here e http docs python org dev 3 0 using unix html If you re using OS X your instructions are here e http www python org download mac e http docs python org dev 3 0 using mac html Choosing the Right Python Version You will find the different installers include a number after the word Python This is the version number When I started writing this book those numbers ranged from 2 3 7 old but still usable through 2 5 2 the previous stable version to 3 0 the new version about to be released At the same time as version 3 0 was released the Python team also put out version 2 6 which is an upgraded version of Python version 2 for people who want or need to stick with the old way of doing things but still want to benefit from general fixes and some of the new features introduced in version 3 0 The Python language is continuously evol
286. mall insult a flesh wound a deep gouge ragged tearing of skin blood to spurt everywhere 202 CHAPTER 9 E CLASSES massive head injuries savage laceration a fractured rib cage facial mutilation the skull to split in two life changing a scar internal bruising serious blood loss other debilitating injuries chronic concussion leaving a severed limb dangling at a horrible angle multiple fractures a broken neck the rupturing of internal organs a decisive mortal blow class GameObject Generic Game Object One class to rule them all The class sets up the properties which are available to all GameObject sub classes It has no methods of its own The idea is that the other game objects Players Things and Locations are all sub classes of GameObject features shared by all GameObjects should go here Text properties name desc gender and race Base stats strength brainz speed and charm Derived stats life magic prot and gold Plus an inventory list def setName self name self name name title def getName self return self name name property getName setName def setDesc self desc self desc desc capitalize def getDesc self return self desc desc property getDesc setDesc def setGender self gender gender gender lower if gender startswith f self gender female elif gender star
287. mas if there is more than one of them Again text strings have to be enclosed in quotes or else the interpreter will treat the string as if itis a variable Functions often return a value of some kind so it is usual to catch this value by assigning it to a variable like this variable function Don t forget the brackets after the function name Functions are covered in much greater detail in Chapter 6 19 20 CHAPTER 2 DESIGNING SOFTWARE Your final script should look something like Listing 2 6 Listing 2 6 A Refined hello_world py Problem Get the computer to output a message Target Users Me Target System GNU Linux Interface Command line Functional Requirements Print out a message User must be able to input some text Testing Simple run test expecting a message to appear expecting message input text Maintainer maintainer website com 1 Print out a friendly message print Hello World 2 Input some text some text input Type in some words 3 Print out the text we just entered print some text Note The behavior of the input function has changed in Python 3 You need to use the raw input function to get the same effect in older versions Testing the Design Now it s time to save the file again and test it by running the script with the Python interpreter as before If you get any error messages at this stage you probably made a mistake typing the script Common
288. me more bread and cheese once again she returned home happy and contented Her step mother began to get suspicious so the next day she sent her out as before but this time sent a man along to watch her He saw the little black ewe feed the girl with bread and cheese and reported this back to the step mother Then the step mother said to her husband that she would like some lamb for dinner and so had the little black lamb killed and carved up for the pot As plan A had failed Cinderella s stepmother reverted to her original plan and set her to work in the kitchen The sisters treated her worse than before telling her that all she was fit for was to scour the pans clean the knives and clean out the grates In between times they would hit her over the head with the skimmer pop pop pop telling her That s what you deserve and that s all you can expect my lady So that was how it was She was made to do all the dirtiest work and soon she was up to her ears in grease and her face was as black as soot Every so often one or other of the sisters would hit her over the head with the skimmer pop pop pop until her head was sore with bruises The next day Cinderella spent all day crying her little eyes out a funny little old woman came to the kitchen door begging for alms and asked her what was the matter When the little girl told her all about what had happened to the little black lamb the old lady told her she was not to cry but to go and
289. ment which checks to see if the module is being run with name main Otherwise the code in the if else block would be run during module import as well Since execution in Python can mean following some control flow and sending output to the user or it can mean defining new classes it s all the same to the interpreter it s important to remember that no magic is at work in modules no magic in the name main block none in weak internal variables and none in class or method definitions It s not magic it s just Python 251 252 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Combining Modules into Packages Before we can make a package out of our modules we have to see what a package is so let s do that now Understanding Packages One way of looking at a package is as a hierarchical directory structure containing potentially very many py files with similar files found in the same place Another is as a convenient way of preventing the names of many different modules from conflicting both on disk and also when imported The two are equivalent For example if you wanted to distribute your pirate speak code you would first of all want to put strings py and convert py in the same directory purely for convenience you would also want to avoid subjecting users of your code to problems from two files sharing the same name Someone might have a string formatting module called strings or might do a lot of automatic
290. module in the previous section the methods were still attached to the strings module that is we run them by prefacing each method s name with strings In Listing 11 2 all the methods are added to the current program s scope rather than only adding the strings object to the scope and keeping the methods inside it Save this code as convert py in the same directory as strings py Listing 11 2 Importing from the Module in Listing 11 1 Module to utilize existing string conversion functions on lots of text Import the contents of our strings module from the local directory from strings import Import the Python random module from the standard library import random Probability of calling yarr and me hearties functions weightings 0 2 0 3 def random pirate stub Randomly turn a stub into a piratified sentence choice random random if choice lt weightings 0 return yarr stub elif choice lt weightings 0 weightings 1 return me hearties stub return stub def to stubs text Convert a string into stub sentences Piratify first to make every sentence an exclamation text piratify text Now split discarding any empty strings return s for s in sen strip for sen in text split if s 243 244 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES def convert string content Convert a string to pirate speak return join random pirate txt for txt in
291. n ring scroll wand gt gt gt inventory sort Reversing the order of your list can be done using the list reverse method which reverses the original list gt gt gt reversed inventory lt listreverseiterator object at Ox81abfoc gt gt gt gt inventory reverse gt gt gt inventory wand scroll ring potion emerald 10 gold coins CHAPTER 5 M USING LISTS Many sequence manipulation functions rather than returning another sequence return another object in the above example an iterator object is returned This object can be used in a for loop just the same as a sequence The reversed function returns a new reversed list iterator object gt gt gt for item in reversed inventory print item wand scroll ving potion emerald 10 gold coins Multidimensional Lists Lists can be multidimensional that is to say nested inside other lists Lists containing other lists can be used as a way of storing a matrix or table of information A 3x3 matrix could be created by assigning a list of lists to a variable gt gt gt matrix 11 12 13 21 22 23 31 32 33 Processing Large Amounts of Data Easily It is common practice to iterate through a list using a for loop This construction for var in list is known as a list traversal It is appropriate to use a for loop because even if you don t know how long your list is your computer sure does using the bu
292. n Python Common Gateway Interface CGI scripts The simplest way to find out if your server supports Python is to upload a Python script to your cgi directory and see if it works In the top level directory of your web site shown in Figure 8 2 you will probably have an html folder where the pages of your web site reside along with a cgi folder which may well still be empty If the directory isn t this easy to spot you will need to ask your hosting provider for the location of your cgi directory 173 174 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB mainwebsite_cgi 4 096 mainwebsite_html 4 096 G stuff 4 096 f bash_logout 24 3 bash_profile 230 bashrc 124 3 skins ini 25 Figure 8 2 Web site top level folder Copy Listing 8 3 into a text file and save it as hello cgi Upload this file to your web site s cgi directory Make sure the web site copy of hello cgi has the correct permissions set as I showed you in the myscript py example All users probably need to be able to read and execute the file but only you should be able to write to it It may be possible to limit permissions further as often the web server has a dummy user set up who is used to run programs You will need to check all these details independently as they will vary from situation to situation Listing 8 3 hello cgi usr bin env python coding UTF8 Simple CGI script import time now time strft
293. n a time it wasn t your time nor my time pages more text here and if they live happy so may you and me n If you are dealing with a large body of text like the following example you may wish to deal with it in smaller pieces file readline size reads a single line from the file up to the next newline character n The size argument is the maximum number of bytes to read including the trailing newline and an incomplete line may be returned An empty string is returned only when the end of the file is encountered immediately gt gt gt text open story txt CHAPTER 7 WORKING WITH TEXT gt gt gt text readline Once upon a time it wasn t your time nor my time but a very long time ago there lived a gentleman and a lady in a very beautiful part of the country They had only one daughter who was very pretty and good and both her parents were very fond of her When the girl was about five years old her mother died The father was heartbroken over the loss of his wife and left the little girl pretty much to herself She cried endlessly because she did not understand where her mother had gone or why she did not come to her n Files are their own iterators so you can also iterate through lines in a file using a for loop Each iteration returns the same result as file readline and the loop ends when the readline method returns an empty string gt gt gt for line in text print line I ll let y
294. n another script which I had prepared with a few little TODOs of its own fix style py Python script to be checked gt rpcombat py Looks like a Python file 0K TODO Warning wrong interpreter invoked Warning no text encoding declaration 16 10 2008 TODO rpcombat py line 17 Fix up interpreter call and file encoding 16 10 2008 TODO rpcombat py line 192 rewrite the character sheet output using string format 16 10 2008 TODO rpcombat py line 217 shop stock list format 16 10 2008 TODO rpcombat py line 291 Alternate option to read characters from file 153 154 CHAPTER 7 WORKING WITH TEXT 16 10 2008 TODO rpcombat py line 332 save rpcharacters to file Application Details File rpcombat py Version 0 2 Prototype Last Modified 16 10 2008 t AUTHORS Author maintainer website com Maintainer maintainer website com Credits Having a list of TODOs like this is rather useful I can now work through my TODO list item by item This saves constantly trawling through the file to remind myself of what I intended to fix next I m going to ignore the first item for now as that involves explaining how to make scripts executable on their own which TIl get into in the next chapter but we could take care of some other items now Formatting Data On line 192 of rpcombat py I have a note that
295. n be retrieved using the accessor methods getName and getDesc Accessor methods are a means of accessing attributes and are named after the attribute by convention with get set or del prepended to the method name So far my Player type seems to work a bit like a dictionary except that the values become automatically capitalized when they are created Later on in this chapter I ll show you how you can make your new type behave exactly like a dictionary if you so wish Who Is self The main difference between methods and functions is this additional parameter called self self refers back to the object upon which the operation is taking place so in this instance self refers to player1 The name of the object is passed to any methods called on it along with any additional arguments It is also used in the class definition where values are assigned to attributes Attributes are like variables contained within the object such as self name These attributes can be accessed from outside the object as in this instance player1 name In most cases however you don t want to access them like this because you will usually want to process or format the data using a class method like setName or getDesc It is good practice to always use methods to access attributes see the Properties section later in this chapter The idea is that an object should be like a black box you don t need to know what the box contains as long as you know or can find
296. n hashes gt gt gt profile keys Brainz Name Gender Race Charm Muscle Speed Desc gt gt gt profile values 0 Adam y TS 0 0 0 gt gt gt profile items Brainz 0 Name Adam Gender Race Charm 0 Muscle 0 Speed 0 Desc 87 88 CHAPTER 5 M USING LISTS Note These three methods return views instead of lists Views can be iterated over and support membership tests in much the same way as lists Dictionaries can also be created using the built in dict function Without arguments the function will create an empty dictionary Also the function will accept arguments to create dictionaries out of other iterable entities such as lists keyword arguments or other dictionaries It is also possible to create a blank copy of a dictionary using the dict fromkeys iterable value None method gt gt gt new profile profile fromkeys profile gt gt gt new profile Brainz None Name None Gender None Race None Charm None Muscle None Speed None Desc None gt gt gt profile Brainz 0 Name Adam Gender Speed 0 Desc Race Charm 0 Muscle 0 Dictionaries also respond to the built in functions len max and min gt gt gt max profile Speed gt gt gt min profile Brainz gt gt gt len profile Deleting Items To remove an indiv
297. n in profile inventory profile weapon stock weapon else profile weapon 0 20 50 See if player has any armour profile armour 0 0 50 for armour type in armour types if armour type in profile inventory profile armour stock armour type print profile Name is now ready for battle Add new player to list of players players append profile Combat print print Then let the combat begin print vel max 23 vel min 1 dam max 23 Loop while both players are still alive while players o life gt 0 and players 1 life gt 0 for attacker player in enumerate players target int not bool attacker life left players target life Calculate velocity of blow CHAPTER 5 M USING LISTS attack speed layers attacker Speed weapon speed layers attacker weapon 2 attack chance random randint 1 players attacker Brainz attack velocity attack speed weapon speed attack chance target prot players target prot armour speed players target armour 2 target velocity target prot armour speed velocity attack velocity target velocity 2 if trace print t velocity if velocity gt 0 if velocity gt vel max vel max velocity hit type int 7 velocity vel max if hit type gt 7 hit type 7 if trace print t tHit hit type print players attacker Name hits hit type players target Name TU else if
298. nary dictionary key value Don t forget to enclose the key name in quotes or the interpreter will treat it as if it is a variable Dictionaries can also be traversed using a for loop but the loop will iterate only through the keys CHAPTER 5 M USING LISTS gt gt gt for key in profile print key Brainz Name Gender Race Charm Muscle Speed Desc If you want the value use profile key as you have seen before However if you need to ensure that the key exists in the dictionary you can check using key in profile which returns True or False Otherwise Python will throw an error If you do not know whether the key exists already the best approach is to use the get key default method This method takes two arguments the key that you re looking for and a default value if that key is not found gt gt gt profile get Name Adam gt gt gt profile get Gender not known gt gt gt profile get Life not present not present Note that if the key is present but only contains an empty string you ll get the empty value returned rather than the default message gt gt gt for item in profile profile get item not present We can access the dictionary s contents as a whole using the keys values and items methods which return the keys the values and the key value pairs respectively You ll notice that the items are presented in an apparently random yet consistent order see the note o
299. nd you should be able to find it on the Web Python has an active and dedicated community of both core coders and end use programmers and it makes sense for you to take as much advantage of that as you can But once you ve felt the benefit of this community you might wish to consider giving back wherever possible through discussing testing and providing feedback on other people s projects and maybe even one day contributing code with your own programming skills Jargon Busting Here are the terms from this chapter e Bytecode This compiled equivalent of a textual Python script is turned into an internal computer readable representation and where possible stored on disk in a pyc file so that Python can omit the compile process in future e Contributed This term describes code that is available on the Web separately from the Python installation Contributed modules are maintained by the Python developer community 259 260 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES e Distutils This is the first of two standard methods of installing contributed Python software e Easy Install This is the second of two standard methods of installing contributed Python software e Egg This archived format for Python packages allows you to download and install them as a single file for simplicity e Import When you import you put the objects from a module into the current scope so they can be accessed by other objects in that s
300. nditional expression are often expendable entities which are only required for as long as the loop is active Rather than keep thinking up different names this kind of integer counter is usually named i or j by convention Caution Be very careful about reusing variable names It is often better to come up with a unique and more explanatory name 65 66 CHAPTER 4 E MAKING CHOICES Here is what this simple program prints oo P 16 32 64 128 256 512 Two things are important to remember in this sort of construction Any variable used in the conditional expression must be initialized before the execution of the loop Also there must be some way of updating this variable within the loop otherwise the loop will just go round and round forever which is called an infinite loop Caution Python won t warn you if you have created an infinite loop and you will have to find some way of killing the running program if you make this mistake In text mode this is as simple as pressing Ctrl C or Ctrl 2 but killing the program might not always be so simple You have been warned It is possible to use different sorts of variables in the conditional expression Let s consider the problem of calculating the average of several numbers input by the user The main problem here is that I don t know how many numbers will be input The solution is to use what is called a sentinel value to control the loop Rather than using the count
301. ndow manager You should create only one root widget for each program and it must be created before any other widgets The rest of the application has been gathered up into the HelloTk class As the class is constructed the window becomes populated with widgets The first item is a frame which will contain the two buttons The first argument is the container this widget is to be packed inside in this instance it s the root window The Tkinter Frame instance is stored in a local variable called frame Calling the pack method on frame tells it to size itself to fit the contents and then make itself visible A similar process is used to create the buttons except that Tkinter Button takes more arguments buttons need to have some text or an image to use as a label to indicate their usage and they also need a command to be run when they are clicked The first button runs the self say hi method and the second runs the frame quit method of the Tkinter Frame object which quits the application You can specify the direction of packing using the side option here the buttons are packed as far to the left as possible If this option is left out side defaults to Tkinter TOP Having set all that up we just need to launch the root window with all the widgets you just packed into it and to enter into the Tkinter event loop which handles events from the user and those queued by Tkinter itself The program will stay in the event loop until you close
302. nduaeduanceenduanduancks 170 Using exec and DS eP Peer e pene er ePaper rr creer errnrerT sbr rset rer pprr S gt nn rer seri sptrrtrr ent ery errr emrEnrenry 172 Putting Python on the Web 2 ovis152255 s0 52ccsscedsesnsensandsnssnnsdnsnesnedanspssodsuessesdesansenseeuecauentuae 173 Jargon Busting ssssesessssssesusnsusnsnunnsusnonnrnnununnnnunununnnnunununnnnununuonnnnnununnnnnnununnnnnnnnnnnnnnnnnn 179 SUMMAN ar prairie anias deas Speers AAAA as eaaa naaa A AAE Ra Aade Aae S aade renters rere tr 179 Chapter 9 ClaSS S viva cssscsisccsassasessnssenentistsnnsatsesssviscesnssaisecasusveiuesentsnssesassancssisermncsnies 181 resi f 018 ss 41114010 21a tae ener eer eeeeeen en em rere eeeere rere rere ddaa ERANLA aiaa Edaini aeia 182 When Should Classes Be Used c csscsscscsscsecsecsecsessecesecsessessesassaesaesaesaesaesassaseasons 185 Gea oe Co ee ee 191 PAESPANG AI OU E TTT TT 200 Jargon BUSTING icccccecccccccccccccsceeccsccsccsceecenccseesenecenceatesceateaseatesceatescsstescnstencestesenecenceatescnans 219 SUMMAIV a ee er ee ee re ey ee eee 220 Chapter 10 ExCe pul vissisccssssssscsntsesesessstastccstesnssssstsnscatesasessssacsenivesessassessestivasusesivn 221 When Something Goes Wrong cscscssssssecsessecsessessessesaesaesaesaesaesaesassaesassassaseaseatons 221 Glass6S 0f EXCEDHON S es re ea 224 A Final Note on Pythonic Exception Handling ccsssssssssessessesessesessesesaesaeeasens 238 dargon BUSTING 02sec
303. ne gt gt gt matrix 1 1 two gt gt gt matrix 2 2 three gt gt gt matrix one data data data two data data data three gt gt gt value for row in matrix for value in row if value is not data one two three The for statements within the list comprehension have to come in the same order you would put them if you were using nested loops So for row in matrix comes before for value in row which gives a rather counter intuitive reading It is the resultant value of the left hand expression that becomes the item of data in the new list the for and if statements that follow it should run in the order they are to be processed If you get confused try to find ways you can reduce the level of nesting required Sets Sometimes you may need to use a data structure to test for membership For example we are processing a group of names how do we know if we ve already come across the same name This is where we can use sets using the if item in set statement Notice that creating a set automatically removes any duplicate entries Similarly it is also useful to find out what is common or different between two sets of items gt gt gt a set apples oranges bananas apples oranges gt gt gt b set avocados mangos apples grapes mangos gt gt a set apples oranges bananas gt gt gt b set avocados apples grapes mangos
304. need to be added to your script to be able to run it directly you will still need to have the correct version of Python installed in order for it to work If you wish to create a stand alone Python application for a Windows compatible computer you will need to obtain the py2exe compiler The instructions in this section apply to Unix compatible computers only First you need to add either of the following two lines to the very beginning of your script if you haven t already choose the one where your Python distribution is located usr bin env python or usr bin python This tells the operating system where to look for the Python executable The first version leaves it up to your operating system to decide which Python version to use the others directly point to the Python executable you want You can check the location of the executable with which For these examples I will assume you have access to a Bash prompt via the terminal There are some important variations in the way different operating systems or platforms are set up to run Python programs so you will probably need to read up on the specific details for your system 161 162 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB which python usr bin python If you have an alternative version of Python installed you might want to use the following instead usr bin python3 0 This way you can keep scripts around on your system that use di
305. ng a uniform interface e state A unique configuration of information in an object Summary You have covered most of Python s main coding constructs now and have started on the path of object oriented programming You have learned about concepts with brash new names like polymorphism inheritance and encapsulation You have grasped the basic usage of attributes methods and their related properties Now you are happily creating subclasses and overloading operators using magic methods If I have done my work well you will also understand the meanings of all these terms The last remaining constructions you really need to know are the ones involved in handling Errors and Exceptions You saw an example ofatry except construction already in Listing 9 2 Chapter 10 will fill in the details Beyond that the power of Python lies in its sprawling standard library The remaining chapters of this book will give you insight into the functions and classes contained in some of the more commonly used modules CHAPTER 10 Exceptions Things rarely go quite as expected particularly in the realm of programming An exception is what happens when Python encounters an error that would make it impossible or dangerous to continue executing the code As you have already seen dealing with errors is an integral part of the discipline of programming In this chapter you will be learning how to handle exceptions as and when they happen using try except st
306. ng methods from the final point where the exception reached you the programmer in the lt module gt right back to the original source of the exception in f3 A good rule of thumb if you re also using code from a module see Chapter 11 for more information on how to do this is to read a traceback as far as the last thing you wrote yourself and look there for any mistakes you might have made Programming errors can also occur in the standard library of course but they re quite rare You can also use the traceback module to extract tracebacks from errors while you handle them so that for example you can log the full traceback somewhere and then continue with your program We won t discuss that technique here but modules are discussed in the next chapter and Python s standard library of modules is documented in quite a lot of depth and with examples on the Python web site Exception Chaining and Tracebacks Tracing an unhandled exception can get more difficult if one exception is handled only for another to be invoked in the very try statement that s doing the handling The biggest worry from the debugger s perspective is that the original exception having been dealt with might be inaccessible However 235 236 CHAPTER 10 EXCEPTIONS tracebacks provide a way of establishing from one exception if it was raised in the context of handling a different exception this is called exception chaining If one exception is bein
307. ng of a value to be dynamically specified 133 134 CHAPTER 7 WORKING WITH TEXT Editing Strings Strings as you have probably gathered by now can t be edited in place but they do have some useful methods that will return a new edited version of the string You often need to clean up the beginning and end of the string to remove extraneous whitespace or punctuation especially if you re trying to compare some user input with a stored value This is done with the string strip chars method This returns a copy of the string with chars removed from the beginning and the end if the sequence is found If no arguments are given string strip will remove whitespace characters by default gt gt gt sent4 A much much longer sentence gt gt gt sent4 strip A much much longer sentence You can choose to only strip characters from the start of the string using string 1strip chars gt gt gt sent3 1strip e XamPLe tEXt or from the end of the string with string rstrip chars gt gt gt sent4 rstrip sentence A much much longer You can replace all occurrences of one substring with another using string replace old new count The optional argument count determines how many occurrences of the substring are replaced gt gt gt sent3 replace EX wis eXamPLe twist This last method is extremely useful Finding Strings Sometimes you might want to know whether a string
308. ng rpcombat py to Reduce Repetition scsssssessessesessseseseseeeeeeeees 108 Jargon BUSTING acc cscs ce cca cl ck eh eek saci abtacececectascaceshcctaschcesuesteatencedcesceatssctenesdeateacnnccadeateaceats 123 S mmMmaN BPE Ce aOR Cre rere EE rT SPP aaa aaa BOE PIT ESL aeai 124 Chapter 7 Working With TeXt sssssssscssssssessssssseessssssssnsseseeeeeesssssssnsseeeneeessssess 125 Strings and TUNES srciiiciicccssncseccssnsetusaansaduasinisasiassestastansanbeisaaisesasiitslatsdsuaiaiieserevesennensacats 125 Matching Patterns Using Regular ExpreSSiOns csssscsesscsesscssssessesssesseeseeesesees 135 JC eee ee 141 APDIICAUONS cerier E 145 Jargon BUStING ccccscescesceseseeseeseeseeseeseeseeseeseeseeseesessaesassaesaesassaesassassassassassaesaseassaseas 159 SUMMANY esecsccs seas esicaccdacecaxecactcaxacactcanadactcaneiactsanatastianaiactcaneiactiaeiiianeiaecaeiecciaeatcaeent 160 www allitebooks com CONTENTS Chapter 8 Executable Files Organization and Python on the Web 000sss 000 161 Making Programs Executable as Stand Alone Applications cscseseseseseeeees 161 Organizing Your Project ccccecssssessesesesseeseesesseeseesaesassaeseeseeseesassaesassassassansassateassass 164 Writing Stylish So tL eee eee een eer ee renee eer retre rer Tee eerr teeter rater tee rrer errr reer Tenet enn 165 Importing Modules asset enidecsceccdentdendsncctentdonnd nedtactiondd nedstendncedvan ceardkcaceandee
309. ng with objects mercifully simple you really need to consider only the methods you want the object to support When using your own custom classes you can create the necessary methods as you need them It is also possible to create special magic methods which allow you to use standard operators like lt and with your custom classes and you can replicate the behavior of other existing types such as lists and dictionaries allowing slice notation indexing and iteration So it is possible to get your custom objects to behave in similar ways to the built in objects you have already encountered This overloading of operators to work with many different forms of objects is known as polymorphism CHAPTER 9 E CLASSES Note Obviously if the method you want to use is not present you will get an error In Chapter 10 will show you how to catch and handle exceptions while the program is running Namespaces and Why We Need Them Each object has its own namespace which is implemented by a dictionary object This is important to prevent name collisions You may already have variables called name and desc in the body of the program for example but they won t conflict with player1 name or player1 desc Methods and attributes can be accessed by following the name of the object with a dot followed by the name of the item you wish to access Each name refers to an object Names are added to the namespace by an operation called name binding T
310. ning these phrases Each phrase in the set needs to be of a compatible grammatical structure mood and tense otherwise the final output won t sound right The preferences section is minimal most important is the trace variable which controls output of the trace table during execution Set this variable to True and you will get the workings out printed to the screen as you run the script I needed to set up an empty list to contain the character profiles it is called players The character profile is held in a temporary dictionary called profile 91 92 CHAPTER 5 M USING LISTS The first four fields are input by the user I have provided a very basic form of validation in cases of choices from a fixed list the script guesses the correct answer on the basis of the first letter and the other fields are capitalized The next four statistics are generated using the random module the method random randint start end will return a pseudorandom integer between the start and end of the specified range much like rolling polyhedral dice The combat stats are worked out from the random values returned and are checked to be within the range 1 to 100 if not the value is rerolled Finally this section prints out the completed character profile I moved the shopping section into the character generation loop for simplicity and the stock list is displayed using a for loop Notice how the price is extracted from the tuple values using an additional
311. nsseeesessaesseseaeees 55 Dealing with Logical ErrOrs ccccscscseseeseesecsessecsecsessessecaesaesaesaesaesaesaesassassaesaseaseaseass 57 Using Conditional Code in the Application cccsssssssssesessessessesesesesseeseeseeseeeaeees 61 Now Repeat That sooo cect dca ec na teciesee weeny cane seei ce recueeae cheese ceetsesicerecsansericseaseetsetuceessenseeeececs 65 Jargon OUGUINGet eee eer oe eee eee ea ene oan ena mete a 73 SUMMA a esaea nee ner reer ere nr aeaea AEL ALEE LEASES eaaa SSAA A Eae AAA aS AERALA LEEA 74 Chapter 5 Using LS Ss esscescetce sens saterics cacececessnncene cd caasus sausreckoivssssusauseuisetcessdesstssewestcreus 75 Working with Python Data StruCtures ccccsssssssessessessssesseessesseesseeseesseeseesessanees 75 a E E E E E E E E EEEE 79 LISIS ee ee ee 80 A ern ern A A ee ee ee 85 eg aT Fs Ui e PP OED oP PATE Ae OBOE P BOD EOD O P REE OPE DOPED POET EE ED 86 A Simple Role Playing Combat Game scscsesssesesssssessesesesseeseesessessessesseseaseaneaees 89 Jargon BUSUN O o aasaran annaia arda aana araara aa aaisa arta 99 STULL A A 100 Chapter 6 Functions ssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnmnnn 101 Accessing Privileged Information cccscsssssssssssessesssessseseessesesseesaesessassassassaseateas 101 Working with Variable SCOpGC csscsscsscssssessrssesserssessesseessesssessesseeseeeseesaeeseesaeeaeees 105 Refactori
312. nternal contig value return config def set config val Set an internal config value Global means global to this module not truly global global config config val CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES Let s try this out gt gt gt from internals import gt gt gt return config True gt gt gt set config foo bar gt gt gt return config foo bar gt gt gt config hasn t been imported try it config Traceback most recent call last File lt stdin gt line 1 in lt module gt NameError name config is not defined gt gt gt Assigning to it locally won t change the module variable config where local gt gt gt return config foo bar gt gt gt And assigning in the module won t change it locally set config where internal gt gt gt config where local gt gt gt But if you now import it explicitly the stored module variable appears from internals import config gt gt gt config where internal gt gt gt It s the same object modifying it locally modifies it in the module config test success gt gt gt contig test success where internal gt gt gt return config test success where internal gt gt gt Local assignment still doesn t change the module object config new True gt gt gt return config test success
313. o document the solution and see how this then feeds back into the next round of the development cycle Designing Your Own Software Why Bother There are many reasons that you might want to write your own software the chief one being that the applications you have available don t do what you want in the way that you want You may already have an idea of something you wish to achieve which led you to pick up this book in the first place Perhaps you need a solution for work maybe you have a killer game idea that you d like to realize you might want to organize some data in a web page or perform some system administration task You might want to control a gadget you just built or automate any number of repetitive jobs Possibly you already have the program you need if only it would do this one extra thing or maybe you just enjoy the idea that writing your own software is possible I m not going to try to tell you why you should want to get involved in this peculiar sport I ll assume you have worked that out for yourself already 10 CHAPTER 2 DESIGNING SOFTWARE Let s start by asking some questions Never be afraid to ask questions Why bother doing all this Why not just go straight ahead and start writing code Coming up with a clear design first along with a clear idea of what answers you expect to see cuts down on a lot of wasted effort and makes your programs easier to document troubleshoot and maintain It is important to
314. o date when the code changes delete old and irrelevant comments Comments should be complete grammatical statements HHH Stub of code or development note to be removed before the program is released This function doesn t do anything note there may be a def function here one day HHH return def naming conventions functions lower case with underscores Variable naming Choose names that people might be likely to guess whole words are good but don t make it too long Use singular names to describe individual things use plurals for collections Single letters are only useful for throw away variables variable names lower case with underscores a b c 0 1 2 int variables a b c X Y Z 0 5 1 65 2 42 float variables x y z result i j for i in int variables for j in float variables if result is not None return result def main func Docstring The title should make sense by itself A function s docstring should Summarize the function s behaviour 168 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Document its arguments Return values Side effects Exceptions raised and restrictions on when the function can amp cannot be called Optional arguments and keywords should also be explained if they are part of the interface result naming conventions return result if name main results main func print nPoints awarded for style
315. o produce an even more accurate result It may be necessary to redesign all or part of the program from scratch for this you have to go back to the drawing board start again with the top level design sketching the process out in pseudocode again translating that into Python code rerunning the script and testing it against the same set of values as before until there are no longer any discrepancies Notice that I have bumped up the version number to 0 2 every time you refine your design you will be creating a new minor revision of your application so you should increment the number after the decimal point to reflect this The number before the decimal point is the actual version number The standard practice is to increment this number to 1 once the program is ready to release into the world at large Some larger projects use three numbers separated by points so version 1 3 2 would be the second minor revision of the third major revision of version 1 and so on Now that you ve seen conditional statements and used them in our application let s consider another type of control flow statement loops CHAPTER 4 E MAKING CHOICES Now Repeat That Often you will want to repeat the same process over a range of different values Writing this out longhand is not only tedious but contrary to the principles of good programming Ideally you should have to write out a set of instructions or piece of information only once This can be achieved using a
316. o something To remember what these methods were supposed to do I need to go back to the original pseudocode The resulting Python file is fairly substantial but not unmanageable as shown in Listing 12 3 For larger programs you would probably want to split the program up into smaller files possibly separating the form from the functionality I ve omitted large chunks for brevity and clarity so you can see the important bits of the program the complete example can be found in the code download Listing 12 3 gtkinfo py usr bin env python Example windowed app Usage gtkinfo py Target System GNU Linux Interface Gtk Functional Requirements Display some system information in a window version 0 1 maintainer maintainer website com status Prototype date 12 01 2009 Import modules import gtk import pango import os commands class GtkInfo Code removed for clarity See the download for full listing Create a new window self window gtk Window gtk WINDOW TOPLEVEL self window set title Gtk Info self window set border width 6 self window set default size 600 400 self window set resizable True self window connect delete event self delete event Create a vertical box to pack the widgets into self vbox1 gtk VBox False 0 self window add self vbox1 Create menubar self menubar1 gtk MenuBar self menubar1 set pack direction gtk PACK DIRECTION LTR self menubar1
317. o the screen which is called print in Python So your first problem translates fairly easily into executable code as you can see in Listing 2 5 You can print any text you like Hello World is the default Listing 2 5 First Steps for hello_world py Problem Get the computer to output a message Target Users Me Target System GNU Linux Interface Command line Functional Requirements Print out a message User must be able to input some text Testing Simple run test expecting a message to appear expecting message input text Maintainer maintainer website com CHAPTER 2 M DESIGNING SOFTWARE 1 Print out a friendly message print Hello World 2 Input some text 3 Print out the text we just entered This script can be run by typing python hello world py on the command line You did remember to save it didn t you The only line in this script that is actually executed by the interpreter is print Hello World Everything else is either ignored as a comment or assumed to be a docstring in the case of the block of text at the beginning If you change the text between the quotes in the call to the print function the Python interpreter will print out whatever you tell it to Well done You just wrote your first Python program It s about as basic as can be but you can now bask in the satisfaction of having got Python to do something Further Refining the Design Constantly editing the script to g
318. object speeds up the pattern comparison process because the pattern does not have to then be compiled each time it is used You might also need to use this approach where a function specifically requires a regular expression object to be passed to it rather than just a pattern string gt gt gt pattern W a Sm w gt gt gt regex object re compile pattern gt gt gt regex object split string Z Deja vo hp Doing this creates a chance to save and name your patterns for later use and may also help to make your code more readable if you name your patterns appropriately Note The options for the flags argument are covered in the official Python documentation They apply to more advanced uses of re which shall let you explore at your leisure This chapter provides just a brief introduction Letting the Characters Escape If your users are particularly grammatically expressive you may have to deal with sentences containing all kinds of random punctuation There will be circumstances when you couldn t possibly edit the strings yourself to put backslashes in front of all the characters that are not ordinary letters or numbers Fortunately re escape pattern will escape all the nonalphanumeric characters in pattern for you gt gt gt re escape string Wea t Wo Z VG VEAN AN Veal 2 i WV DAE WAA WA VALUAN VA Ka WABA AVAA Manipulating Strings Using Regular Expressions As you
319. obvious In this instance the Player must be able to buy but the Shop must also be willing to sell Suddenly the whole dynamic of the program changes If you study Listing 9 2 it will be apparent that the code on the page merely sets up lots of objects and prepares them for action The game itself doesn t start until four lines from the end I standardized the Thing object to fit in with the names used on other GameObjects Now the only difference between Things Weapons and Armour is that they are technically of a different type a feature that I made use of to filter inventory lists using the built in issubclass function Next I filled in the details of the Location object A game location only really needs a name a description and importantly an inventory which I am using to represent the location s physical CHAPTER 9 CLASSES contents In order to be able to use the in operator with my new object I defined Location contains item so that it looks in Location inventory to find matches the inventory will be treated as the object s contents for all intents and purposes It has one additional attribute Location commands which is a dictionary of command words mapped to executable strings I have also included a mechanism for retrieving Locations from stored values in case I need it in the future I decided that most of the in game commands should be a function of the Location In order to facilitate this I defined a few method
320. ode what you have just written which in turn means more time to think about how you could improve and expand your work The other great thing is that you can use Python for pretty much any task you can think of be it large or small simple text driven script or major graphical application It does have its limitations but by the time you come up against them you will already have become a competent programmer and will be able to make an informed choice about where to go next Introducing the Interactive Shell Although this book is primarily aimed at showing you how to write stand alone scripts and applications that is fully fledged programs that don t require anything other than having Python installed for them to work Python also works in interactive mode which allows you to enter Python commands using a command line interface This is a great resource for immediate Python gratification You need to locate and start up your computer s command line interface sometimes known as the terminal or terminal emulator or on machines running Windows the DOS prompt This will provide you with a command line at which you can start typing commands Interactive mode can be started up by typing python at the command line If Python is properly installed you should see something like this python Python 3 1 1 1311 74483 Aug 17 2009 17 02 12 GCC 4 2 3 Debian 4 2 3 5 on linux2 Type help copyright credits or license for more inf
321. ods e object setitem self key value This method allows assignment to self key so that the value can be retrieved by object getitem e object delitem self key This method allows deletion of self key This is brought into play when the del statement is used on part of an object and should delete the element associated with that key Obviously you only need to include these methods if it is appropriate to allow values to be modified using index notation It may be more appropriate to use accessor methods get and set or use the properties function I ll come to that later My Player type needs to allow the setting of values using indexes solimplemented setitem like this def setitem self key value self dict key value return There are a few additional methods that are recommended for types that are supposed to emulate containers Both mappings and sequences should implement the object iter self method to allow efficient iteration through the container This method is called when an iterator is required for a container It should return a new iterator object that can iterate over all the objects in the container For mappings it should iterate over the keys of the container and should also be made available as the method keys for sequences it should iterate through the values You should also include the object reversed self method to complement it This is called by the built in function re
322. of material 1 15x0 95 m due to the allowance needed for hemming I would need 2 3 m of material if I measured it lengthways but I would use less material if I measured it along the width which would give 1 9 m I know the roll of material is 1 4 m wide I will probably want to round the length up to the nearest half meter when I go to buy it e Inthe second case 100x200 cm each curtain needs to be 150 cm wide 0 75x200 However because that is greater than the material width I will have to allow an extra 20 cm for the join as well as 20 cm for the hems making 190 cm so each curtain has to be 1 15x1 9 m If I measure the material lengthways I would need 4 6 m But I could get away with measuring it along the width so 3 8 m would be the minimum length required e Ifthe window is twice as deep as it is wide 200x100 cm it would only make sense to measure it lengthways giving an answer of 4 3 m e The fourth case 200x200 cm is interesting because either way round it requires two widths of material Measuring it along with width would seem more efficient requiring only 6 8 m of material except that I will end up with a very funny looking pair of curtains with a horizontal seam in the middle By measuring lengthways I could get away with only buying three lengths of cloth which would give a measurement of 6 45 m e can t use this material to make curtains in the fifth case 200x300 cm each curtain has to be 2 15x2 25 m so
323. of text known as documentation strings to your own code and these strings will then show up in an online help utility This information can easily be turned into web pages to provide a useful reference I will show you how to do this in Chapter 2 it is one of the unique features of Python and is worth learning how to use early on Jargon Busting This chapter introduced several technical terms with which you may not be familiar these are defined as follows Command line interface CLD This mechanism is for interacting with a computer by typing in commands to a text based interface such as the terminal or DOS prompt Compile This process turns a sequence of instructions in programming language which is readable by humans in theory into machine code which can be directly executed by the computer Python does this automatically in the background when you run your script In some other languages this has to be done as a separate step before you can execute the program Documentation strings This block of text on the first line of a function module class or method definition is also called a docstring Docstrings are used by Python s built in tools for automated testing and to provide help documentation See Chapter 2 Interpreter This program turns a sequence of instructions in a script into something that a computer can execute on the fly effectively compiling the code as the program is running rather than as a separat
324. open story txt rb lt io BufferedReader object at Oxb7ba990c gt The b mode returns a different kind of object to the default file object one that contains the same information in byte format You might want to use this if you wanted to handle image or audio data Write mode w lets you change the contents of the file completely Append mode a only lets you add information to the end This last option is useful for creating log files The buffering argument used with w or a can either be 0 or 1 If it s 0 your data is written straight to your hard drive Ifit s 1 Python creates a temporary file to store the text in before it gets written out The data is not written to disk unless you explicitly call file flush or file close You probably won t need this option immediately Reading from and Writing to Files The most basic method you can use to access the file s contents is file read size This reads up to size bytes from the file less if the read hits the end of the file before it gets to size bytes If the size argument is negative or not given the whole file will be read in as a single string The bytes are returned as a string object unless you opened the file as a binary object in which case you ll get the raw bytes If you are reading a plain text file containing ordinary characters you might not notice a great deal of difference gt gt gt text open story txt gt gt gt text read Once upo
325. ormation gt gt gt The details may be different according to your operating system The first line shows the Python version number and the compilation date of the version you have installed The last line gt gt gt is your Python prompt This indicates that the Python interpreter is running and waiting for your input The Python interpreter is the program that reads Python programs and carries out their instructions Interactive mode can be useful for testing out individual commands line by line experimenting with new ideas and getting help try typing help and don t forget the parentheses they are important To get out of the interactive Python interpreter on a Linux system you can use the quit or exit functions or else you must hold down the Ctrl key and the D key at the same time in Windows you need to use Ctrl Z and then press Enter CHAPTER 1 INTRODUCING PYTHON Choosing to Code with a Text Editor My preferred method of creating Python scripts is to use a plain text editor with a few basic programming features Programs like notepad especially notepad2 BBedit gedit NEdit and Kate are ideal for this task A variety of editors are available that offer specific enhancements for programmers such as syntax highlighting which is useful for showing coding errors immediately as you type Old school hackers may prefer to use Vi or Emacs Python does not dictate what software you use to create the code the choice is
326. ory space eventually but explicitly closing it is cleaner Often the data in a file write operation is stored in a buffer until the file is closed You can make sure the data gets written out to the file without closing it by using file flush A closed file cannot be read or written any more Calling close more than once is allowed CHAPTER 7 WORKING WITH TEXT Applications One of the most common uses of string manipulation is to convert text into different formats You may have a plain text file for example that you want to display as a web page Web pages often use Hypertext Markup Language HTML which encloses text between pairs of tags in angle brackets These tags can indicate paragraphs headings bold and italic text and so on Converting Text The pseudocode for a script to convert a text file into a basic HTML page would be very simple as shown in Listing 7 1 Listing 7 1 Pseudocode for a Text Conversion Script Get filename from user input Open file Manipulate text Write out web page I have used the string replace string split and string rstrip methods to create a suitable output filename and title for the page using the input filename and stripping the txt ending The manipulations in the body of the script are performed by re sub The challenge here is to find suitable patterns in the source text that can be matched precisely enough to avoid unwanted parts of the text being matched but
327. ou try this one out yourself as the output would fill up the rest of this chapter Alternatively you can read the lines of a file into a list for later consumption using file readlines sizehint If the optional sizehint argument is present instead of reading up to the end of the file whole lines totaling approximately sizehint bytes are read Some file like objects may not implement file readlines Writing to the file is simple As long as you opened the file in w or a mode file write str writes a string to the file gt gt gt out file open story html w gt gt gt out file write content 522 gt gt gt out file close If you opened the file in write mode the first write will overwrite the entire file and any successive writes will follow on from where the last write left off If you opened the file in append mode the writes will begin at the end of the file gt gt gt out file open story html a gt gt gt out file write content 522 gt gt gt out file close You can also write a list of strings to the file using file writelines sequence The sequence can be any iterable object producing strings though typically it s a list of strings writelines does not add line separators so you ll have to remember to put in the extra n characters yourself Finding Your Way Around Files The place in the file that the interpreter has read up to is represented by the number of bytes characters
328. ould not define other numerical operators I will explain these special numerical methods later in the chapter Properties There are several different ways of accessing an object s attributes you have already seen accessor methods get and set being used It is often the case that you will want to make sure additional actions are taken such as validation when getting and setting values rather than assigning and retrieving arbitrary raw data However the code that uses this object shouldn t need to know about the details This is where the property fget fset fdel doc function comes in Note Actually it s not a function but a class but let s not split hairs at this stage class Player def setName self name self name name capitalize double underscores in front def getName self return self name double underscores in front name property getName setName This allows you to use the name property as follows gt gt gt player1 Player gt gt gt player1 name inkon gt gt gt player1 name Inkon The property function automatically wraps the attribute access so you can be sure the additional actions in this case capitalization take place every time The property function takes four optional arguments You can include none of these which will make the attribute inaccessible If you want the attribute to be read only use just one argument the get method If you want read write acce
329. out what methods it supports This black box approach is known as encapsulation 183 184 CHAPTER 9 CLASSES Tip You can get an idea of what methods and attributes an instance of an object supports using the built in function dir In this instance dir player1 will return a list of names available within the namespace of that object Identity An object s identity never changes once it has been created you can find out an object s identity using the built in id function and compare the identity of two objects using the is operator gt gt gt player1 Player gt gt gt player2 Player gt gt gt player1 is player2 False gt gt gt id player1 136928972 gt gt gt id player2 136930284 The id function returns an integer representing its identity which you can think of as its location in memory Every object has a different id Determining an Object s Type An object s type determines the operations and methods the object supports and can be discovered using the built in type function gt gt gt type player1 lt class Player gt The type and id functions should really be used only for investigation and debugging Inside a program the recommended approach is to try the method you want and see if it works This is known as duck typing The premise is that if it looks like a duck and quacks like a duck you can assume it is a duck for programming purposes This approach makes worki
330. overview 108 109 readable code 111 113 testing 109 111 re findall pattern string flags method 140 refining 49 defined 74 regular expressions characters with special meaning in 140 141 defined 159 finding patterns at beginning or end of string 138 139 letting characters escape 139 manipulating strings using 139 141 matching character sets and alternatives 138 matching subpatterns 137 138 object 139 291 292 INDEX repetition reducing by refactoring rpcombat py matrix refactored 113 123 overview 108 109 readable code 111 113 testing 109 111 replacement field names 130 repr function 157 re split0 method 136 re split pattern string maxsplit method 139 re sub method 145 re subn pattern repl string count method 140 resultant defined 99 results output 101 defined 124 returning 25 return statement 102 reversed function 83 195 rich comparison methods 197 rich document format RDF 258 just width fillchar method 129 role playing combat game 89 98 rollQ function 103 round brackets tuples and 79 round function 59 rpcombat py reducing repetition by refactoring matrix refactored 113 123 overview 108 109 readable code 111 113 testing 109 111 rpcombat py write_out 157 S Scale widget Tkinter 264 scope 185 defined 124 script 6 Scrollbar widget Tkinter 264 Search and Replace function 109 SecurityError exception
331. ow many players or 2 max players int reply This is a global variable players def roll sides dice 1 Dice rolling simulator sides Number of sides the die has 115 116 CHAPTER 6 FUNCTIONS def def def dice number of dice to be rolled defaults to 1 Returns a random number between dice and dice sides weighted towards the average result 0 for rolls in range 1 dice result random randint 1 sides return result ziply seq None Create a matrix of matches from a sequence Takes one argument seq which should be a sequence of length gt 1 Returns a tuple of tuples matches opponents list seq opponents reverse matches actor target for actor in seq for target in opponents if target actor return tuple matches join with and sequence Join up a list with commas and and between last two items Takes a sequence and returns a sentence if len sequence gt 1 last item sequence 1 sentence join sequence 1 sentence sentence and last item elif sequence lt 1 sentence whole lot of nothing else sentence sequence 0 return sentence fix gender gender phrase Replace the word them with gender specific pronoun Takes two arguments gender a string which can be male female or something else phrase the string to be modified Returns a string with non gender specific pronouns replaced by gender specific ones
332. ow to go about the constant refinement and simplification of your ideas that will lead to an effective solution that is easy to understand I ll be introducing you to pseudocode the software design language and showing you how to turn it into Python documentation strings to create a framework around which you can grow an effective and maintainable piece of software Your first challenge which I ll be using as an example for this chapter is to get the computer to respond to acommand and print out some kind of response fairly basic stuff which you could probably guess at a solution for However it s always good to start as we mean to go on so I m going to break this down in terms of the software development cycle In order to get off the starting blocks it is necessary to identify the problem through a process of asking the right questions and then begin developing a solution initially by creating a wish list of the things you want your new program to do and getting feedback from potential users You will use all this information to create your initial program design In the next part of this chapter you will be able to start turning that design into working code As soon as the design is coded you will want to begin refining and testing it starting with defining the results you expect to get and then figuring out how to detect and correct errors In the final stage of the software development cycle you will discover how to use docstrings t
333. ow you can manipulate entire sequences of data at once CHAPTER 5 Using Lists In this chapter you will learn how to deal with multiple items of data by processing them in sequence First I ll give you an overview of the data structures that Python provides that can contain more than one item These include Strings tuples lists and dictionaries There are a few actions that you can perform on all of these data types so I ll cover those before going into the details of creating and manipulating each type separately In the second half of this chapter you ll be making a simple role playing combat game You will use these data structures to contain your player s statistics and inventory and to translate the numerical results of the combat calculations into descriptive phrases You ll be using what you learned about loops in the previous chapter to work with the individual items in those data structures In the process I ll cover some of the design and development issues you may have to face when creating more complex applications and you ll see how Python can be used to make repetitive number crunching and text manipulation not only fast and painless but also entertaining Working with Python Data Structures So far you have learned to work with individual pieces of data to produce some simple results Real world data is usually in lumps or groups and it would be useful to work with such groups without having to make lots of repetitive st
334. own words that describe the actions that the program needs to take in order to produce the desired result If you are working as part of a team your company may have a standard way of writing pseudocode but you don t need to worry about such considerations while you are learning Over time you are likely to find that your pseudocode naturally adopts a Python like syntax Initially the important thing is that you can express your ideas clearly without having to worry about the rules of the language yet The pseudocode is presented in comments in the order you want it to happen as shown in Listing 2 3 Listing 2 3 Pseudocode Example Do the first thing Do the next thing Do the last thing www allitebooks com CHAPTER 2 M DESIGNING SOFTWARE Each line is commented out so that the interpreter knows not to execute any of these statements This is your top level design which is a general description of how you are going to solve this problem Each statement is referred to as a step When a step is performed it is said to be executed Using Indentation and Subgroups Python is very strict about indentation Python regards any space or tab at the start ofa line as an indentation Every indentation means something Without going into too much detail at this stage pay careful attention to how indentation is used in the examples The most important thing to remember is not to mix tabs with spaces as the interpreter thinks these are two d
335. ox The global dave value would remain inaccessible Variables in this situation are known as shadowed globals and are demonstrated in Listing 6 2 If this is ever a problem you can usually get round it by passing the global value to the function under a different name Listing 6 2 Dave s Shadow Side print dave tfred tpete dave 0 fred 1 pete 2 def cardboard box global fred pete dave fred pete print Inside the box print dave t fred t pete cardboard box print Outside the box print dave t fred t pete dave fred pete Inside the box 33 Atei CATS 2 Outside the box 0 t 1 t 2 CHAPTER 6 M FUNCTIONS Tip The vars function can also return a symbol table for which you need to provide the name of the object as an argument Using the global Statement Although you can access global variables from inside functions it is always recommended to declare that you are using these variables using the global statement see Listing 6 3 If you ever need to change a global variable from inside a function using the global statement is a must Listing 6 3 Dave Goes Global usr bin env python Dave the cardboard box A simple scope example fred pete 1 2 def cardboard box global dave dave fred pete return vars print Inside the box print cardboard box fred dave print Outside the box print vars And here s the output
336. pellbook maintainer Print out results print format info spellbook TODO Check for existence of AUTHORS ChangeLog NEWS README and TODO files in the same directory as file TODO if no AUTHORS file write out author s to AUTHORS TODO Check TODOs and Warnings If fixed append Changelog else append TODO This is what happened when I ran the script on itself fix style py Python script to be checked gt fix style py Looks like a Python file 0K TODO 16 10 2008 TODO fix style py line 90 Test if file is valid python 3 0 if not run 2to3 on file 16 10 2008 TODO fix style py line 103 Turn comments into proper docstring 16 10 2008 TODO fix style py line 148 Check for existence of AUTHORS ChangeLog NEWS README and TODO files in the same directory as file CHAPTER 7 WORKING WITH TEXT 16 10 2008 TODO fix style py line 149 if no AUTHORS file write out author s to AUTHORS 16 10 2008 TODO fix style py line 150 Check TODOs and Warnings If fixed append Changelog else append TODO Application Details File fix style py Version 0 1 Prototype Last Modified 16 10 2008 AUTHORS Author maintainer website com Maintainer maintainer website com Credits Inspired by Duncan Parkes remark about inline TODO comments i And to compare I ran it o
337. pplication your test code can also benefit from this During our discussion we covered passing data to functions and working with variable scope Variable scope allows us to keep information where it belongs whether it belongs in the global scope of an application or in a function only All this means you can now refactor your early Python programs to make them easier to work with and more elegant To prove this we refactored one of our sample applications CHAPTER 7 Working with Text It s likely that you ll be working with text an awful lot in your Python programs because that s mainly how your program will interact with its users and a lot of the data it will work with will be in the form of text There are numerous features of Python that help you work with text and I ll cover many of them in this chapter Specifically Pll cover splitting and joining strings which are the most basic string operations you ll carry out That ll lead me onto formatting editing and searching for strings all vital for a lot of Python programs Finding strings is often linked to regular expressions and Python provides us with a wide array of regular expression functionality much of which I ll cover Finally I ll explain how to work with the files on your file system another common programming task Strings and Things Most commands in Python 3 will work in exactly the same way as in Python 2 There are however some important changes
338. pter you will learn about classes and see how to create and implement your own 179 CHAPTER 9 Classes Python is an object oriented language You have already seen what can be achieved using the methods provided by the basic data types Central to the concept of object orientation is the class a template for creating your own data types A data type may be a person for example each of whom has unique aspects that might be specified by data such as LastName FirstName Age Sex and EyeColor You might think of a class as a noun representing objects that are described by unique aspects These aspects are technically referred to in OOP as attributes Class definitions are sometimes referred to as templates as their entire purpose is to serve as a blueprint or model of class objects for you to use in your programs Classes form the basis of a whole new style of programming You have already moved the focus of your programs from simple top down scripts to function driven applications Classes enable you to move the action into custom built objects which can contain both data attributes and commands methods Creating objects involves a further stage of abstraction from function driven programming and provides a means of modeling real world constructs as single entities in your code Object orientation can help to reduce complexity particularly in long programs It also encourages programmers to re use existing tested code because it is pos
339. r locale dir bindtextdomain app name locale dir class Window1 SimpleGladeApp def def def def def def def init self path guixample glade root window1 domain app name kwargs path os path join glade dir path SimpleGladeApp init self path root domain kwargs Window1 new new self print A new s has been created self class name Window1 new Window1 custom methods Write your own methods here Window1 custom methods Window1 on window1 delete event on window1 delete event self widget args print on window1 delete event called with self s widget get name Window1 on window1 delete event Window1 on new1 activate on new1 activate self widget args print on new1 activate called with self s widget get name Window1 on new1 activate Window1 on openi activate on openi activate self widget args print on openi activate called with self s widget get name Window1 on openi activate Window1 on save1 activate on save1 activate self widget args print on save1 activate called with self s widget get name Window1 on save1 activate Window1 on save asi activate on save asi activate self widget args print on save asi activate called with self s widget get name Window1 on save as1 activate def if CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Window
340. r The single equals sign is the assignment operator It can be combined with other operators to perform more complex assignment operations e Built in A built in element is an innate part of the programming language as opposed to something that has to be imported from a module Built in elements are part of Python s standard library e Comparison operators These operators compare two values e lt Less than e lt Less than or equal e gt Greater than e gt Greater than or equal e Equal Not equal e is Tests object identity e is not Tests object identity e Conditional statement This section of code is performed if a certain condition evaluates as True A conditional statement is a form of control flow statement e Critical values These values exist at the edges of the permissible ranges set for an application In particular these are values that would cause changes or unexpected results from the normal running of the program e Flowchart This is a graphical method of designing and analyzing the control flow of a program 73 74 CHAPTER 4 MAKING CHOICES e Iteration An iteration is single run through a loop e Loop A loop is code construction that repeats e Loop body The body is the suite of instructions to be repeated in a loop e Null statement The pass keyword creates a null statement which tells the interpreter to move on to the next statement e Operands These are the expressions on
341. r of curtains To start you need to define the problem again calculate how much material to buy given the size of the windows The functional requirements are that the user must be able to input the measurements of the window and get back the required length of fabric in meters and the total price of the fabric to be bought To get a proper idea of how you might go about making a pair of curtains you might want to talk to real curtain makers watch them in action or at least get them to show you how they calculate the amount of material they need It might also be worth investigating your local fabric shop Following some research my top level design looked like this Prompt the user to input the window measurements in cm Add a bit for the hems Work out how many widths of cloth will be needed and figure out the total length of material for each curtain in cm still Actually there are two curtains so we must double the amount of material and then divide by 10 to get the number of meters Finally work out how much it will cost And print out the result H HH HH H HH F Before any calculation is possible you will need to know how wide the roll of material is and how much it costs per meter I have assumed a width of 140 cm and a price of 5 units of currency per meter for this example I can use the input function to get the window height and window width from the user The input function returns a string so I need to convert that into
342. ram development known as abstraction which means you get to create your own functions CHAPTER 6 Functions In the previous chapters you have learned how to assign variables do sums perform comparisons set conditions and make choices In the preceding chapter I showed you how to store process and retrieve greater amounts of information by grouping the data into lists and dictionaries You have learned enough vocabulary to enable you to design programs that can solve a wide variety of problems Now it s time to up the game to a new level The skills you have acquired so far are more than enough for writing top down scripts like the examples in the previous chapters No matter how long the scripts become execution still starts at the top of the page and stops at the bottom The only problem is that if you want to do something more than once the section has to be repeated verbatim This also means that modifying the code will become difficult as you can often find yourself having to rewrite the whole script to change one small thing It would be great to be able to break down the program into manageable pieces of functionality and luckily Python gives us a way to do this using functions In this chapter I ll cover how to write functions and segment your code into manageable pieces In the process lIl explain how to refactor one of our example applications Accessing Privileged Information The first design considerations in mak
343. rately sequence Just a list of words for word in sequence print word Application Note Like while for can take an else statement which is executed at the end unless the loop is exited with a break statement For the final example in this chapter I want to take the problem of removing all the punctuation from a line of text input by the user This is going to be a simple command line application again The top level design sketched out in pseudocode might look like this Enter a text string Loop through characters in text string if character not one of 3 lt gt write out character elif character new line To test out this example I will need to come up with a suitable text string that contains all the available punctuation Something along the lines of the following will probably do the job nicely One long string with various punctuation like this this and a list of lt arithmetic and comparison symbols gt including dubious email addresses and Stuff that might be Python code which evaluates as something other you don t want Another sentence Blah97635098q6v4ib5uq The first problem with our design is that if the character is a period it will get written out rather than translated into a new line so I need to reverse the order of the if and elif statements Second it would be much neater if I could replace all the punctuation with spaces there
344. re compiling that is you can run your script as soon as you have saved it without having to perform any intermediate steps Python is also a useful language in that it can be used to deal with a wide range of problems with relative ease and it has a good selection of preconstructed modules to deal with many of the more common computer tasks I find it similar to playing with conceptual building blocks the elements of the language are fairly simple to grasp but they can be combined in pretty much any way you can think of to create apparently complex and graceful structures One of the other great features of Python is that it is self documenting You still have to write the documentation yourself but you can include blocks of help text known as documentation strings or docstrings within the structure of the program itself I will be using this particular feature to keep all the design ideas together in one file at the outset and allow us to think on paper Subsequently you will be able to look back over old code and know what you were trying to achieve long after you have forgotten why you wrote it in the first place Eventually these docstrings will be used to communicate how your program works to other programmers Identifying the Problem Before you start coding anything you need to have an idea or a problem that you wish to solve It is quite useful to start with a familiar repetitive task that you suspect could be performed more efficiently u
345. remember that you are trying to make life easier not more complicated for yourself and others Asking the Right Questions Why bother is an important programming question that should be applied to every project module function or line of code before you begin There is a strong possibility that someone somewhere has already written the software you need or at least something very similar that you could convert to your needs with the addition of a couple of lines of code Close on its heels comes the next question Isn t there a simpler way to do this Laziness is the programmers ally you probably won t have to write very much from scratch and you won t need to learn everything by heart before you begin Generally speaking you can look up most things as you go along The art of programming has much more to do with figuring out a logical procedure to get something done than memorizing lines of code indeed one of the most common practices involved in coding is simplification Your first task is about as simple as it gets but as the book progresses you will be facing much more complex problems as well as coming up with ideas of your own Using Python to Solve Problems Python is similar in style to pseudocode the language programmers use to design software with the difference that you can run it on your computer and actually get results It s easy to read being close to English fun to learn and doesn t requi
346. return profile The profile dictionary is unpacked into string format using the keyword argument gathering operator and the items are referenced by their respective keywords rather than positional indexes I have reduced twelve calls to the print function down to one and now I only have to deal with a single format string to edit it further As you can see this code is much easier to read and understand Once I d tested it out I then removed the old code that is marked out in comments in Listing 7 5 I want to do something similar to the code block starting at line 217 as shown in Listing 7 6 Listing 7 6 Stock List Formatter Display shop stock list with prices Deleted code TODO shop stock list format print print lt SHOP gt for item in stock print t item stock item 0 print lt 0 gt print print You have profile gold gold Hit stock list 0 s 10 1 gt 3 format item stock item 0 for item in stock shop lt SHOP gt 0 lt 0000 gt 155 CHAPTER 7 WORKING WITH TEXT You have 1 gold format n join stock list profile gold print shop I made this in two parts First the stock list is formatted separately using a list comprehension Second I drop this comprehension into a shop format Now I can be much more precise about how everything lines up as sho
347. ring To align text on the left use string ljust width fillchar gt gt gt x ljust 37 example text And to align text on the right use string rjust width fillchar gt gt gt x rjust 37 4 example text Or you can pad the string out with zeros to the left to fill a field of the specified width using string zfill width gt gt gt s3 zfill 37 0000000000000000000000000eXamPLe tEXt Finally string expandtabs tabsize can be used to expand all the tab characters in a string using spaces If tabsize is not given a tab size of 8 characters will be used by default gt gt gt s2 one ttwo tthree gt gt gt s2 expandtabs one two three Advanced Formatting One of the nice new features of Python 3 is that strings have a new method string format args kwargs that allows you to drop the variables given as an argument into a specially prepared string rather than having to go through the complicated process of joining lots of individual strings up The string is prepared by putting references to the format arguments in curly braces at the point they are to be inserted in the 129 130 CHAPTER 7 WORKING WITH TEXT string These replacement field names can be numbers positional arguments i e args or names keyword arguments i e kwargs The format is similar to the one you would use to pass variables into a function gt gt gt
348. rning as the preceding if and elif statements would have caught either of the two possible results It is possible to nest if statements to allow for more possibilities and you can leave out the elif or else statements if you don t want anything to happen unless the condition is satisfied In other words sometimes you want to do something if a condition is satisfied but do nothing if it is not satisfied After the final statement the indentation must go back to its original level this will indicate to the interpreter that the conditional block has come to an end Python marks out blocks of code using indentation alone it doesn t use punctuation marks like the curly braces you may see in other languages This unique feature of Python means you have to be extra careful about indentation If you do get it wrong you ll find out soon enough as the interpreter will complain loudly gt gt gt if c print c Cc t 1 indent bad File lt stdin gt line 4 indent bad AN IndentationError unindent does not match any outer indentation level Conditional statements provide a powerful way of checking that your data is what you expect it to be this is called validation Validation is usually performed when the data is first fed into the computer and also when that information is written out to a file or database record Checks could include making sure that all boxes have been ticked and all questions answered that the input contains th
349. roduce variables at random places in your scripts Beware Python won t complain unless you try to use a variable before you have actually assigned it a value but it s really easy to lose track of what variables you are using and where you set up their values in the script There are two really sensible practices that will help keep you sane when you start to create large numbers of different variables One is to set up a bunch of default values at the start of each section where you will be needing them keeping all the variables you are going to use together in one part of the text like an ingredients list The other is to keep track of the expected types and values of your variables keeping a data table in your design document for each program that you are writing Python needs to keep track of the type of a variable for two main reasons Chiefly the machine needs to set aside enough memory to store the data and different types of data take up different amounts of space some more predictably than others The second reason is that keeping track of types helps to avoid and troubleshoot errors Once Python has decided what type a variable is it will flag up a TypeError if you try to perform an inappropriate operation on that data Although this might at first seem to be an unnecessary irritation you will discover that this can be an incredibly useful feature of the language as the following command line example shows gt gt gt b 3 gt g
350. rogram into the new one by hand or worse rewrite them from scratch Python has a number of indispensible standard methods that your programs can always access However you re bound to end up encountering some problems a few times and their solutions won t be part of Python s core built in functionality For example one of your programs might need a class to interact with spreadsheet files or another might make use of a method to calculate the distance between two points on the earth s surface It would be impractical for Python to have to hand every last bit of functionality you might need The Python distribution would be enormous and it would grow over time The Python interpreter would take a long time to start as it found all of the functionality and brought it into memory and again this procedure would only get longer And a running interpreter would take up huge amounts of memory This is clearly not satisfactory given how rarely the average program would use most of this functionality Luckily Python has a solution for these problems It provides a flexible easy to use system for modularizing your code that is for reusing your own code for optionally including extra functionality from the standard Python distribution and for bringing in any other contributed code from other Python developers The structure of modules and packages lets you keep your projects organized smaller and easier to understand In this chapter we l
351. rompt user to make a purchase purchase input Please choose an item or type done to quit CHAPTER 6 M FUNCTIONS If the item is in stock and the player has enough gold buy it if purchase in stock elif purchase done or purchase return profile inventory and profile gold gt 10 else print We don t have purchase in stock return purchase That passed its initial testing so I decided to move on immediately I was also getting fed up with having to choose a weapon when the character obviously had only one weapon in inventory First I needed to retrieve a list of what weapons the character has obtained This was relatively easy to work out because everything in the inventory apart from the armor can be used as a weapon so I constructed a list comprehension to return the members of profile inventory not in the armour types set I made the following actions conditional on the length of the list If the resultant list only contained one entry that item is automatically chosen If there are fewer the character wields a fist by default Otherwise the user is prompted to choose a weapon as before I wondered whether I could use the same list comprehension technique to set up the armor statistics So I replaced the following code for armour type in armour types if armour type in profile inventory profile armour stock armour type with this armour stats stock item for item in pro
352. rt sys import time now time strftime Y m d the time function yields system time date etc def get args result t join str i str arg for i arg in enumerate sys argv return result def get input result sys stdin read return result def output args data arguments n join args output string n join Datestamp now nCommand line arguments sys stdout write output string return arguments nStdin data if name main 171 172 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB arguments get args data get input output arguments data In Listing 8 2 the first function get args performs a list comprehension on sys argv The second function get input reads data in from stdin Their results are bundled together and printed prettily by the output function Listing 8 2 does much the same job as the original hello world py but this time using the sys module to perform input and output instead of the built in input and print functions Tip Some modules are better documented than others You can get a very brief overview of the available methods and attributes of any Python object using dir object or help object Using exec and eval While we re on the subject of dynamic applications I d like to look at two rather useful but dangerous built in functions exec string and eval string The exec function was
353. s First I must convert the string into a number Otherwise I will get an error if I try to perform arithmetic on a text string curtain width float window width 0 75 20 CHAPTER 4 E MAKING CHOICES print t curtain width curtain length float window height 15 print t t curtain length Now I need to work out how many widths of cloth will be needed and figure out the total length of material for each curtain in cm still widths curtain width roll width print t t t widths total length curtain length widths print t t t t total length Actually I have two curtains so I must double the amount of material and then divide by 10 to get the number of meters total length total length 2 10 print t t t t total length Finally I need to work out how much it will cost price total length price per metre print t t t t t price Each different variable is preceded by a different number of tab stops t so the information goes into a separate column Now I can properly test the design Here are the results of the first test I ran Enter the height of the window cm 100 Enter the width of the window cm 100 width height widths total price 95 0 115 0 678571428571 780357142857 15 6071428571 780357142857 You need 15 6071428571 metres of cloth for 78 0357142857 Immediately several things become apparent I was expecting an answer of something more like 1
354. s 77 78 CHAPTER 5 M USING LISTS The string type has the method join which takes a sequence as an argument and returns a single string made up of the elements of the sequence joined together with copies of the original string between them It might seem more logical to write sequence join separator but that would be a method belonging to the separator type Try reading the statement backward if you have trouble making sense of this gt gt gt join fruit avocados bananas oranges grapes mangos You could pretty this up by assigning sep first and then calling sep join fruit if you prefer this could be useful if you are using the same separator elsewhere and you want to make sure that you are splitting and joining all your lists with the same separator Readability of code is an important consideration The assignment in the following example could be to vegetables split sep in this case Splitting up a string to form a list can be done with stringToSplit split separator max which is much more logical to write This is a string method so it works on a string variable This method takes an additional argument that specifies the maximum number of splits allowed gt gt gt vegetables carrots potatoes onions leeks celery gt gt gt vegetables vegetables split gt gt gt vegetables carrots potatoes onions leeks celery The preceding code allows to you process each word in
355. s Set to True to trace variables trace False max players 2 This is a global variable players 93 CHAPTER 5 M USING LISTS Generate characters while len players lt max players print print New Character print Create empty profile dictionary profile Name Desc Gender Race Muscle 0 Brainz 0 Speed 0 Charm 0 life 0 magic 0 prot 0 gold 0 inventory Prompt user for user defined information Name Desc Gender Race name input What is your name desc input Describe yourself gender input What Gender are you male female unsure race input What Race are you Pixie Vulcan Gelfling Troll Validate user input profile Name name capitalize profile Desc desc capitalize gender gender lower if gender startswith f profile Gender female elif gender startswith m profile Gender male else profile Gender neuter race race capitalize if race startswith P profile Race Pixie elif race startswith V profile Race Vulcan elif race startswith G profile Race Gelfling elif race startswith T profile Race Troll else profile Race Goblin Generate stats Muscle Brainz Speed Charm profile Muscle random randint 3 33 random randint 3 33 random randint 3 33 profile Brainz random ran
356. s Text Formatted text display Allows you to display and edit text with various styles and attributes and supports embedded images and windows Toplevel A container widget displayed as a separate top level window If you want to develop applications further using Tkinter The full documentation can be found at http www pythonware com library tkinter introduction index htm CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Saying Hello with PyGTK Tkinter can be used to make functional interfaces but the presentation is rather basic Python s Gtk bindings provide access to the themeable and potentially prettier PyGTK graphic toolkit You need to have python gtk2 installed from there you ll follow a process similar process to the one you used with Tkinter import the module into your program and use the methods and widgets it provides Again the main application is abstracted into the HelloGtk class see Listing 12 2 and the object it creates is represented by the main window which is displayed on the screen Listing 12 2 hellogtk py usr bin env python HelloGtk Example windowed app Usage hellogtk py Target System GNU Linux Interface Gtk Functional Requirements Display a GUI window containing two buttons which write a message to stout when pressed version 0 1 maintainer maintainer website com status Prototype date 12 01 2009 Import modules import gtk class HelloGtk This is a
357. s this to speed up handling import statements across a lot of interconnected modules However the caching means that you ll typically find yourself quitting the interactive prompt and running it again to clear Python s internal cache of imported modules Help is at hand however in Python s own imp module gt gt gt import convert gt gt gt import imp gt gt gt convert weightings 0 20000000000000001 0 29999999999999999 gt gt gt Now edit the weightings in convert py by hand Save the file to disk and try importing again import convert gt gt gt convert weightings 0 20000000000000001 0 29999999999999999 gt gt gt The values haven t changed so force reload with imp reload imp reload covert lt module convert from convert py gt gt gt gt convert weightings 0 10000000000000001 0 59999999999999998 249 250 CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES The imp module has other advanced functionality for finding loading and handling modules However you should generally only need the variations on the import keyword discussed previously in this chapter the functionality exposed by imp is generally only useful when modules are being re created or recompiled during a program s execution Note also that multiple module reloads are additive changing a variable name in a module can leave the old one lying around after a reload For example if you ve imported the conver
358. s attributes from the parent class it is based on Using the preceding examples let s take a look at how this unfolds I need to create a Weapon class and come to think of it I ll probably need an Armour class as well And there are some other things that the Player can interact with that don t fall into the category of either Weapon or Armour I could create three separate classes of Weapon Armour and general Things but now that I think about it Weapons and Armour are specific instances of Things so I will create a base class called Thing Each Thing needs three basic attributes price strength and speed These attributes need to be assigned possibly from a table of predefined values when the object is created For now I will give them default values gt gt gt class Thing price 0 strength 0 speed 0 gt gt gt box Thing gt gt gt box price 0 gt gt gt box lid Traceback most recent call last File lt stdin gt line 1 in lt module gt AttributeError Thing object has no attribute lid I have created a box which is an instance of Thing It has a price strength and speed but no lid Now I can create a subclass of Weapon like so 189 190 CHAPTER 9 M CLASSES gt gt gt class Weapon Thing print Weapon made Weapon made gt gt gt axe Weapon gt gt gt axe price 0 I could assign values to the Weapon laboriously one by one gt gt gt axe price 75 gt gt
359. s for the Location type First the Player must be able to get into the object so Location enter player adds the Player to Location inventory and starts up a mini command line The command line is dealt with by Location interpret command player which finds the relevant command in Location commands formats it with any additional arguments and the sends the whole thing to exec The try except statement wrapping will be covered in Chapter 10 It follows that all the combat commands should go in the Location too so I moved most of the combat section in the main body into Location combat which controls the main loop and into Location resolve conflict attacker target which deals with the individual strokes The calc damage function became Location damage attacker target velocity Using properties considerably simplifies item access what was players target Name in the previous version becomes target life in this one Finally Location exit player removes the Player from Location inventory I created a subclass called Shop to implement some different game commands and provide a contents list with prices In most of its details a shop is just like any other Location with the sole difference that trading may take place and combat cannot The program now has only seven global functions some of which could be further abstracted I added the trace text function as a wrapper to the print function to provide verbose debugg
360. s illustrated in Table 2 1 Table 2 1 An Example Data Table Identifier Description Type some text User input text Text string 23 24 CHAPTER 2 DESIGNING SOFTWARE This information could be copied into the original txt file hello world txt in this instance if all you need is to provide yourself with a reference for future modifications Complex applications often come with several text files and extensive user documentation I ll come back to these later The form and content of the user manual is really up to you The important point to keep in mind is that the software design cycle does not end until all the documentation has been brought up to date Jargon Busting Here are the terms introduced in this chapter Argument This is a value you pass to a function or to a procedure so it has some data to work with Assignment This is the operation of setting a variable to a value Delimiter This punctuation mark is typically used to separate a text string or other piece of data from surrounding characters Escape This is a method of indicating that the next character in a text string should be processed in a different way Function A function is a block of code that performs a calculation accepts zero or more arguments and returns a value to the calling procedure when it is complete Indentation This refers to the use of spaces and tabs at the start of a line of code or text except you want to use sp
361. s in such a way as to deal with the problems that originally caused them using Python s core handling structures to turn exceptions and other problematic situations into straightforward control flow finally clauses This part of a try statement block is executed regardless of whether or not an error occurs Even if the error is not handled by a relevant except clause the finally clause will still execute After that Python s exception framework raises any unhandled errors to the next method up and so on These clauses are useful for any tidying up that must be performed locally regardless of the nature of the error Look before you leap LBYL philosophy In this cautious programming style situations that might raise exceptions are tested for in advance and only attempted if they are possible This philosophy is useful in languages other than Python where exception handling is either cumbersome or detrimental to program performance Pythonic This term describes a program that is structured in a way that is most amenable to both the ideas and principles that have informed Python s internal development and to the nature of the Python community s established coding conventions 239 240 CHAPTER 10 EXCEPTIONS Raise When you raise an exception you both create an exception object and cause Python s core exception framework to begin passing it up from the current method through the method that called it and the met
362. s of information you may need to use quite a complicated set of words numbers and relationships to describe even the simplest real world entity in virtual world terms CHAPTER 3 VARIABLES AND DATA TYPES Python provides a variety of ways of combining these simple data types to create more complex data types which I ll come to later in this book First you need to know about the fundamental building blocks that are used to define your data and the basic set of actions you can use to manipulate the different types of values In the Beginning Was the Void Python has a special predefined value called None which represents nothing zilch nada The value None has its own type and is useful where you want to create a variable but don t want to specify its value and therefore its type just yet Assigning values such as 0 or to a variable will create an int or str type variable You can assign the value None like this which produces a NoneType variable Don t forget it starts with a capital N information None For the next few examples the real world information that I ll be attempting to model in virtual form will be fantasy characters for a role playing game To create some continuity in my fantasy world I need to be able to keep track of the characters names plus some descriptive information such as what kind of fantasy creature each is I also need to create some statistics to represent strength speed and so on to provid
363. s yarr This is a test Yarr this is a test gt gt gt print strings me hearties It uses the strings module It uses the strings module me hearties CHAPTER 11 REUSING CODE WITH MODULES AND PACKAGES After you try this out lookin the my modules directory You should now see a new file called strings pyc we ll discuss this later in the chapter in the Python s Internal Behavior section You can now use this module to convert short sentences into pirate speak at the Python prompt But there s a limit to how much you would want to paste into print statements in this way and this script will only ever convert one sentence at a time In short although the methods are useful in the context of a program they aren t convenient for the end user Let s see how to overcome this shortcoming Using Your Module in Another Program Wouldn t it be great if you could embed these low level conversion methods in another higher level method one that could take a large block of text decompose it into sentences and randomly include piratical interjections Then you could get it to rewrite your e mail content before sending it to a friend and celebrate Talk Like a Pirate Day in style Let s build this next using the import statement to keep the low level methods in strings py Look at the code in Listing 11 2 which uses a slightly different form of the import statement to avoid cluttering up the code When we used the
364. se statements or exception handling It will all be executed the first time anything is imported from the package The Universe of Python packages As has been noted throughout this chapter packages are a great way to offer reusable code and Python itself comes with a number of packages that make your life easier We ll look at them in the next section and then examine some third party packages that have been contributed by the Python community Standard Modules Python is distributed with a standard library of modules that provide useful if basic functionality The precise list of modules varies depending on what operating system you are using primarily in the system modules for example Unix and Linux Python distributions have support for POSIX system calls Unix users and groups and terminal command prompt functions whereas Microsoft Windows Python has modules to access the Windows registry and Windows installer files However broadly speaking the list of standard modules is the same across distributions and is quite long This does mean that there s a big toolkit of modules for you to play with Always check the functionality provided by standard modules before writing your own or downloading contributed code Importing from the standard library is as easy as importing from modules you ve written yourself the standard library is on sys path so Python s import statements will always be able to find the modules in it In fact
365. selection def delete event self widget event data None Quits the application gtk main quit print You called a event return False Bye Finally we start the application def main gtk main if name main application GtkInfo main Using Glade and tepache to Build Interfaces Only certain sorts of programmers find building GUIs from scratch exciting and there is a kind of logic in the idea that graphic interfaces should be made in a graphic interface If you are using Linux you can create your GUI using Glade and use a utility called tepache to generate a Python framework from it You then just have to fill in the blanks such as menu names and error messages tepache guixample glade written file guixample py The preceding code snippet will produce a page of code like the following as well as a file called SimpleGladeApp py The py file builds the GUI according to the instructions in the corresponding glade file usr bin env python coding UTF8 Python module guixample py Autogenerated from guixample glade Generated on Thu Jan 22 17 30 21 2009 Warning Do not modify any context comment such as They are required to keep user s code import os import gtk from SimpleGladeApp import SimpleGladeApp from SimpleGladeApp import bindtextdomain 279 280 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS app name guixample app version 0 0 1 glade di
366. sequences lists mappings dictionaries and Booleans true or false values These are the staple ingredients of all Python programs The value None has a type all of its own NoneType The rest of this chapter will introduce you to words and numbers However first we need to talk about Python s dynamic typing features Working with Dynamic Typing In Python once a value has been assigned to a variable the interpreter will then decide what sort of value it is i e a number some text or some other relevant quality This is known as dynamic typing it has nothing to do with how many words per minute you can input from the keyboard Unlike some other languages it is not necessary to declare what your variables are before you use them This is both a blessing and a curse The main advantage is that you don t really have to worry exactly what type an item of data is so long as it behaves the way you want it to 29 30 CHAPTER 3 M VARIABLES AND DATA TYPES Dynamic typing makes it much easier to handle different types of unpredictable user input The interpreter can accept user input in many different forms to which it assigns a type dynamically This means that a single piece of code can be used to deal with words numbers or any other data type and that the programmer doesn t need to decide what type the data will be in order to assign it to a variable Not needing to declare variables before you use them makes it tempting to int
367. sessssseeeeeeessssssssssnens 1 Chapter 2 Designing Software sssasssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nna 9 Chapter 3 Variables and Data TypeS sssssnnnnsnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nn 27 Chapter 4 Making ChoiCeS sssssssnsnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 49 Chapter 5 Using LiStS sssnnsssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnna 75 Chapter 6 Functions sssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nnna 101 Chapter 7 Working with TeXt ssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 125 Chapter 8 Executable Files Organization and Python on the Web 00sssss000 161 Chapter 9 ClaSS OS i viscsesissescestsaicssaseancsuisusnsiassesnentecasesnnsssincnsissassuassnssvensensensisanesnies 181 Chapter 10 EXCQ QUO sive iscsiscsivesssanssenatescnessannaxenavancsesasunansnacnsasesnsexexansaunanesenensasxanvs 221 Chapter 11 Reusing Code with Modules and Packages sccsssssssssssssssessssssssess 241 Chapter 12 Simple Windowed Applications ccccssssssssssssssssssssssesssssseeesssseens 261 LL Geert ere errrerr crete ere reer tr ter errr cer reer errr err trrreerrrerrrer rere errrrrerrrr errr rrr rer rer er 283 www allitebooks com www allitebooks com Contents About the AWhOr sia seca sects enisi
368. set child pack direction gtk PACK DIRECTION LTR CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Here is where we start creating the separate menus and wiring the application together Note how we connect each action to a method Each method will be explained as we work our way through the rest of this example s code FILE menu self file gtk MenuItem label File use underline True self file menu gtk Menu self save as gtk ImageMenultem stock id gtk save self save as connect activate self on save as self file menu append self save as self save as show self quit gtk ImageMenultem stock id gtk quit self quit connect activate self on quit self file menu append self quit self quit show self file set submenu self file menu self file menu show EDIT menu self edit gtk MenuItem label Edit use underline True self edit menu gtk Menu self copy gtk ImageMenuIltem stock id gtk copy self copy connect activate self on copy self edit menu append self copy self copy show self edit set submenu self edit menu self edit menu show HELP menu self help gtk MenuItem label Help use underline True self help menu gtk Menu self about gtk ImageMenuIltem stock id gtk about self about connect activate self on about self help menu append self about self about show self hhelp gtk ImageMenultem stock id gtk help self hhelp connect activate self on
369. setProt def setGold self value self gold int value def getGold self return self gold gold property getGold setGold def setInv self contents self inv contents def getInv self return self inv def strInv self flatlist str item for item in self inv text join with and flatlist return text inventory property getInv setInv class Player GameObject Role Playing Character 205 206 CHAPTER 9 M CLASSES Takes an optional dictionary as an argument The dictionary Player dict must contain the output of repr Player if dict is not provided the Player will self generate prompting for name desc gender and race Player has two methods Player buy purchase Player strike target def init self store if store print nNew Character n Prompt user for user defined information Name Desc Gender Race name desc input What is your name input Describe yourself gender input What Gender are you male female unsure race input Race Pixie Vulcan Gelfling Troll Set game statistics self self self self self self self self setName name setDesc desc setGender gender setRace race setStrength setBrainz setSpeed setCharm Calculate derived statistics self life int self strength self speed 2 roll 49 1 2 if 0 else self if 0 else s
370. sible to create objects that inherit the majority of their attributes and methods from an existing class leaving the programmer to add or override the pieces that specifically need to be different Using the principle known as inheritance specific objects can be created out of general ones producing a family tree of objects similar to the classification of natural species Imagine you have three base classes Animal Vegetable and Mineral these classes would contain general attributes that could be applied to any object of the same class You could then derive further subclasses from these base classes so Animal could become the parent to several child classes such as Mammal Reptile Fish Bird Insect and so on Each of these children would contain new attributes specific to their type but they d also inherit the general attributes of an Animal You could further subclass Mammals into Dog Cat Pig and so forth When you want to use an Animal in your program you could for example create an instance of Dog called rover which inherits the bark method that all Dogs have In your code issuing the command rover bark could then result in something like Woof being printed to the screen Note This is just an example don t recommend creating such a large family of subclasses in actual practice you should create only the classes you need to use 181 182 CHAPTER 9 M CLASSES As in the real world you don t need to know all
371. sing a simple script Your first problem is to find out how to communicate with your computer using Python and how to get your computer to communicate back Using this as an example I will briefly explain the stages of the software development cycle that most programs will go through in course of their development You don t have to slavishly follow all the suggestions in this chapter but you may find that they provide a useful structure for developing more complex applications and to get you back on track if CHAPTER 2 M DESIGNING SOFTWARE you find yourself getting lost It is also useful to be able to design your program before having to think about the details of coding it in a formal computer language The first stage of writing any new program is to grow a design out of the initial problem a sequence of actions that will calculate the correct result and create the required output Once the idea has passed the Why bother test it is necessary to create a simple framework on which to hang the various bits of code that you come up with Without such a framework it is often difficult to know where to begin and virtually impossible to construct anything but the simplest of scripts without it getting messy very quickly The great news is that no specialized knowledge is required at this stage The process can be started off by answering a few simple questions e What do you want the software to do e Who will be using it e What system wi
372. so now it s time to code the design see Listing 4 8 Listing 4 8 stringclean py usr bin env python Clean up text string replacing punctuation with spaces 71 72 CHAPTER 4 MAKING CHOICES Target System GNU Linux Interface Command line Functional Requirements Loop through the characters in a string Removing all non alpha numeric characters Lines break at full stops It would be great to have a single space between each word Testing Run test Test values One long string with various punctuation like this this and a list of lt arithmetic and comparison symbols gt including dubious email addresses and Stuff that might be Python code which evaluates as something other you don t want Another sentence Blah97635098q6v4ib5uq test Expected results One long string with various punctuation like this this and a list of arithmetic and comparison symbols including dubious email addresses and Stuff that might be Python code which evaluates as something other you don t want Another sentence Blah97635098q6v4ib5uq version 0 1 maintainer maintainer website com status Prototype punctuation 3 lt gt print print print input string input Enter a text string output string space flagged False for char in input string if char print output string output string elif char not in punctuation output strin
373. ss you must define two arguments get and set The third argument is used to delete the attribute and the fourth argument also optional is used to provide a docstring for the property You can provide these as keyword arguments if say you want to include a docstring in a read only property CHAPTER 9 CLASSES Customizing Attribute Access There may be times when it is not appropriate to use properties if you are using old style classes for instance In such cases the following methods can be defined to customize the meaning of attribute access e object getattr self name This method is called when an attribute lookup has not found the attribute in the usual places where name is the attribute name This method should return the computed attribute value or raise an AttributeError exception e object getattribute self name This method is automatically called when the attribute name is accessed It does not work with older version of Python e object setattr self name value This method is called when an attribute assignment is attempted where name is the attribute name and value is the value to be assigned to it e object delattr self name This method is called when an attempt is made to delete the attribute name Emulating Numeric Types The methods listed in Table 9 2 can be defined to emulate numeric objects I ll just give you a brief overview here because it s a lot of information to take in all in on
374. ss to a wealth of detail about the module When we wrote strings py and convert py we took care to add documentation strings wherever possible So either module will produce detailed documentation at the interactive prompt as you can see in the following example gt gt gt import strings gt gt gt help strings Help on module strings NAME strings Module to convert strings to pirate speak FILE home jp chi1 my modules strings py FUNCTIONS me hearties str Appends me hearties to the end of a stub sentence piratify str Generic piratify of whole paragraphs by exclaiming everything yarr str Appends Yarr to the start of a stub sentence and piratifies it END Finally your module can contain a reference to the module variable name This was first discussed in Chapter 8 and its most common use is to put code into your module that only runs if the module is executed on its own at the command prompt At the bottom of convert py add the code from Listing 11 4 Then you can execute the whole file at the command prompt in different ways as suggested in the output block below the listing Listing 11 4 Executable Block to be Added at the End of convert py appended at the bottom of convert py if name main import sys if len sys argv gt 1 Convert file given on command line print convert file sys argv 1 else Convert any content piped into the program print convert string sys stdin rea
375. st reverse method 82 lists 75 100 creating 80 dictionaries 85 86 89 defining 86 88 deleting items 88 89 sorting 89 using 89 list comprehensions 84 85 manipulating 108 modifying 80 81 multidimensional 83 overview 75 processing large amounts of data 83 84 queues 82 role playing combat game 89 98 sequences accessing items in 75 76 determining if items are in 77 79 sets 85 sorting 82 83 stacks 82 terminology 99 100 tuples 79 80 changing values in 80 creating 79 80 list sort method 82 list traversal 83 local data 105 locals function 105 local variables defined 124 logical errors 57 61 logical operator defined 46 Look before you leap LBYL 238 defined 239 loop body defined 74 loops 65 73 defined 74 overview 65 for statement 69 73 while statement 69 break keyword 67 68 continue keyword 67 68 nesting conditional statements 69 overview 65 67 M magic methods 184 191 magic variable 37 major revisions 64 manipulating lists and dictionaries 108 map function 84 mapping 29 defined 46 99 methods 195 matrix 83 defined 99 max seq 79 Menubutton widget Tkinter 264 Menu widget Tkinter 264 Message widget Tkinter 264 methods 77 181 190 191 defined 99 220 minor revisions 64 min seq 79 modes files 141 142 modification procedure 108 defined 124 modularization defined 124 modules 24 90 241 260 INDEX creating 242 243
376. st that the basic call works first and then proceed to design the new function The design for this one is fairly simple 1 Pop off the last item of the list 2 Join the rest of the list with commas 3 Add the word and followed by the popped off item I don t want to use list pop here for two reasons if the function was sent a list this method would modify it in place and I would lose the last entry as a side effect of this function Also I want this function to be generalized enough that I could send it a tuple instead so I used slice notation last item sequence 1 sentence join sequence 1 sentence and last item sentence This solution worked fine with lists that had more than one item but it still added and to single item lists So I added an if else clause so that single item lists would pass through Implementing the gender fix looked slightly more complex Again I began with an empty function definition def fix gender gender phrase Replace the word them with gender specific pronoun print fix gender called with arguments gender phrase return As the chances of the actual phrase coming up naturally were rather small I inserted a dummy call into the buy equipment function to test my new function test phrase profile Name buys themself some equipment print fix gender profile Gender test phrase Having worked out the details of how to call the function filling in
377. suitable weapons CHAPTER 5 M USING LISTS and model hand to hand combat between characters Testing methods trace table and play testing Test values Ez Tall Pixie female Inkon Small Troll male Expected results All statistics should be integers in the 1 99 range Apart from that this script needs play testing Limitations Too many to mention version 0 1 maintainer maintainer website com status Prototype Import modules import random set up constant data stock shield 15 20 50 sword 60 60 40 dagger 25 30 50 halberd 80 80 30 club 15 30 30 flail 50 70 45 hammer 99 100 20 cuirass 30 45 20 armour 101 100 0 lantern 10 5 30 pole 10 5 50 rope 10 5 70 armour types set shield cuirass armour hits hits bashes smites whacks shreds mutilates lacerates annihilates misses misses nearly hits fails to connect with swipes wildly at flails ineffectually at gets nowhere near nearly decapitates self instead of hits self on the foot to the amusement of damage report small insult flesh wound deep slash ragged gash savage laceration fractured rib cage smashed up face split skull life changing a scar bruising serious blood loss total debilitation chronic concussion a severed limb multiple fractures an amputated head Preference
378. t modify list global list gt gt gt print global list dave pete fred gt gt gt print result None Here the local symbol table for modify list will contain only local list as the function doesn t create any variables itself and doesn t return any either Refactoring rpcombat py to Reduce Repetition Now that you understand global and local scopes you are ready to start refactoring Refactoring is the process of rewriting your application to make it easier to maintain or more efficient but keeping the same behavior Let s go back to rpcombat py and examine the refactoring possibilities The first bits of code that stand out are the repeated calls to random randint This particular function gets called several times in different ways with different values The trick here is to come up with a function that will work in all these instances and be flexible enough to work with future changes As these future changes are as yet unknown the only way we can cater for them is to make write a generic piece of functionality that can replace these individual pieces of code with calls to that function This is called generalization It may be useful to look at the original design to recall what the original intention of the code that needs refactoring In this case I was trying to model the rolling of some dice The interesting part of the problem is that the number of dice and the number of sides that the dice have will ne
379. t gt c word gt gt gt trace False gt gt gt bt c Traceback most recent call last File lt stdin gt line 1 in lt module gt TypeError unsupported operand type s for int and str gt gt gt c trace Traceback most recent call last File lt stdin gt line 1 in lt module gt TypeError unsupported operand type s for str and bool Here I attempted to perform operations on incompatible types You re not allowed to adda number to a word or take a yes no answer away from it It is necessary to convert the data to a compatible type before trying to process it You can add words together or take numbers away from each other just like you can in real life but you can t do arithmetic on a line of text The tracebacks are Python s way of alerting you to a potential error in your logic in this case a TypeError This tells me that I need to rewrite the code to make it clear what type of information I want to put in and get out of the equation The purpose of data types is to allow us to represent information that exists in the real world that is the world that exists outside your computer as opposed to the virtual world inside We can have the existential conversation about what is real and what is not some other time The previous example uses variables of type int whole numbers and type str text It will quickly become apparent that these basic data types can only represent the simplest unit
380. t gt gt fruits avocados bananas oranges grapes mangos gt gt gt fruits 2 oranges The first thing you will notice is that indexing is zero based that means you start counting at 0 An index of 2 accesses the third item in the list the first item would be referenced with 0 So you can use integers 0 through to the number of elements in the sequence minus one 0 ton 1 as indices Negative indices count backward from the end gt gt gt fruits 1 mangos You can grab sections of a sequence using slices Slicing is used to fetch multiple items from a sequence Slices are written using the same notation as an index but this time with two or three integers separated by colons The first value is the inclusive starting point and the second number is the exclusive end point of the slice So s 0 2 means that the slice will start from index 0 and stop just before index 2 i e fetch items at positions 0 and 1 The third value is optional and specifies an additional step value which may be negative so instead of picking out a sequential list you can retrieve every other or every nth item and you can also retrieve them backward if you need to So s i j step will retrieve a slice of s starting from i and up to but not including j and taking the specified step gt gt gt fruits avocados bananas oranges grapes mangos gt gt gt fruits 2 2 oranges gt gt gt fruits 2
381. t doesn t have to recompile the file Python s actual running speed doesn t change The converse of this is also true so if you really have to reload a module or recompile its bytecode version as long as you don t do it too often you shouldn t see too much of a performance hit Python also keeps track of the timestamp of the original text file it records it in the bytecode That way if you edit the text Python notices the change in timestamp and recompiles the bytecode Finally Python actually executes your module This might come as a surprise because you re importing modular code into your program and you don t want it running itself as if it were a separate process But don t worry this is execution in a much broader sense When Python interprets a def foo statement or class bar statement it s actually executing that statement it bundles up the method or class definition in its internal representation and hangs that bundle off the current scope under the name foo or bar respectively A consequence of this behavior is that all the code in your modules really is executed Any control flow method calls and so forth in your module outside of class or method definitions are also executed at this time Indeed your module might need to run some setup tasks at the point of import so this is potentially useful behavior This is why if you want to run your module as a standalone file you have to put that code in an if else state
382. t field is a placeholder for a variable value in a formatted piece of text The field name is replaced by the value of the variable it refers to Format specifier The format specifier field contains a specification of how a value in a replacement field should be presented Greedy This type of regular expression will match as many characters as possible Nongreedy This type of regular expression will match as few characters as possible Regular expression This refers to a pattern that can match a piece of text Tags These additional marks are added to a text file to specify the manner of formatting Tags are used by markup languages to indicate paragraphs headings bold and italic text and so on Tarball This refers to an archived group of one or more files The files need not be compressed however An archived and compressed file might end in tar gz Unicode This method of defining characters is suitable for languages throughout the world It allows for the representation of the many different characters required to make up international character sets i e alphabets Whitespace This refers to any character that prints out as space in text including tabs t newlines n and so on as well as the actual space character This is equivalent to the regular expression t n r f v Wildcard The character can match any other character in a pattern match 159 CHAPTER 7 WORKING WITH TEXT Summary As you ve seen tex
383. t is integral to most Python programs and you saw how often of our examples use it We take text input from users manipulate that text and display messages in response This is why Python comes with so many text related features In this chapter you learned how to split and join strings format edit and search for strings use regular expressions to search for patterns within strings and work with the files on your file system We then applied much of this in our example application 160 CHAPTER 8 Executable Files Organization and Python on the Web Up to this point you ve been running your scripts via the python interpreter This is all well and good but it would be nice if we could get the scripts to run by themselves just like real programs so that s just what we ll look at first l ll then cover how to organize and spruce up your code because clean organized code makes you more efficient and gives you time to write even more lovely Python code This leads us onto some of the dynamic features of Python modules and the exec and eval functions These features give you more flexibility and choice when writing Python applications The final topic in this chapter and another whole world of choice is Python on the Web I ll go over a simple CGI form to show you how to process data on a web form Making Programs Executable as Stand Alone Applications Assuming you are using a Unix compatible computer a couple of things
384. t module and then you change the name of the convert method to change and use imp reload you will see the following behavior gt gt gt import convert imp gt gt gt convert convert lt function convert at 0xb7c1a82c gt gt gt gt Rename the convert method to change and reload imp reload convert lt module convert from convert py gt gt gt gt convert change lt function change at Oxb7c6f62c gt gt gt gt convert convert lt function convert at 0xb7c1a82c gt If you also edited the method when renaming it convert convert will still exhibit the old functionality The old definition is not stored anywhere permanently though so if you restart the interactive prompt the old method will be lost If you have renamed convert to change you might want to change it back to avoid any confusion when following the examples in this chapter Python Module Internals When you ask Python to import a module it performs three main tasks First it finds the module file on disk by looking in a number of specific directories Next it pulls the module into memory interpreting it into an internal representation Finally it executes this representation Each of these phases is worth a bit of explanation so you have more of an idea of what Python s doing for you should anything go wrong Python will find any module in a directory on the PYTHONPATH This has a different use from that of the PATH discussed
385. t of a better name I m going to call this program GtkInfo so to organize the files I have created a folder in Projects called gtkinfo see Figure 12 3 gt Ei ile Edit View Places Help Name Size MIME Type Type wy AUTHORS 23 bytes text x authors author list a ChangeLog 482 bytes text x changelog ChangeLog document rwx J index html 2 9 KB text html HTML document aadebug txt 3 0 KB text plain plain text document NEWS 110 bytes text plain plain text document amp Alsa png M Hardware png A Kernel png a Qjackctl png Sound png 1 4 KB image png PNG image 1 6 KB image png PNG image 1 9 KB image png PNG image 613 bytes image png PNG image 2 6 KB image png PNG image E gtkinfo pyc ea gtkinfo py w README g 9tk_info svg Hi Mixer svg 5a Save_As svg 12 9 KB 16 0 KB 299 bytes 44 5 KB 13 4 KB 18 6 KB application x python bytecode text x python text x readme image svg xml image svg xml image svg xml gtkinfo v 16 items Free space 693 3 MB Figure 12 3 The gtkinfo directory Python bytecode Python script README document SVG image SVG image SVG image The original Bash commands were taken from http alsa opensrc org index php Aadebug all I have done is turn the sequences of Bash instructions into Python strings Creating the GUI As in the previous example the entire graphic interface is defined as a single class GtkInfo The graphic interface is built by the
386. t of this book I will focus on the specifics of the Python programming language and you will learn how to construct more complex applications 25 www allitebooks com CHAPTER 3 Variables and Data Types In the previous chapter you learned that a variable is a unit of data with an identifier which is held in your computer s memory it can be changed by putting a new value into it or modifying the value that is already there In this chapter I will be introducing some of the different types of variable that are available for you to use in your programs and Ill be showing you how to build them into the expressions and statements of Python that will allow you to turn your designs into working code This is where you start to do some real programming You will be creating two programs from scratch in this chapter one to manipulate and format simple text strings and a script that performs a mathematical calculation All this is made possible by using variables Using variables allows you to specify a calculation or method for getting a result without having to know what values those variables will refer to beforehand Any information that is put into the system must be turned into a variable before you can do anything to it and it will be the contents of a variable that finally get sent back to the user that called the program Choosing Good Identifiers Identifiers are the names used to identify things in your code Python will regard any wor
387. t out a message Considering Other Needs or Limitations You may also want to include other needs or limitations of the software which are called nonfunctional requirements Nonfunctional requirements define other constraints and qualities required of the software such as accessibility usability by the intended users cost effectiveness compatibility with other systems security and performance You probably won t need to worry too much about such things if you are just programming for your own amusement Defining Acceptable Results You need to give some thought as to how the software will be tested Early on it will be enough to simply run the program and see if it works but as your scripts grow in complexity you will find it useful to formalize the methods used for testing It may be worth finding or creating some data to use as an example for testing purposes and you may find it useful to create a table of values that you expect to find at various points in the process In Chapter 11 I ll be showing you how to create built in test suites which can make a large part of the process automatic It s generally considered good practice to write your tests before you do any coding but it often happens that the tests and code actually get written at the same time For now a note of the expected output will do fine Considering Maintenance Any piece of software that is going to be used more than once will require some degree of mainten
388. t strings is just a different way of writing a single string In other words you can t write a string using a string and a variable together like this Note It is possible to join strings together using the sign In the context of strings this is called the concatenation operator an operator is a command that often consists of a symbol or combination of symbols placed between two variables known as operands just like you would write a mathematical equation The situation is further complicated because the sign is also used to add numbers together as you will see in the next section this is called overloading Using the sign to concatenate words can slow down the execution of your program to an alarming degree For this reason recommend that you don t use this method if you need to join lots of strings some more efficient ways of joining strings together will be covered in Chapter 7 Using Quotes Character is the term used to describe a single letter number or punctuation mark Strings of characters intended to be displayed as text are known as string literals or just strings If you want to tell the interpreter that a block of text is meant to be displayed literally as text you have to enclose the text in quotation marks these can take several different forms A text string enclosed by double quotes and all on one line A text string enclosed by single quotes all on one line again A text string including mult
389. t tChange change type print inflicting a damage report damage type and life changing change type Inflict damage on target players target life damage Check whether target is still alive or not if players target life lt 0 Print loser print print players target Name collapses in a pool of blood Remove loser from players list del players target print End this round of combat immediately break if trace print print t tmax damage velocity dam max vel max min vel min print Print winner print players 0 Name wins the fight Any further changes that I can think of right now get noted in a new TODO file in the Projects subdirectory It s really sensible to keep sane hours and know when to stop It will probably be obvious from here that you could continue the process of abstraction until the only line left in the main body is something like game and everything else is contained within a function There are some valid reasons why you might want to do reduce your code to only one line and it could be said that the next level of evolution in programming style object oriented programming takes it even further Before expanding into those dizzy horizons further refinements that can be made at this level Function oriented programming is a major change in approach and there are many tips tricks and potential pitfalls to be aware of CHAPTER 6 FUNCTIO
390. t will import from the first module which matches the import statement As you can see the list always begins with an empty string so Python will always look in the current directory first Next in the list are the colon delimited terms in the PYTHONPATH if it has been set Finally if no match is found in any of these locations the standard library locations are always appended to the list so those libraries are always available to Python However any similarly named module in the current directory or on the PYTHONPATH will override them When the module is found the Python interpreter has to do some work to turn your text into something that matches Python s internal representation of code This compilation happens before it actually executes the contents of the module and exposes the functionality As it only needs to be done once assuming a module is not changed during Python s execution cycle then it tries to save the compiled version as a pyc file the c is for compiled You should have seen pyc files while following the examples in this chapter The compiled files are in bytecode which is platform independent this means that they don t depend on your operating system That s only really useful if your files are on a network share or are otherwise accessible by people running Python under more than one operating system Note that when Python saves a compiled file it s just speeding up its future compilation phase because i
391. tProt self self prot int self getSpeed self getBrainz 2 roll 49 1 2 if 0 lt self prot lt 100 pass else self prot int rol1l 33 3 def getProt self return self prot def setGold self self gold int roll 40 4 def getGold self return self gold def setInv self self inv def getInv self return join self inv def generate self Role Playing Character generator Takes no arguments Returns a new Player object print print New Test Character print Prompt user for user defined information Name Desc Gender Race name input What is your name desc input Describe yourself gender input What Gender are you male female unsure race input What Race are you Pixie Vulcan Gelfling Troll CHAPTER 9 E CLASSES self setName name self setDesc desc self setGender gender self setRace race self setMuscle self setBrainz self setSpeed self setCharm self setLife self setMagic self setProt self setGold This may look complex but consider that I have moved the functionality of the old generate rpc function entirely into the Player type Now new players can be generated in just two lines in the main body player1 Player player1 generate Parents and Children Inheritance In a simple sense children inherit features from their parents So in a similar sense you might have a class object that inherit
392. the actual code became really straightforward if gender female sentence phrase replace themself herself elif gender male sentence phrase replace themself himself else sentence phrase replace themself itself return sentence 112 CHAPTER 6 FUNCTIONS Having ascertained that the function call worked as expected it was now time to put a call in place to fix the bothersome output of the missed shot handler which follows the velocity calculation in the combat section I simply needed to wrap a reference to misses miss type up in a call to the new function fix gender players attacker Gender misses miss type The best way to test this function was to choose a weak weapon like a club for both combatants to increase the chance of a miss The results were satisfactory It would be easy enough to expand the range of replacements if I needed to at any point in the future but it wouldn t tick any boxes to spend any more time on it at this stage The Matrix Refactored The last issue I wanted to look at was how to make it possible to have more than two combatants I wanted to be able to create a list of matches and iterate through that for the combat section Before making the big change in functionality I made a couple of abstractions in the hope of increasing readability and ease of future development Again I started with a couple of empty declarations def calc velocity attacker
393. the built in round function price total length price per metre print t t t t t round price 2 And print out the result print You need round total length 2 meters of cloth costing round price 2 The following results represent three tests of the new script Enter the height of the window cm 200 Enter the width of the window cm 200 width height widths total price shorter wider 170 0 215 0 False True 4 3 You need 4 3 metres of cloth costing 21 5 MAKING CHOICES 63 64 CHAPTER 4 MAKING CHOICES Enter the height of the window cm 200 Enter the width of the window cm 300 width height widths total price shorter wider 245 0 215 0 False True 6 45 32 25 You need 6 45 metres of cloth costing 32 25 Enter the height of the window cm 300 Enter the width of the window cm 200 width height widths total price shorter wider 170 0 315 0 False True 6 3 31 5 You need 6 3 metres of cloth costing 31 5 This new version of the program almost gets there The tests on 200x200 cm and 200x300 cm produce expected results which shows that I m on the right track But the last test for 300x300 cm still underestimates the amount of material I d need This is probably because I haven t properly accounted for the additional material required for joins If you are familiar with the process of curtain making or even good at algebra you could refine the process further t
394. the documentable contents of the module are output Next any modules you might need are imported Each import should be on a separate line and modules should be grouped according to where they come from Only then does the real coding begin One of the reasons for this ordering is that some items need to be set up before other parts of the program can use them modules have to be imported before they are called on to perform their duties variables need to be assigned before they can be referenced and functions must be declared before they are called Additionally before you use anything it must be in scope An import an assignment ofa variable and a function definition are all just ways of getting a variable in the current namespace It follows that the next things to be set are constants and global variables that is data that won t change its value or need to be available to several different parts of the program This is followed by the definition of any classes or functions that you are going to need The main body of the program comes last of all and in this example is very short and slightly cryptic Listing 8 1 style_guide py usr bin env python coding UTF8 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Docstring Short title and summary on first line followed by a new line style guide py A Docstring should be usable as a Usage A description of the script s function command line synta
395. the last good habit I d like to introduce you to before I move on to discussing the Python language in depth As mentioned earlier in this chapter Python is self documenting to a certain extent Modules classes and functions may be documented by placing a text string as their first statement just as I have done with the specification details in this example The simplest way to test this is to start Python in interactive mode from the same directory as the hello world py script python Python 3 1 1 1311 74483 Aug 17 2009 17 02 12 GCC 4 2 3 Debian 4 2 3 5 on linux2 Type help copyright credits or license for more information The three chevrons are the Python prompt which means the interpreter is ready to receive your first command Next you need to import the script you have just written as a module using the import statement gt gt gt import hello world Hello World Type in some words Some Text Some Text You will see that the program runs in exactly the same way However this method allows us to easily access the documentation you have created help is a built in function that takes one argument the name of the module class function keyword or topic that you want to find out about Notice that I didn t use the py extension or enclose the name of the module in quotes this is because it became a CHAPTER 2 M DESIGNING SOFTWARE Python object when I imported it so I need to refer to the script as
396. the role of the user what that user wants the software to do and what benefit might be gained from it User stories are prioritized by the user to indicate which features are most important and are then used to work out an estimate of the time it will take to develop those features by the developer The developer may then create use cases out of these stories and tests to determine whether the requirements have been fulfilled Use cases are longer more detailed descriptions of features that are required by specific users in specific situations You just need enough information to work out the basic functionality that will be required from the program you are about to write Try to keep it concise Devising a Solution Armed with all this information let s return to the text file to complete the first stage of the software design The design stage can often be the most time consuming part of the programming process It consists of several stages in which the output from one stage becomes the input to the next Often the output from one or more stages is fed back through the same process several times This is known as recursion when it happens inside a program The whole process of software design has a similarly recursive nature To celebrate this fact let s go back to the initial problem Understanding the Problem This time around you re going to fill in a little more detail and turn your text file into a very simple user requirements doc
397. thing like Listing 2 1 Listing 2 1 hello_world txt Problem Get the computer to output a message Target Users Me Target System GNU Linux Interface Command line Functional Requirements Print out a message User must be able to input some text Testing Simple run test expecting a message to appear Maintainer maintainer website com Breaking Down the Solution into Steps Next you start the process of turning your wish list into a program In this stage you will design a logical flow of actions which will hopefully produce the desired outcome First you need to turn the text file into something the Python interpreter can understand by saving it as a Python py file Use the Save As function in your text editor to save the file with the same name but this time with a py extension This example will become hello world py Now the interpreter needs to know what this text is otherwise it will attempt to interpret the first thing it finds as a command There are a variety of ways of marking out blocks of text as comments or text strings so that the interpreter doesn t attempt to execute them Comments The hash symbol is used to mark comments Comments are completely ignored by the Python interpreter All text between the sign and the end of the line is ignored This formatting is a convenience so you the programmer can write little notes to yourself to remind yourself what this part of the code is supposed to do or to f
398. this is the directory where an individual user s personal files are kept This keeps them separate from operating system files and the files of other users Summary A lot of information has been covered in the short space of this chapter You have learned how to create stand alone applications organize your projects lay out your code use modules to accept command line arguments and input from stdin execute and evaluate arbitrary strings and create a custom namespace Finally you have learned how to write a simple CGI script that can receive input from a web based form You can now create proper programs that will work in the same way as any other application you have installed on your system Your end users don t need to know anything about what language you wrote it in or the details of the code they only need to know what commands are available via the interface you have created for them It s up to you whether the program is self explanatory or in what form you provide help files If in doubt there is nothing wrong with plain text Caution You now have the power to mess up really badly Of course you will probably get away with many minor errors and indiscretions but now is a really good time to start considering security issues during the testing phase You are now ready for the next coding paradigm shift full object oriented programming Nearly everything in Python is an object of some type or another and in the next cha
399. thout putting the module name and dot in front of it like so gt gt gt argv e The list is empty because I m looking at it from the command line As I mentioned the import statement provides an alternative version in case importing something like this would cause a clash with a name that you are already using in the script gt gt gt from sys import argv as arguments gt gt gt arguments Ded This gives you a lot of choice in the way you import functions and objects into your programs Python has a lot of specialized and extended functionality that can be imported from modules which have been prewritten and tested by other programmers You have already seen the re and time modules in action and have imported your own scripts to view their help output In Chapter 11 I ll give you an overview of some of the most commonly used modules Listing 8 2 shows a simple example of how to use command line arguments and how to read information from stdin and write it to stdout CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Listing 8 2 sysargs py usr bin env python coding UTF8 Using command line arguments This script can receive command line arguments and data from stdin The output of any command can be fed into the script via the pipe command echo Foo sysargs py bar baz Datestamp 2008 11 12 Command line arguments 0 sysargs py 1 bar 2 baz Stdin Foo impo
400. tical order It seems that sequences have some kind of implicit sort order even if they haven t been sorted yet It turns out that it is possible to use this implied order to compare sequences using the same comparison operators that are used to compare the fundamental data types gt gt gt fruits gt vegetables False gt gt gt vegetables gt fruits True gt gt gt vegetables fruits False The sequences fruits and vegetables both contain five members Nevertheless the Python interpreter is able to determine that the two sequences are not the same and decide that one is greater as a whole than the other Note that empty sequences will always return False in when tested in Boolean expressions This is where the similarities between strings tuples and lists end Tuples A tuple is an immutable ordered group of items or elements Think of tuples as useful little sealed packets of information A tuple is specified as a comma separated list of values which may be enclosed in parentheses On certain occasions the parentheses are required so when in doubt use parentheses The values need not all be of the same type A value can also be another tuple Creating a Tuple You can create a tuple with zero items using an empty pair of round brackets gt gt gt blank tuple If you want to make a tuple that only contains one item you must follow the item with a single comma item 79 80 CHAPTER 5 M USING
401. till not perfect Somehow I d like the pattern to match potential commas and spaces if they exist before the opening chevron Matching Subpatterns I don t want to match just any old character here I m looking for a specific pattern but the problem is that it may or may not be present The solution is to enclose the subpattern in parentheses followed by a question mark which makes the subpattern optional gt gt gt pattern gt gt gt re split pattern string fetes None Z ny my D j vu 5 et Yu This kind of works but we ve collected a lot of extraneous items Putting inside the parentheses at the beginning stops the subpattern from being returned but it is still counted as part of the match gt gt gt pattern gt gt gt re split pattern string 5 Z D j V Yu Regular expressions are not easy on the eyes As you can see we ve already built up a pattern that looks like a sentence in some alien language I have to be honest I m afraid it gets worse If you want to match an actual punctuation mark or special character e g or or brackets you ll have to escape each character with a backslash For example would match an actual period Some ordinary characters also have a special meaning if preceded by a backslash Table 7 5 shows these special characters Table 7 5 Escaped Characters with Special Meaning in Regul
402. tion call This statement executes calls a function Recall from Chapter 2 that a function is a named block of code that performs a task accepts zero one or more arguments and returns a value to the calling procedure when it is complete A function call consists of the name of the function followed by a list of arguments enclosed in parentheses e Function definition This statement creates a new function specifying its name its parameters and the statements it executes e Generalization When you generalize code you replace a piece of code that is only useful in specific circumstances with something that can be used more generally e Global These are the variables defined in the main program e Keyword arguments These values are passed to a function by name 123 124 CHAPTER 6 FUNCTIONS e Lambda expression A lambda expression is a disposable one line function without a name e Local This variable is defined inside a function and can be used only inside its function e Modification procedure This function changes its arguments inside the function body Only mutable types can be changed by modifiers e Modularization To modularize a program you split it into several files or subprograms e Object An object is a bundle of attributes variables and related methods functions Software objects are often used to model real world objects you find in everyday life Almost all entities in Python are objects or p
403. to remind us that their behavior is somewhat different to sequences Note Dictionary like structures are referred to in some other languages as associative arrays or hashes because the values used to define the keys must be hashable values Hashed keys are small values calculated from the actual key and are an efficient way of storing potentially large keys This makes value lookup much quicker Python organizes the information in the dictionary according to these hashes rather than an alphabetical list It follows that if you change the contents of a hashable value the resultant hash would also change and the computer would no longer be able to locate the value it contains Now you know why keys have to be of an immutable data type Defining Dictionaries Dictionaries are created by specifying key value pairs separated by colons inside curly braces gt gt gt profile Name Desc Race Gender Muscle 0 Brainz 0 Speed 0 Charm 0 gt gt gt profile Name Adam gt gt gt print profile Name has profile Brainz Brainz Adam has 0 Brainz You can refer to items in a dictionary using the familiar square bracket notation Remember we accessed items in a list as item i where i is the position of the item In dictionaries there are no positions because all we have are keys and their respective values Hence we just use the key itself to access its corresponding value in the dictio
404. to stubs content def convert file filename Convert a file to pirate speak return convert string open filename read Next save some text that you d like to convert to pirate speak You can call the file whatever you want but for the purposes of the example output I ve called it example txt Still in the same directory as all of these files run the interactive prompt and type the following commands gt gt gt import convert gt gt gt convert convert string He sleeps all night and he works all day He sleeps all night and he works all day me hearties gt gt gt open example txt read Mister Pudey the very real problem is one of money I m afraid that the Ministry of Silly Walks is no longer getting the kind of support it needs You see there s Defence Social Security Health Housing Education Silly Walks They re all supposed to get the same gt gt gt convert convert file example txt Yarr mister Pudey the very real problem is one of money I m afraid that the Ministry of Silly Walks is no longer getting the kind of support it needs me hearties You see there s Defence Social Security Health Housing Education Silly Walks Yarr they re all supposed to get the same The convert module can now be used in conjunction with the strings module to convert large blocks of text to pirate speak You can change the weightings variable in convert to make particular piratical convers
405. to the menubar and its show method run The menubar also has to be packed into place The exact command varies from widget to widget but the principle remains the same Next I made a container for the two main panels of the application on the left is a list of options and the right hand pane displays the retrieved information The process of filling the space proceeds in a similar manner to the menus First I created two scrolled windows to contain the views setting the to CHAPTER 12 SIMPLE WINDOWED APPLICATIONS gtk POLICY AUTOMATIC so that the scrollbars will disappear if the information fits inside the space In order to display the list of options I will use the gtk TreeView object which provides a file browser like display Actually I m just going to use it to display a list rather than a tree but that doesn t matter immediately I can fill it with information later Next I connect the cursor changed signal to a self on treeview1 cursor changed method stub so I can get a response when the user clicks a different option All this functionality is packed into place and then filled with relevant information by self create treelist The second scrolled window is filled with a gtk TextView widget which displays the text buffer that I created earlier Again all the widgets are packed into place and the horizontal box is displayed The last piece in the puzzle is a Quit button which is packed into the bottom of the vert
406. tput is different to allow a price list to be displayed def init self name Somewhere store super init name store 215 216 CHAPTER 9 M CLASSES self commands look print 0 s buy player buy 0 def str self stock list O s 10 1 gt 3 format item item gold for item in self inventory if issubclass type item Thing view lt name s desc s Stock List o s lt 0000 format n join stock list vars self return view def trace text Verbose output for trouble shooting purposes Takes same arguments as print if trace on print lt lt text return def join with and sequence Join up a list with commas and and between last two items Takes a sequence and returns a sentence Make sure all the list items are in string format sequence str item for item in sequence if len sequence gt 1 last item sequence 1 sentence join sequence 1 sentence sentence and last item elif len sequence lt 1 sentence whole lot of nothing else sentence sequence 0 return sentence def roll sides dice 1 Dice rolling simulator sides Number of sides the die has dice number of dice to be rolled defaults to 1 Returns a random number between dice and dice sides weighted towards the average CHAPTER 9 E CLASSES result 0 for rolls in r
407. ts Muscle Brainz Speed Charm profile Muscle roll 33 3 profile Brainz roll 33 3 profile Speed roll 33 3 profile Charm rol1l 33 3 Work out combat stats life magic prot gold life profile Muscle profile Speed 2 roll 49 1 2 magic profile Brainz profile Charm 2 roll 49 1 2 117 118 CHAPTER 6 FUNCTIONS def prot gold profile Speed profile Brainz 2 rol1 49 1 2 roll 40 4 Validate stats if 0 lt life lt 100 profile life else profile life if O lt magic lt 100 profile magic magic else profile magic if 0 lt prot lt 100 profile prot prot else profile prot roll 33 3 profile gold gold life roll 33 3 roll 33 3 Output the character sheet fancy line lt 4 gt print print fancy line print t profile Name print t profile Race profile Gender print t profile Desc print fancy line print print tMuscle profile Muscle tlife profile life print tBrainz profile Brainz tmagic profile magic print tSpeed profile Speed tprotection profile prot print tCharm profile Charm tgold profile gold print return profile buy equipment Purchase an item of equipment Takes no arguments This function modifies the current ch
408. tswith m self gender male else self gender neuter def getGender self return self gender gender property getGender setGender 203 204 CHAPTER 9 M CLASSES def setRace self race race race capitalize if race startswith P self race Pixie elif race startswith V self race Vulcan elif race startswith G self race Gelfling elif race startswith T self race Troll else self race Goblin def getRace self return self race race property getRace setRace def setStrength self self strength roll 33 3 def getStrength self return self strength strength property getStrength def setBrainz self self brainz roll 33 3 def getBrainz self return self brainz brainz property getBrainz def setSpeed self self speed r011 33 3 def getSpeed self return self speed speed property getSpeed def setCharm self self charm roll 33 3 def getCharm self return self charm charm property getCharm def setLife self value CHAPTER 9 E CLASSES self life int value def getLife self return self life life property getLife setLife def setMagic self value self magic int value def getMagic self return self magic magic property getMagic setMagic def setProt self value self prot int value def getProt self return self prot prot property getProt
409. turn Gtk has another preformed dialog for the About dialog box so I can fill in the on about stub using that For the help pages I used a different approach The os system method allows external applications to be launched in a fairly straightforward way it just takes the command line as an argument After all that trouble packing widgets into boxes making them functional turned out to be much easier def on about self widget args About Dialog What it says on the box print About Gtk Info logo self icon dialog gtk AboutDialog dialog set name Gtk Info dialog set version str version dialog set authors maintainer dialog set documenters maintainer dialog set logo logo comment A graphical interface for displaying system information dialog set comments comment response dialog run if response 6 dialog destroy return def on help self widget args we Help Launches help page in a web browser os system usr bin sensible browser index html return The on treeview1 cursor changed method is called when a new selection is made and it calls get selection which you saw previously The delete event method is called when the Quit menu option is clicked def on treeview1 cursor changed self widget args Qption panel 278 CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Gets the icon that received the click and displays relevant information self get
410. tware These numbers may refer to major or minor changes and are generally assigned in increasing order The method used for assigning these numbers is referred to as a versioning system Summary This chapter has explained some of the technical terms that you will encounter as you study the art of programming and has introduced you to some sources of further information to help you as you grow in understanding So far you have made sure that the right version of Python is installed on your system and you know how to get to the Python command prompt You should have selected a text editor to use for writing and saving your scripts but you haven t written any code yet In the next chapter I will show you how to start designing programs from the initial seed of a problem that you wish to solve CHAPTER 2 Designing Software If you are anything like me you impatiently started reading this book at Chapter 3 became really confused about halfway through and decided to start at the beginning and read it all properly this time This chapter explains the bit that most manuals leave out how to actually turn your problem into a working program The first stage of the software development cycle is the creation of a program design In this chapter I ll demonstrate how to break down your ideas into manageable chunks understand each problem that you are trying to solve build up a sequence of actions keep track of what you ve done and above all h
411. u have self gold gold left else print You don t have enough gold to buy that return purchase def strike self target Calculate velocity of hit or miss Takes one argument target is another Player object This method looks up values from the players and returns a weighted semi random integer representing the velocity of the strike weapon self weapon CHAPTER 9 E CLASSES armour target armour attack chance roll1 99 attack velocity self speed weapon speed attack chance target velocity target prot armour speed roll target brainz velocity attack velocity target velocity return int velocity class Thing GameObject Tools and Treasure Takes four mandatory arguments Thing name gold strength speed where name is a string and gold strength speed are integers between 0 99 Things are largely immutable and have no public methods Attributes may be retrieved using index notation def setStrength self value self strength value def getStrength self return self strength strength property getStrength def setSpeed self value self speed value def getSpeed self return self speed speed property getSpeed def init self name gold strength speed self setGold gold self setStrength strength self setSpeed speed self setName name self setDesc def str self return str self name def repr self stats Thing name r gol
412. ual characters including spaces and punctuation Other forms of sequence are tuples and lists Tuples and lists are sequences of data items the chief difference between them being that lists are editable in place whereas tuples are not the distinction will be fully explained in Chapter 5 It s possible to use either in a for statement They are constructed as follows tuple sequence1 1 2 3 list sequence2 1 2 3 I ll show you how to use and construct lists and tuples fully in the next chapter The for statement then makes it possible to process several pieces of data in only a couple of lines of code So in the example below the variable banana contains each element in the splits sequence in turn as the for loop goes through each iteration gt gt gt splits Fleagle Beagle Drooper Snorky gt gt gt for banana in splits print banana Fleagle 69 70 CHAPTER 4 E MAKING CHOICES Beagle Drooper Snorky Using a for statement allows us to dispense with variables in loop conditions and means we can simply go through a sequence and do the same actions on each member of the sequence in turn This turns out to be incredibly useful for example we can go through a line of text and print each character out on a separate line of its own text input Type in some words for character in text print character Or we can iterate through a list of items of data and process each one sepa
413. ubtracted and multiplied the same as in the real world using the and operators the is the multiply sign This creates an expression An expression is a piece of code that has to be evaluated i e worked out before you can discover its value You can assign expressions to variables using statements like this gt gt gt muscle 2 3 gt gt gt brainz 7 3 gt gt gt speed 5 4 gt gt gt strangeness muscle brainz speed gt gt gt strangeness Note When you are using Python in interactive mode you don t need to use the print function to see the value of a variable You will need print if you want to get output from a script All this works much as expected until the last line where strangeness equals 75 How did that happen Surely 5 4 9 multiplied by 20 would give 180 But no what happens here is this 2 3 7 3 4 5 CHAPTER 3 VARIABLES AND DATA TYPES How does Python decide that the arguments to the are muscle and brainz speed rather than just muscle and brainz What happens here is that Python works out the arguments to before it works out the arguments to and it picks as these arguments the smallest bits of the expression that make sense in this case brainz and speed The multiplication operator is said to bind tighter than the addition operator Understanding Operator Precedence What you have just seen is an example of something much more general
414. umbers yet 2 32 gt 5 1 5 0 2 gt 7 2 12 0 2 gt 3 3 15 0 CHAPTER 4 E MAKING CHOICES gt 8 4 23 0 gt 1 gt gt gt Nesting Conditional Statements You are allowed to nest loops and other conditional statements in Python probably infinitely but it is best to keep the number of levels of nesting to a minimum For one thing it s very easy to get confused about which option the program is taking at any particular point Also having lots of indented blocks within other indented blocks makes your code difficult to read can slow down the program s execution and is generally considered bad style If you have come up with a design that involves two or three layers of looping you should probably start thinking about redesigning it to avoid the excessive depth of nesting Using the for Statement The other control flow statement I want to introduce is the for statement which is constructed in a similar manner to the if and while statements Its construction is for element in sequence followed by an indented suite of instructions During the first iteration of the loop the variable element contains the first element in the sequence and is available to the indented suite During the second iteration it contains the second element in the sequence and so on To understand how this statement works you need to know about sequences The simplest sequence in Python is a string which is a sequence of individ
415. ument In a professional situation this would become the agreement between the programmer and the client who is paying for the software to be written For now you re just going to use it as a means to keep a record of the original purpose of this software Make sure that the problem is stated clearly and that you have identified the issue you wish to resolve correctly Often there can be hidden subtleties in the problem that look straightforward when written down in your natural language 13 14 CHAPTER 2 DESIGNING SOFTWARE but require additional clarification when you come to turn them into programming language or code If the problem is not defined well enough to make a decision at that point you will need to return to this stage and clear up the details before you can make any further progress Knowing What the Software Needs to Do The things you want the software to do are known as functional requirements in the trade Simple examples include printing a message adding up some numbers up or sending an e mail form You might have additional details to add for example maybe the user must be able to input some text numbers larger than 200 cannot be entered or the update must be performed over the Internet Functional requirements specify how a system should behave For your first exercise Hello World you will be asking the software to print out a simple message so your first functional requirement is that the computer should prin
416. un it Listing 10 2 Testing Division Methods with Basic Exception Handling Method without exception handling def divide by x y return x y Method with exception handling def divide by safe inaccurate x y try return divide by x y except ZeroDivisionError return 10000000000000000 print Safe 3 2 print Unsafe 3 print Safe 3 0 0 g format divide by safe inaccurate 3 2 0 g format divide by 3 2 0 g format divide by safe inaccurate 3 0 I N I print Unsafe 3 0 0 g format divide by 3 0 Listing 10 2 will produce the following output Safe 3 2 1 5 Unsafe 3 2 1 5 Safe 3 O 1e16 Traceback most recent call last File basic handling py line 15 in lt module gt print Unsafe 3 0 0 g format divide by 3 0 File basic handling py line 3 in divide by return x y ZeroDivisionError int division or modulo by zero In Listing 10 2 we cause the unsafe method to raise an exception only at the very end the program terminates at the first uncaught exception If there were any code after the exception is raised it wouldn t be run It s important to remember that exceptions are created within both divide by and divide by safe when the division operator notices that its second argument is zero But the 223 224 CHAPTER 10 EXCEPTIONS try except individe by safe inaccurate tells the method how to handle the exception so it is rais
417. us to mark out an entire block of text along with all the line breaks I m going to use triple quotes in a specialized way to create a documentation string The block of text in Listing 2 1 is going to form the basis of your documentation so let s enclose it in triple quotes A text string that occurs as the first statement of any module function or class becomes a Python docstring which makes the text available as part of the built in documentation for this application It won t do anything yet but Listing 2 2 is now a legal Python file the interpreter won t complain if you try to run it Listing 2 2 Creating a Framework for hello_world py Problem Get the computer to output a message Target Users Me Target System GNU Linux Interface Command line Functional Requirements Print out a message User must be able to input some text Testing Simple run test expecting a message to appear Maintainer maintainer website com This is just a comment Organizing the Tasks into Steps Now you are prepared to start designing your program The program design is initially sketched out in pseudocode which is the design language that can be used as an intermediary step to turn the user requirements document into a piece of Python code There is nothing special about the form of this language your own natural way of expressing ideas is exactly what is needed in this situation Pseudocode is simply a series of logical statements in your
418. versed to implement reverse iteration It should return a new iterator object that iterates over all the objects in the container in reverse order The membership test operators in and not in are normally implemented as an iteration through a sequence However container objects can supply the following special method with a more efficient implementation which also does not require the object be a sequence The object contains self item method is called to implement membership test operators It should return True if the item is in self False otherwise For mapping objects it should consider the keys of the mapping rather than the values or the key item pairs for sequences it should search through the values If you want to create a fully dictionary like object you also need to include the standard mapping methods keys values items get clear setdefault pop popitem copy and update These methods should behave in a similar manner to the methods included in Python s standard dictionary objects Mutable sequences should provide methods append count index extend insert pop remove reverse and sort like Python s standard list objects 195 196 CHAPTER 9 M CLASSES Finally sequence types should implement concatenation using the addition operator and repetition using the multiplication operator by defining the methods add radd iadd mul xrmul and imul they sh
419. ving version 3 0 has become the norm and has evolved into version 3 1 1 The new version which I ll refer to as version 3 0 because all 3 x versions are simply refinements on the original plan of 3 0 includes several changes to the programming language that are incompatible with version 2 x x is any number you like which I will refer to in the rest of this book as the old version Most of the language is the same however so the differences between the versions of Python will be noted in the text as those subjects are covered Examples in this book are for Python 3 0 except where noted CHAPTER 1 INTRODUCING PYTHON Further information about the latest version of Python 3 0 also known as Py3k or Python 3000 is available at http www python org download releases There may be some differences running Python on other operating systems which I will do my best to point out where relevant Otherwise the examples of code will work the same way This is one of the many good points of Python For the most part this book will concentrate on the fun part learning how to write programs using Python The official Python documentation is plentiful free and well written and you should read it alongside this book It is available on at http www python org doc Understanding the Jargon Throughout this book will be doing my best to demystify clarify and explain the various bits of technical terminology that you will frequently encounter
420. which a floating point number should be rounded The final character determines how numerical data should be presented These characters will work on integers You should not specify a precision value if you expect an integer Table 7 3 Integer Format Specifiers Option Description b The binary option outputs the number in base 2 c The character option converts the integer to the corresponding Unicode character before printing d The decimal integer option outputs the number in base 10 o The octal format option outputs the number in base 8 x This hexadecimal format option outputs the number in base 16 using lowercase letters for the digits above 9 X This hexadecimal format option outputs the number in base 16 using uppercase letters for the digits above 9 n The number option is the same as d except that it uses the current locale setting to insert the appropriate number separator characters None This option is the same as d Here s an example that formats the same number in different ways gt gt gt 0 gt 8b format numbers 1 101010 gt gt gt 0 gt 8c format numbers 1 gt gt gt 0 gt 8d format numbers 1 There are some additional type characters to represent floating point numbers and exponents CHAPTER 7 WORKING WITH TEXT Table 7 4 Float Format Specifiers Option Description e This exponent notation specifier prints the number in scientific notation using the l
421. who are accustomed to the command line but this audio information that could be useful to complete beginners too so I want to create a graphical point and click version I m going to use the bits of Bash script as they are by importing the commands module commands getoutput allows me to run Bash commands and pass the results back to Python This program will also use os system to launch external applications The pseudocode outline for such an application might look like this Assign BASH commands to strings named lt option gt func Create GUI Create Icons Create clipboard Create a new window Create menubar FILE menu Save gt Create save As dialog amp specify file to open for writing Quit gt Quit the application EDIT menu Copy gt Copies selected information to the clipboard HELP menu About gt What it says on the box Help gt Launch browser with help page Create option list panel Create list of options get selection from option list Create command line according to option chosen Execute command line and display in text output panel CHAPTER 12 SIMPLE WINDOWED APPLICATIONS Create text output panel Create QUIT button Display GUI and wait for user signals The immediate issue that needs facing here is that this design is going to require several additional files to be available to the application several icon images and an HTML help page For wan
422. wn in the following printout of the character sheet and shop listing lt 7 ss H OR 44 H gt Ez Pixie female Tall and wild lt 0 ss Fr 844 H gt Muscle 46 life 30 Brainz 37 magic 30 Speed 28 protection 23 Charm 46 gold 67 lt SHOP dagger 25 shield 15 club 15 armour 101 pole 10 halberd 80 hammer 99 cuirass 30 flail 50 rope 10 sword 60 lantern 10 lt You have 67 gold Storing Data The last two TODOs on lines 291 and 332 go together I want an Alternate option to read characters from file andto save rpcharacters to file How hard can that be I wondered I ll spare you the statistics of how many brain cells I fried over the years trying to figure out these tasks and cut to the chase There is a major gotcha involved in retrieving data from text files whatever format your data was originally in as soon as you write it to a text file it becomes a text string Got that I ll run it by you again In this example I want to save the list of player profile dictionaries to a file The dictionaries contain a variety of data types two tuples a list several strings and lots of integers Once saved to a text file these all become string values with no special meaning to the Python interpreter In fact all this data becomes one big string I could write a really complicated function that isolates the different types of 156 CHAPTER 7
423. x with my home directory shown in Figure 8 1 If I want to make an application available to the whole system I prefer to make a link in bin as I did in the myscript py example Also I could add 1ib python3 0 to my PYTHONPATH which is a list of directories that are searched for modules to be imported by the import statement PYTHONPATH HOME 1ib python3 0 PYTHONPATH export PYTHONPATH Putting my private directory first means it will be searched before the others CHAPTER 8 E EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB File Edit View Places Help Name v Size MIME Type Type a gt e doc 5 items x directory normal folder v Bir 2 items x directory normal folder b io python2 5 175 items x directory normal folder b ea python3 0 44 items x directory normal folder v E Projects 7 items x directory normal folder b es html_update 32 items x directory normal folder b 2 mrnatural 7 items x directory normal folder b ed multimediainfo 28 items x directory normal folder 7 b 3 pythagoras 25 items x directory normal folder v ea rpcombat 7 items x directory normal folder ie CHANGELOG 119 bytes text plain plain text document ie NEWS 22 bytes text plain plain text document al README 1 3 KB text x readme README document e rpcombat py 12 4 KB text x python Python script Ey rpcombat pyc 11 5 KB application x python bytecode Python bytecode TODO 12 bytes text plain plain text document aT usb y AUTHO
424. x and parameters notes about the algorithm used and a decent usage example This script takes no arguments and prints out a useless message message It should contain The closing triple quote should be on a line by itself Version information goes immediately after the docstring with a blank line before and after These variables with double leading and trailing underscores are magic objects which should only be used as documented version 0 1 status Prototype date 31 10 2008 maintainer maintainer website com credits Thanks to everyone Import any modules we might need Standard library imports import os import re Related third party imports local application library specific imports Globals and constants Set variables whose values won t change before any others Global private global This variable is only used inside the program Class declarations would normally happen about here Classes are covered in the next chapter def code layout Tndentation 167 CHAPTER 8 M EXECUTABLE FILES ORGANIZATION AND PYTHON ON THE WEB Use 4 spaces per indentation level Never mix tabs and spaces In fact just use spaces line length Limit all lines to a maximum of 79 characters for big blocks of text 72 chars is recommended You can use the implied line continuation inside and or a backslash to wrap long lines of text Always keep comments up t
425. y My base class Thing has a method called setValues which sets up a bunch of internal attributes and returns no value This process is known as initialization and it can be easily converted into a constructor method class Thing def init self price strength speed self price price self strength strength self speed speed Now new objects of type Thing can be created with a single instruction that passes it the three required arguments box Thing 0 0 0 How cool is that Now I want my Weapon subclass to be set up slightly differently so I need to createan init method for that as well In the previous example Weapon had its own setValues method that overrides the method set on its parent object but this involves rather a lot of repetitive code It would be much better if I could callthe init method of the superclass Thing and then add the one line that needs to be different class Weapon Thing def init self price strength speed Thing init self price strength speed self damage strength Notice that I need to pass the same arguments to Thing init Calling a special method like this directly referencing its name with double underscores is called referencing an unbound method There is an alternative method super which is preferred The super function is smarter than calling an unbound method and usually does the right thing when dealing with unusual situations such as inheritance from mu
426. y as other characters in a regular expression unless you need to catch square brackets or dashes You can specify alternative patterns after the choice operator which is also known as the pipe symbol gt gt gt pattern W a Sm w gt gt gt re split pattern string ety Z D j V Yu Finding Patterns at the Beginning or End of a String Sometimes you will be looking for a pattern to occur specifically at the beginning or end of a sequence of characters The caret can be used to specify the start of a sequence and the dollar sign can be used to indicate the end Note that the caret works as a negation operator inside square bracketed character sets so it can t be used to indicate the start of a sequence gt gt gt pattern a gt gt gt re split pattern string ry 1 Z9 a D j V a3 hu CHAPTER 7 WORKING WITH TEXT gt gt gt pattern p gt gt gt re split pattern string at Z a D j Vu a Creating a Regular Expression Object You can compile a regular expression pattern into a regular expression object using re compile pattern flags This returns a pattern object on which you can call all the previous methods but you no longer need to provide the pattern as an argument You might want to do this if you need to use this pattern to perform lots of matches all at once compiling the pattern into an
427. y from one of the standard types it is better to subclass that type and override the methods that you need to work differently In order to implement basic container like behavior you need to include two specific methods e object len self This method should return the length of the object that is the number of items contained within the object expressed as an integer gt 0 This integer is returned by the built in len function If you don t definea bool method or nonzero method the object will return False in a Boolean context e object getitem self key This method should return the value corresponding to the given key as in self key For sequence types the accepted keys should be integers and slice objects For a mapping you can use any type of key Note The special interpretation of negative indexes is up to the getitem method I implemented both of these methods for my Thing type as I want to be able to access items as if using a dictionary However I need my Things to be immutable once set the values can t be changed My Weapon and Armour types will also inherit this behavior but it only needs to be specified once in the parent Thing class 194 CHAPTER 9 E CLASSES def len self return len vars self def getitem self key names vars self try item names key except item 0 return item If I wanted to make a mutable type I would need to implement another two meth
428. y inefficient combined with lots of calls to the print function or statement in Python 2 using the operator can potentially slow your program s execution to a crawl Python isn t that slow Often it works out better to manipulate a list of words and then use string join sequence to return a string that is a concatenation of the strings in the sequence This method is the opposite of string split the data you want to manipulate is in the sequence of the argument and the string that you call the method on is just the string of characters you want to use to separate the items This could be a space or an empty string gt gt gt sO example gt gt gt s1 text gt gt gt sep gt gt gt sep join so s1 example text Remember that string join is expecting a sequence of strings as an argument gt gt gt sep gt gt gt sep join potrzebie p o t r z e b i e You may need to convert other data types into strings and join up any sublists first so that you present the outermost join with a list of strings gt a 1 gt gt gt b 2 37 gt gt gt sep join skidoo a b Traceback most recent call last File lt stdin gt line 1 in lt module gt TypeError sequence item 1 expected str instance list found This problem is easily fixable with a little ingenuity gt gt gt sep join skidoo sep join str a str b skidoo 1 2 37 This is where you need to
429. yer in enumerate players target int not bool attacker with matches ziply range 0 len players for attacker target in matches This retrieves a matrix of list indices from the ziply function which is then iterated through for each round of combat Once this change has been tested and passed with the two contestant setup then and only then is it time to find out what happens if I increase the max players count In theory that is all I should have to do now I shouldn t have to go searching through the code to replace random variables because I sensibly put this declaration clearly in the preferences section at the beginning of the code page Of course it may not be quite that simple I have already been staring at one line that is clearly expecting only two players I increased the value to three and continued with the testing The new method worked surprisingly well but there was an issue with the conflict resolution as I expected The combat ends when the first character dies with that character s attacker declared as the winner I now need to rewrite the code so that the combat continues until there is only one player left standing The most logical and robust way of doing this seemed to be to remove the dead character from the player list using del players target and make the while loop dependent on that list having more than one entry using while len players gt 1 This is much cleaner The program isn t perfect
430. yle py fix style py Purpose Python script style amp version checker Target System GNU Linux Interface Command line Functional Requirements Automatically go through all lines in a Python file pulling out TODO comments and appending TODO file check amp update AUTHORS file check for existence of README NEWS and ChangeLog So all documentation can be auto created from main python files Print warnings and prompt before any changes are made version 0 1 status Prototype date 16 10 2008 maintainer maintainer website com credits Inspired by Duncan Parkes remark about inline TODO comments import re import time Datestamp Now time strftime d m Y Set up some regular expressions for later use Putting them all in one place makes editing and troubleshooting much easier filename format re compile py version format re compile 0 9 0 9 0 9 status format re compile Prototype Alpha Beta RC Stable date format re compile 0 3 0 9 01 0 9 0 9 4 email format re compile todo format re compile s s TODO todo FIXME fixme s Dictionary to hold namespace variables of file spellbook version 0 0 status date 33 13 2023 WORKING WITH TEXT 149 150 CHAPTER 7 WORKING WITH TEXT maintainer author credits def get file
431. you will be able to come back to your programs and see immediately what they were supposed to do and what your original intentions were this makes maintaining programs much simpler too OK let s make sure your system is ready for you to start running Python programs Running Python on Various Systems Python can be installed and run on most operating systems including Windows Mac OS X or OS 2 Linux and Unix If you are running Mac OS X or a GNU Linux system you will probably have it installed by default I would recommend using a system of this kind which already has Python set up as an integral part The book was written using a Debian GNU Linux system and therefore the examples will CHAPTER 1 INTRODUCING PYTHON work exactly the same on any Debian based system such as Ubuntu In fact the differences between Linux variants are few so you ll be equally at home with other distributions Installing on Windows If you are using Windows you will probably have to install Python and configure certain settings correctly before you can get to grips with the examples in this book For that you will need to refer to the specific instructions for your operating system on the following Python web pages e http wiki python org moin BeginnersGuide Download e http www python org doc faq windows e http docs python org dev 3 0 using windows html First you need to download the official installer alternative versions for Itaniu

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