Wiley Python: Create - Modify - Reuse
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1. def functionName paramenterl parameter2 default_value lt code block gt return value optional Note two elements in the preceding example Parameters As you can see parameters can simply be a variable name making them required as part of the function call or they can have a default value in which case it is optional to pass them in the function call OQ The return statement This enables the function to return a value to the code that called it The nice thing about this is that you can run a function and assign its output to a variable Here s an example of a function definition gt gt gt def getname name return name is very hungry gt gt gt Calling a Function To call a function simply enter the function name with the function signature functionName paramenterl parameter2 If a parameter has a default value in its definition then you can omit that parameter when you call the function and the parameter will contain its default value Alternately you can override the default value by entering the value yourself when you call the function 11 Chapter 1 A Python Primer For example if a function were defined as follows def jimsFunc age name Jim Then you could call the function in any of the following three ways jimsFunc 23 jimsFunc 42 James jimsFunc 42 firstName Joe In the first example I simply took the default value for the first parameter in the sec2. A Python Primer This chapter provides a quick overview of the Python language The goal in this chapter is not to teach you the Python language excellent books have been written on that subject such as Beginning Python Wrox 2005 This chapter describes Python s lexical structure and programming conventions so if you are familiar with other scripting languages such as Perl or Ruby or with compiled programming languages such as Java or C you should easily be up to speed in no time Getting Started Of course the first thing you need to do is install Python if you don t already have it Installers are available for Windows Macintosh Linux Unix and everything from OpenVMS to the Playstation no I m not kidding Obtaining Python and Installing It If you go to www python org download you can find links to download the correct version of Python for your operating system Follow the install instructions for your particular Python distribution instructions can vary significantly depending on what operating system you re installing to What Version Number to Install Although the examples in this book should work for any Python version above 2 0 it is best to install the latest stable build for your operating system For Windows which is the environment I primarily work in the latest stable version is 2 51 There is an alpha build of Python 3 0 available as of this writing but other than just looking at it for fu
3. gt gt gt Note some points about Python s implementation of class programming as demonstrated in the preceding example Q If we were inheriting from other classes those class names would have been inside the parentheses of the class name1 definition In this case there is one class method setmyname If we wanted to create a constructor for the class it would be named __init__ Q To create an instance of a class you simply assign a variable to the class definition as in jimname namel Q Attributes are accessed with familiar dot notation instance variable attribute such as jimname name 14 Chapter 1 A Python Primer Summary This chapter provided a brief tour of the Python language including the following highlights How to get up and running with Python Python s lexical structure Operators expressions and statements Iteration and decision making Functions and modules Oocodoo0dodo Classes and object oriented programming Of course there is much more to the Python language than what this short chapter has outlined Much of it you ll discover as you work through the projects in this book Let s get started 15
4. complete statement However if you type print 12 you ll get a syntax error as shown here gt gt gt print 12 SyntaxError invalid syntax gt gt gt Clearly the system cannot read this because it isn t a complete statement so it results in an error Multi line Statements It is possible to have a single statement span multiple lines You could do this for aesthetic reasons or simply because the line is too long to read on one screen To do this simply put a space and a backslash at the end of the line Here are a few examples name Jim Knowlton sum 12 T3 Iteration and Decision Making There are two basic ways to control the flow of a program through iteration looping and through decision making Chapter 1 A Python Primer Iteration Iteration in Python is handled through the usual suspects the for loop and the while loop However if you ve programmed in other languages these seemingly familiar friends are a little different For Loops Unlike in Java the for loop in Python is more than a simple construct based on a counter Instead it is a sequence iterator that will step through the items of any sequenced object such as a list of names for instance Here s a simple example of a for loop gt gt gt names Jim Joe gt gt gt for x in names Pistia Jim Joe gt gt gt As you can see the basic syntax is for lt variable gt in lt object gt follow
5. is an environment variable set in the system Its value is a list of directories which Python will search for modules Standard library directories The directory in which the standard libraries are located are searched next 13 Chapter 1 A Python Primer Exploring sys path If you ever want to see your system s Python search path all you have to do is bring up the interactive interpreter import the sys module and type sys path The full Python module search path will be returned as shown in the following example gt gt gt import sys gt gt gt sys path C Python25 C Python25 Lib idlelib C Program Files PythonNet e scripts python c python25 C Python25 pyunit 1 4 1 c python25 pamie C WINDOWS system32 python25 zip C Python25 DLLs C Python25 lib C Python25 lib plat win C Python25 lib lib tk C Python25 lib site packages C Python25 lib site packages win32 C Python25 lib site packages win32 lib C Python25 lib site packages Pythonwin C Python25 lib site packages wx 2 8 msw ansi Bee Classes Python is a language that can support both procedural programming and object oriented programming Here is an example of a Python class gt gt gt class namel def setmyname self myname self name myname gt gt gt jimname namel gt gt gt jimname setmyname Jim gt gt gt print jimname name
6. prompt where you can run the Python interpreter you could simply use Notepad on Windows Vim on Linux Unix or TextEdit on Mac and a command line for running programs One nice step up from that is IDLE Python s integrated development environment IDE which is named after Monty Python s Eric Idle and is included with Python It includes the following useful features A full featured text editor Syntax highlighting Code intelligence A class browser A Python path browser A debugger DOCO OOD A Python interpreter environment Chapter 1 A Python Primer In addition to IDLE you do have other options On Windows there is a nice IDE called PythonWin developed by Mark Hammond It can be installed as a full Python distribution from ActiveState s website www activestate com or you can simply install the win32all package to add PythonWin to a standard Python for Windows install PythonWin is a great product very slick and with all the features you d expect from an IDE Other options include an Eclipse distribution for Python called EasyEclipse for Python For my money I d start out with IDLE and then as your experience with Python grows explore other options Lexical Structure Following is a simple Python program It shows the basic structure of many Python scripts which is as follows 1 Initialize variables lines 1 3 2 Do some processing lines 4 5 3 Make decisions and perform actions based on those de
7. resources 567893L Sequence Types String A list of characters Is immutable not changeable This is a string in place Can be represented by single quotes or rs double quotes Can span multiple lines This is an example of a DocString nnn List A mutable changeable sequence of data types 1 2 3 Jim List elements do not have to be like In other 1 2 3 words you could have a float element and an 1 5 2 7 3 0 integer element in a single list Jim Joe Bob Tuple An immutable sequence of data types Other 1 2 3 Jim than the fact that it can t be changed it works 1 2 3 just like a list 1 5 2 7 3 0 Jim Joe Bob Dictionary A list of items indexed by keys d first Jim last Knowlton Identifiers An identifier is a unique name that enables you to identify something Identifiers are used to label variables functions classes objects and modules They begin with either a letter or an underscore and they can contain letters underscores or digits They cannot contain punctuation marks Chapter 1 A Python Primer Operators If you have programmed in other languages the operators in Python will be familiar to you The Python operators are fundamentally similar to those used in other languages In the code shown earlier the conditions evaluated in both if statements involve comparison operators The follow
8. the action associated with an if statement must be indented if variablel Jim print variablel eqiuals Jim You ll learn more about the actual if statement itself later Data Types and Identifiers Python provides a rich collection of data types to enable programmers to perform virtually any programming task they desire in another language One nice thing about Python is that it provides many useful and unique data types such as tuples and dictionaries and stays away from data types such as the pointers used in C which have their use but can also make programming much more confusing and difficult for the nonprofessional programmer Chapter 1 A Python Primer Data Types Python is known as a dynamically typed language which means that you don t have to explicitly identify the data type when you initialize a variable In the code example above the variable name is assigned to the string value Jim However you don t specifically identify the variable as a string variable Python knows based on the value it has been given that it should allocate memory for a string Likewise for the age integer variable and the highschoo1GPA float variable The following table shows the most commonly used available data types and their attributes Data Type Attributes Example Numeric Types Float Implemented with C doubles 5 43 9483 123 Integer Implemented with C longs 1027 211234 Long Integer Size is limited only by system
9. cisions lines 6 10 name Jim age 42 highschoolGPA 3 89 enteredName raw_input Enter your name print n n if name Jim print Your age is age print You had a highschoolGPA GPA in high school if highschoolGPA gt 3 print You had better than a 3 0 GPA good job Keywords Keywords are words that are reserved they cannot be used as variable names In the preceding code the keyword if is used multiple times Chapter 1 A Python Primer The keywords are as follows and del for is raise assert elif from lambda return break else global not try class except if or while continue exec import pass def finally in print yield Lines and Indentation In Python unlike a compiled language such as C line breaks are significant and the end of a program statement is defined by a hard return Program blocks are defined by a combination of statements each on a separate line but with no end of statement character visible and program blocks delimited visually by the use of indentation As shown in the code from the preceding section lines are indented in Python This is not simply a stylistic choice indentation is not just recommended in Python but enforced by the interpreter This is probably the most controversial aspect of Python and it has been the subject of many a flame war online Basically it means that the following code would generate an interpreter error because
10. ed by the code block to be iterated While Loops Awhile loop is similar to a for loop but it s more flexible It enables you to test for a particular condition and then terminate the loop when the condition is true This is great for situations when you want to terminate a loop when the program is in a state that you can t predict at runtime such as when you are processing a file and you want the loop to be done when you reach the end of the file Here s an example of a while loop gt gt gt counter 5 See gt gt gt while x lt counter jougaliane Use Ste pink counter a counter x t 1 0 counter 5 x i counter 5 Soe 2 counter 5 SS 3 counter 5 Se ul counter 5 SS Chapter 1 A Python Primer Break and Continue As with C in Python you can break out of the innermost for or while loop by using the break statement Also as with C you can continue to the next iteration of a loop by using the continue statement What about switch or case Many of you familiar with other programming languages are no doubt wondering about a decision tree structure similar to C s switch statement or Pascal s case Unfortunately you won t find it in Python However the conditional if elif else structure along with other constructs you ll learn about later make their absence not such a big deal Decision Making When writing a program it is of course critical to be able to evaluate c
11. h or remember and you want to use a name to represent it that is meaningful to you simply use the as keyword and import lt module gt as lt identifier gt Reload is another very useful command especially when entering code within the Python interactive interpreter It enables you to reload a particular module without reloading Python For example if you wanted to reload the os module you would simply enter reload os If you re wondering why you would ever want to do that one scenario would be if you have a Python script that runs all the time and it accesses a module on another machine Assuming you always want to ensure that you re running the most current version of the remote module you re accessing you d use the reload command How Python Finds Modules to Load When you use an import statement you don t tell Python where the module that needs to be loaded is located How then does it know where to find the file The answer to that question is the module search path The Module Search Path Python has a predefined priority specifying where it should look for modules known as the module search path When you enter an import command and the name of the module Python checks the following locations in the order shown here 1 2 The home directory This is either the directory from which you launched the Python interactive interpreter or the directory where the main Python program is located PYTHONPATH This
12. ing table describes the operators most commonly used in Python and the ones used in this book Assignment Assignment Operator Operator Symbol Example Numeric Operators Addition x y Subtraction x y Multiplication x y Division x y Exponent Power x y x to the y power Modulo x y the remainder of x y Comparison Operators Greater than gt x gt y x is greater than y Less than lt x lt y x is less than y Equal to x y x equals y Greater than or equal to SS x gt y x is greater than or equal to y Less than or equal to lt x lt y x is less than or equal to y Not equal to or lt gt x y x lt gt y x does not equal y Boolean Operators and and x and y if both are true then the expression is true or or x or y if either is true then the expression is true not not not x if x is false then the expression is true X 15 name Jim Chapter 1 A Python Primer Expressions and Statements Expressions and statements are the building blocks of Python programs They are the equivalent of phrases and sentences in English To understand Python it s critical to understand how to put these building blocks together Expressions Expressions consist of combinations of values which can be either constant values such as a string Jim or a number 12 and operators which are symbols that act on the values in some way The following examples a
13. n I d steer clear of it for the examples in this book in some cases the syntax is very different and the examples in this book won t work with Python 3 0 Chapter 1 A Python Primer The Python Interpreter One of the most useful tools for writing Python code is the Python interpreter an interactive editing and execution environment in which commands are run as soon as you enter them and press Enter On Unix and Macintosh machines the Python interpreter can usually be found in the usr local bin python directory which can be accessed by simply typing the command python On Windows machines the Python interpreter is installed to the c python25 directory for a Python 2 5x installation To add this directory to your path type the following at a Windows command prompt set path path C python25 On a Windows system such as with Unix Linux you simply type python to bring up the interpreter either from the c python25 directory or from any directory if the Python directory has been added to the path When you enter the interpreter you ll see a screen with information like the following Python 2 5 1 r251 54863 Apr 18 2007 08 51 08 MSC v 1310 32 bit Intel on win32 Type help copyright credits or license for more information Se Your Editing Execution Environment Because the minimum requirements for writing and running Python programs are simply an editor that can save text files and a command
14. ond I replaced it with James Modules A module is the highest level programming unit in Python A module usually corresponds to a program file in Python Unlike in Ruby modules are not declared the name of the py file is the name of the module In other words basically each file is a module and modules import other modules to perform various programming tasks Importing Modules Importing modules is done with either the import or reload command Import To use a module you import it Usually import statements occur at the beginning of the Python module Importing modules is a fairly simple operation but it requires a little explanation Consider the following examples import os import os sys from os import getcwd import os as operatingSystem e UNE These examples highlight some variations in how you can import modules This first example is the simplest and easiest to understand It is merely the keyword import followed by the module name in this case os 2 Multiple modules can be imported with the same import command with the modules separated by a comma 12 Reload Chapter 1 A Python Primer You can import specific names only within a module without importing the whole module by using the from lt module gt import lt name gt statement This can be useful for performance reasons if you only need one function from a large module If a module has a name that s difficult to work wit
15. onditions and make decisions Having an if construct is critical for any language and Python is no exception The if Statement 10 The if statement in Python as in other languages evaluates an expression If the expression is true then the code block is executed Conversely if it isn t true then program execution jumps to the end Python also supports use of zero or more elif statements short for else if and an optional else statement which appears at the end if you also have elif statements and would be the default choice if none of the if statements were true Here s an example gt gt gt name Jim gt gt gt if name Jim print your name is Jim elif name Joe print your name is Joe else print I have no idea what your name is your name is Jim gt gt gt Chapter 1 A Python Primer Functions In many ways the principle behind a function is analogous to turning on a TV You don t have to understand all the electronics and communications technology behind getting the TV signal to your receiver in order to operate the TV You do have to know some simple behaviors however such as how to turn it on where the volume switch is and so on In a similar fashion a function gives the program an interface through which it can run program code without knowing the details about the code being run Defining a Function You define a function in Python with the following simple syntax
16. re expressions 10 4 11 4 5 x5 a b Operator Precedence in Expressions When you have a multiple expression like 5 4 7 which operation is done first the addition or the multiplication If it isn t too painful to recall your high school algebra class you might remember learning the rules of operator precedence These kinds of complex expressions require a set of rules defining which expressions are executed first The following list describes the basic rules of operator precedence in Python don t worry if you don t understand all the terms right now they Il be explained as you need them Expressions are evaluated from left to right Q Exponents multiplication and division are performed before addition and subtraction Q Expressions in parentheses are performed first Q Mathematical expressions are performed before Boolean expressions AND OR NOT Statements The statement is the basic unit of programming In essence it says do this to this Statements in Python are not delimited by a visible character such as the semicolon in C or C Every time you press Enter and start a new line you are entering a new statement Chapter 1 A Python Primer For example if you type Print 12 15 into the Python interpreter you ll get the following output gt gt gt print 12 15 27 gt gt gt This is because you told the system to print the result of the expression 12 15 which is a
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