Beginning Python - From Novice to Professional

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CHAPTER 10 ■ ■ ■ Batteries Included You now know most of the basic Python language. While the core language is powerful in itself, Python gives you more tools to play with. A standard installation includes a set of modules called the standard library. You have already seen some of them (math and cmath, containing mathematical functions for real and complex numbers, for example), but there are many more. This chapter shows you a bit about how modules work, and how to explore them and learn what they have to offer. Then the chapter offers an overview of the standard library focusing on a few selected useful modules. Modules You already know about making your own programs (or scripts) and executing them. You have also seen how you can fetch functions into your programs from external modules using import: >>> import math >>> math.sin(0) 0.0 Let’s take a look at how you can write your own modules. Modules Are Programs Any Python program can be imported as a module. Let’s say you have written the program in Listing 10-1 and stored it in a file called hello.py (the name is important). Listing 10-1. A Simple Module # hello.py print "Hello, world!" Where you save it is also important; in the next section you learn more about that, but for now let’s say you save it in the directory C:\python (Windows) or ~/python (UNIX). Then you can tell your interpreter where to look for the module by executing the following (using the Windows directory): >>> import sys >>> sys.path.append('c:/python') 203
204 CHAPTER 10 ■ BATTERIES INCLUDED ■Note In the path of the preceding directory I used forward slashes, while on Windows, backslashes are the norm. Both are legal, but because backslashes are used to write certain special characters (such as newlines), the forward slashes are safer. If you use backslashes, you can use either a raw string (r'c:\python') or escape the backslash ('c:\\python'). What I did here was simply to tell the interpreter that it should look for modules in the directory c:\python in addition to the places it would normally look. After having done this, your can import your module (which is stored in the file c:\python\hello.py, remember?): >>> import hello Hello, world! ■Note When you import a module, you may notice that a new file appears—in this case c:\python\ hello.pyc. The file with the .pyc extension is a processed (“compiled”) Python file that has been translated to a format that Python can handle more efficiently. If you import the same module later, Python will import the .pyc file rather than the .py file, unless the .py file has changed; in that case, a new .pyc file is generated. Deleting the .pyc file does no harm (as long as there is an equivalent .py file available)—a new one is created when needed. As you can see, the code in the module is executed when you import it. However, if you try to import it again, nothing happens: >>> import hello >>> Why doesn’t it work this time? Because modules aren’t really meant to do things (such as printing text) when they’re imported. They are mostly meant to define things, such as variables, functions, classes, and so on. And because you only need to define things once, importing a module several times has the same effect as importing it once. WHY ONLY ONCE? The import-only-once behavior is a substantial optimization in most cases, and it can be very important in one special case: if two modules import each other. In many cases, you may write two modules that need to access functions and classes from each other to function properly. For example, you may have created two modules—clientdb and billing—containing code for a client database and a billing system, respectively. Your client database may contain calls to your billing system (for example, automatically sending a bill to a client every month), while the billing system probably needs to access functionality from your client database to do the billing correctly.
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Beginning Python From Novice to Pro
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viii ■CONTENTS Strings . . . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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110 CHAPTER 6 ■ ABSTRACTION But w
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256 CHAPTER 11 ■ FILES AND STUFF
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358 CHAPTER 17 ■ EXTENDING PYTHON
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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