Beginning Python - From Novice to Professional

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CHAPTER 10 ■ BATTERIES INCLUDED 209 ENVIRONMENT VARIABLES Environment variables are not part of the Python interpreter—they’re part of your operating system. Basically, they are like Python variables, but they are set outside the Python interpreter. To find out how to set them you should consult your system documentation, but here are a few pointers: In UNIX, you will probably set environment variables in some shell file that is executed every time you log in. If you use a shell such as bash, the file is .bashrc, found in your home directory. Add the following to that file to add the directory ~/python to your PYTHONPATH: export PYTHONPATH=$PYTHONPATH:~/python Note that multiple directories are separated by colons. Other shells may have a different syntax for this so you should consult the relevant documentation. In Windows, you may be able to edit environment variables from your Control Panel (in reasonably advanced versions of Windows, such as Windows XP, 2000, and NT; on older versions such as Windows 98, this does not work, and you would have to edit your autoexec.bat file instead, as covered in the next paragraph). From the Start menu, select Start ➤ Settings ➤ Control Panel. In the Control Panel, double-click the System icon. In the dialog box that opens, select the Advanced tab and click the Environment Variables button. That brings up another dialog box with two tables: one with your user variables and one with system variables. You are interested in the user variables. If you see PYTHONPATH there already, select it and click Edit, and edit it. Otherwise, click New and use PYTHONPATH as the name; enter your directory as the value. Note that multiple directories are separated by semicolons. If the previous tactic doesn’t work, you can edit the file autoexec.bat, which you can find (assuming that you have a relatively standard setup) in the top directory of the C drive. Open the file in Notepad (or the IDLE text editor, for that matter) and add a line setting the PYTHONPATH. If you want to add the directory C:\python you type the following: set PYTHONPATH=%PYTHONPATH%;C:\python For information on setting up Python in Mac OS, see the MacPython pages at http://cwi.nl/~jack/ macpython. Depending on which operating system you are using, the contents of PYTHONPATH varies (see the sidebar “Environment Variables”), but basically it’s just like sys.path—a list of directories. ■Tip You don’t have to change the sys.path by using PYTHONPATH. Path configuration files provide a useful shortcut to make Python do it for you. A path configuration file is a file with the file name extension .pth that contains directories that should be added to sys.path; empty lines and lines beginning with # are ignored. Files beginning with import are executed. For a path configuration file to be executed, it must be placed in a directory where it can be found. For Windows, use the directory named by sys.prefix (probably something like C:\Python22), and in UNIX, use the site-packages directory. (For more information, look up the site module in the Python Library Reference. This module is automatically imported during initialization of the Python interpreter.)
210 CHAPTER 10 ■ BATTERIES INCLUDED Naming Your Module As you may have noticed, the file that contains the code of a module must be given the same name as the module—with an additional .py file name extension. In Windows, you can use the file name extension .pyw instead. You learn more about what that file name extension means in Chapter 12. Packages To structure your modules, you can group them into packages. A package is basically just another type of module. The interesting thing about them is that they can contain other modules. While a module is stored in a file (with the file name extension .py), a package is a directory. To make Python treat it as a package, it must contain a file (module) named __init__.py. The contents of this file will be the contents of the package, if you import it as if it were a plain module. For example, if you have a package named constants, and the file constants/__init__.py contains the statement PI = 3.14, you would be able to do the following: import constants print constants.PI To put modules inside a package, simply put the module files inside the package directory. For example, if you wanted a package called drawing, which contained one module called shapes and one called colors, you would need the files and directories (UNIX pathnames) shown in Table 10-1. Table 10-1. A Simple Package Layout File/Directory ~/python/ ~/python/drawing/ ~/python/drawing/__init__.py ~/python/drawing/colors.py ~/python/drawing/shapes.py Description Directory in PYTHONPATH Package directory (drawing package) Package code (“drawing module”) colors module shapes module In Table 10-1, it is assumed that you have placed the directory ~/python in your PYTHONPATH. In Windows, simply replace ~/python with c:\python and reverse the direction of the slashes (to backslashes). With this setup, the following statements are all legal: import drawing # (1) Imports the drawing package import drawing.colors # (2) Imports the colors module from drawing import shapes # (3) Imports the shapes module
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Beginning Python From Novice to Pro
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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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140 CHAPTER 7 ■ MORE ABSTRACTION
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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314 CHAPTER 15 ■ PYTHON AND THE W
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358 CHAPTER 17 ■ EXTENDING PYTHON
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CHAPTER 18 ■ ■ ■ Packaging Yo
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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CHAPTER 29 ■ PROJECT 10: DO-IT-YO
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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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■INDEX 583 ■F Factory class twi
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■INDEX 595 Python C API 365 hand-
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■INDEX 601 TestCase class methods
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■INDEX 603 further exploration 47
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