Beginning Python - From Novice to Professional

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.)