Python Environment Variables

Environment variables—known to some as shell variables, or DOS variables—are system-wide settings that live outside Python and thus can be used to customize the interpreter’s behavior each time it is run on a given computer. Python recognizes a handful of environment variable settings, but only a few are used often enough to warrant explanation here. Table A-1 summarizes the main Python-related environment variable settings.

Table A-1. Important environment variables

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Variable

Role

PATH (or path)

System shell search path (for finding “python”)

PYTHONPATH

Python module search path (for imports)

PYTHONSTARTUP

Path to Python interactive startup file

TCL_LIBRARY, TK_LIBRARY

GUI extension variables (tkinter)

These variables are straightforward to use, but here are a few pointers:

PATH

The PATH setting lists a set of directories that the operating system searches for executable programs. It should normally include the directory where your Python interpreter lives (the python program on Unix, or the python.exe file on Windows).
You don’t need to set this variable at all if you are willing to work in the directory where Python resides, or type the full path to Python in command lines. On Windows, for instance, the PATH is irrelevant if you run a cd C:\Python30 before running any code (to change to the directory where Python lives), or always type C:\Python30\python instead of just python (giving a full path). Also, note that PATH settings are mostly for launching programs from command lines; they are usually irrelevant when launching via icon clicks and IDEs.

PYTHONPATH

The PYTHONPATH setting serves a role similar to PATH: the Python interpreter consults the PYTHONPATH variable to locate module files when you import them in a program. If used, this variable is set to a platform-dependent list of directory names, separated by colons on Unix and semicolons on Windows. This list normally includes just your own source code directories. Its content is merged into the sys.path module import search path, along with the script’s directory, any path file settings, and standard library directories.
You don’t need to set this variable unless you will be performing cross-directory imports—because Python always searches the home directory of the program’s top-level file automatically, this setting is required only if a module needs to import another module that lives in a different directory. See also the discussion of .pth path files later in this appendix for an alternative to PYTHONPATH. For more on the module search path, refer to Chapter 21.

PYTHONSTARTUP

If PYTHONSTARTUP is set to the pathname of a file of Python code, Python executes the file’s code automatically whenever you start the interactive interpreter, as though you had typed it at the interactive command line. This is a rarely used but handy way to make sure you always load certain utilities when working interactively; it saves an import.

tkinter settings

If you wish to use the tkinter GUI toolkit (named Tkinter in 2.6), you might have to set the two GUI variables in the last line of Table A-1 to the names of the source library directories of the Tcl and Tk systems (much like PYTHONPATH). However, these settings are not required on Windows systems (where tkinter support is installed alongside Python), and they’re usually not required elsewhere if Tcl and Tk reside in standard directories.

Note that because these environment settings are external to Python itself, when you set them is usually irrelevant: this can be done before or after Python is installed, as long as they are set the way you require before Python is actually run.

Getting tkinter (and IDLE) GUI Support on Linux

The IDLE interface described in Chapter 2 is a Python tkinter GUI program. The tkinter module (named Tkinter in 2.6) is a GUI toolkit, and it’s a complete, standard component of Python on Windows and some other platforms. On some Linux systems, though, the underlying GUI library may not be a standard installed component. To add GUI support to your Python on Linux if needed, try running a command line of the form yum tkinter to automatically install tkinter’s underlying libraries. This should work on Linux distributions (and some other systems) on which the yum installation program is available.