The sys.path List

If you want to see how the module search path is truly configured on your machine, you can always inspect the path as Python knows it by printing the built-in sys.path list (that is, the path attribute of the standard library module sys). This list of directory name strings is the actual search path within Python; on imports, Python searches each directory in this list from left to right.

Really, sys.path is the module search path. Python configures it at program startup, automatically merging the home directory of the top-level file (or an empty string to designate the current working directory), any PYTHONPATH directories, the contents of any .pth file paths you’ve created, and the standard library directories. The result is a list of directory name strings that Python searches on each import of a new file.

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Python exposes this list for two good reasons. First, it provides a way to verify the search path settings you’ve made—if you don’t see your settings somewhere in this list, you need to recheck your work. For example, here is what my module search path looks like on Windows under Python 3.0, with my PYTHONPATH set to C:\users and a C:\Python30\mypath.py path file that lists C:\users\mark. The empty string at the front means current directory and my two settings are merged in (the rest are standard library directories and files):

>>> import sys
>>> sys.path
['', 'C:\\users', 'C:\\Windows\\system32\\python30.zip', 'c:\\Python30\\DLLs',
'c:\\Python30\\lib', 'c:\\Python30\\lib\\plat-win', 'c:\\Python30',
'C:\\Users\\Mark', 'c:\\Python30\\lib\\site-packages']

Second, if you know what you’re doing, this list provides a way for scripts to tailor their search paths manually. As you’ll see later in this part of the book, by modifying the sys.path list, you can modify the search path for all future imports. Such changes only last for the duration of the script, however; PYTHONPATH and .pth files offer more permanent ways to modify the path.[50]