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Changing the Module Search Path

In Chapter 21, we learned that the module search path is a list of directories that can be customized via the environment variable PYTHONPATH, and possibly via .pth files. What I haven’t shown you until now is how a Python program itself can actually change the search path by changing a built-in list called sys.path (the path attribute in the built-in sys module). sys.path is initialized on startup, but thereafter you can delete, append, and reset its components however you like:

>>> import sys
>>> sys.path
['', 'C:\\users', 'C:\\Windows\\system32\\python30.zip', ...more deleted...]

>>> sys.path.append('C:\\sourcedir')         # Extend module search path
>>> import string                            # All imports search the new dir last

Once you’ve made such a change, it will impact future imports anywhere in the Python program, as all imports and all files share the single sys.path list. In fact, this list may be changed arbitrarily:

>>> sys.path = [r'd:\temp']                  # Change module search path
>>> sys.path.append('c:\\lp4e\\examples')    # For this process only
>>> sys.path
['d:\\temp', 'c:\\lp4e\\examples']

>>> import string
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named string

Thus, you can use this technique to dynamically configure a search path inside a Python program. Be careful, though: if you delete a critical directory from the path, you may lose access to critical utilities. In the prior example, for instance, we no longer have access to the string module because we deleted the Python source library’s directory from the path.

Also, remember that such sys.path settings endure for only as long as the Python session or program (technically, process) that made them runs; they are not retained after Python exits. PYTHONPATH and .pth file path configurations live in the operating system instead of a running Python program, and so are more global: they are picked up by every program on your machine and live on after a program completes.