Troubleshooting python sys.path

Question

The python docs at http://docs.python.org/library/sys.html say that sys.path is...

Initialized from the environment variable PYTHONPATH, plus

Answer 1

I had some issues recently with sys.path and here is how I went about trying to figure out where the entries are coming from. I was able to track all the entries and where they were coming from. Hopefully this will help you too.

• The first that is added C:\WINNT\system32\python27.zip (more details in PEP273).

• Next ones that are added are from entries in windows registry. The entries C:\Python27\DLLs;C:\Python27\lib; C:\Python27\lib\plat-win; C:\Python27\lib\lib-tk come from HOT_KEY_LOCAL_USER/Python/PythonCore/2.7/PythonPath in the registry. More details in Python source code comments here http://svn.python.org/projects/python/trunk/PC/getpathp.c (These entries were the trickiest for me to understand until I found the link above).

• Next, as explained in the site package documentation (link), sys.path is built from sys.prefix and sys.exec_prefix. On my computer both of them point to C:\Python27. And by default it searches the lib/site-packages anywways. So now the entries C:\Python27; C:\Python27\lib\site-packages are appended to the list above.

• Next it searches each of the .pth files in alphabetical order. I have easy_install.pth, pywin32.pth and setuptools.pth in my site-packages. This is where things start getting weird. It would be straightforward if the entries in the .pth files were just directory locations. They would just get appended to the sys.path line by line. However easy_install.pth has some python code that causes the entries listed in easy_install.pth to add the packages list at the beginning of the sys.path list.

• After this the directory entries in pywin32.pth, setuptools.pth are added at the end of the sys.path list as expected.

Note: While the above discussion pertains to Windows, it is similar even on Mac etc. On Mac it just adds different OS defaults like darwin etc. before it starts looking at site-packages directory for .pth files.

In your case, you can start by starting a python shell and checking where sys.prefix and sys.exec_prefix point to and then drilling down from there.

Note 2: If you are using an IDE such as Aptana/PyDev it will add more configurations of its own. So you need to be wary of that.