Question

The Python documentation says that sys.path is “Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.”

But what is the “installation-dependent default” exactly for Windows?

(I know this is probably dependent on how python was compiled, but if all I have is the binary, is there any way to figure out how the default sys.path is constructed?)

Clarification:
I am not asking “What is my sys.path?”. I want to know “how does Python construct sys.path?” Documentation says that sys.path is constructed with sys.path[0] being the script’s current directory, plus whatever Python finds in the PYTHONPATH environment variable, plus some installation-dependent voodoo. So what is this mysterious voodoo part?

Answer 1

Seems like Praveen Gollakota has good info at Troubleshooting python sys.path (repasted here:)

• 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, 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 anyway. 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.