When/how does Python use PYTHONPATH

Question

I'm having some trouble understanding how Python uses the PYTHONPATH environment variable. According to the documentation, the import search path ( sys.path ) is "Initialized from the environment variable PYTHONPATH , plus an installation-dependent default."

In a Windows command box, I started Python (v.2.7.6) and printed the value of sys.path . I got a list of pathnames, the "installation-dependent default."

Then I quit Python, set PYTHONPATH to .;./lib; , restarted Python, and printed os.environ['PYTHONPATH']. I got .;./lib; as expected. Then I printed sys.path . I think it should have been the installation-dependent default with .;./lib; added to the start or the end. Instead it was the installation-dependent default alone, as if PYTHONPATH were empty. What am I missing here?

Answer 1

It always uses PYTHONPATH . What happened is probably that you quit python, but didn't quit your console/command shell. For that shell, the environment that was set when the shell was started still applies, and hence, there's no PYTHONPATH set.