set PYTHONPATH for a directory?

[Neal Becker]

I'm testing some software I'm building against an alternative version of a 
library.  So I have an alternative library in directory L.  Then I have in an 
unrelated directory, the test software, which I need to use the library version 
from directory L.

One approach is to set PYTHONPATH whenever I run this test software.  Any 
suggestion on a more foolproof approach?