Note to self: .pth

I think I finally got .pth files sorted out. I first ran into this with easy_install (here). The Python library docs are here.

A path configuration file is a file whose name has the form package.pth and exists in one of the four directories mentioned above; its contents are additional items (one per line) to be added to sys.path. Non-existing items are never added to sys.path, but no check is made that the item refers to a directory (rather than a file). No item is added to sys.path more than once. Blank lines and lines beginning with # are skipped. Lines starting with import (followed by space or tab) are executed.

The four directories are:

sys.prefix + lib/site-packages
sys.prefix + lib/site-python
sys.exec_prefix + lib/site-packages
sys.exec_prefix + lib/site-python

So, not all directories on the path are searched for .pth files. I made a nested directory:

/Users/telliott/Desktop/temp/x

and put a .pth file in site-packages:

> cat /Library/Python/2.6/site-packages/x.pth /Users/telliott/Desktop/temp/x

To test it:

> python -c 'import sys; L = [p for p in sys.path if "Desk" in p]; print "\n".join(L)' /Users/telliott/Desktop/temp/x

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Of course, I could have just added a path to PYTHONPATH in ~/.bash_profile:

export PYTHONPATH=$PYTHONPATH:$HOME/Desktop/temp

> python -c 'import sys; L = [p for p in sys.path if "Desk" in p]; print "\n".join(L)' /Users/telliott/Desktop/temp /Users/telliott/Desktop/temp/x

You need to restart Terminal to see the effect of changes to PYTHONPATH.

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It's worth emphasizing (as pointed out in the quote) that .pth files can contain code. Here is easy_install.pth:

> cat /Library/Python/2.6/site-packages/easy-install.pth import sys; sys.__plen = len(sys.path) ./Sphinx-0.6.3-py2.6.egg ./docutils-0.6-py2.6.egg ./Jinja2-2.2.1-py2.6.egg ./Pygments-1.1.1-py2.6.egg ./rpy2-2.1.7_20101104-py2.6-macosx-10.6-universal.egg ./numpy-1.5.0-py2.6-macosx-10.6-universal.egg ./Cython-0.14-py2.6-macosx-10.6-universal.egg ./PIL-1.1.7-py2.6-macosx-10.6-universal.egg import sys; new=sys.path[sys.__plen:]; del sys.path[sys.__plen:]; p=getattr(sys,'__egginsert',0); sys.path[p:p]=new; sys.__egginsert = p+len(new)

It seems roundabout, but easy_install.pth is first adding these paths to sys.path, and then moving them to the position in the path specified by sys.__egginsert.