[Nix-dev] Python package works with PYTHONHOME but not with PYTHONPATH

[Nikolay Amiantov]

tl;dr: Why can a Python package behave differently when imported using
PYTHONPATH and PYTHONHOME? Specifically, work with *HOME but not with *PATH.

Hi all,

While packaging Gnome15[1] I stumbled across a strange problem. I've
cleaned up gnome2.gnome_python package to use the default build
system[2]. After that `import gconf` stopped working in the application.

It turned out that the package has been patched to move gconf.so to
site-packages instead of a gtk-2.0 subdirectory, and I have removed this
patch. While investigating why was it needed in the first place I
noticed that on an Arch machine the package is laid out the upstream
way[3] but it works. On a whim I've tried to import the same package in
a wrapped Python and lo and behold -- it worked.

To reproduce this behaviour:
1. Check out my gnome15 branch[4];
2. Run `nix-shell -A gnome15`;
3. Run python2; try `import gconf`; notice it fails;
4. Run $PYTHON; try `import gconf`; notice it works.

The difference is that python2 is a regular Python using $PYTHONPATH
that is automatically set in the build environment, and $PYTHON is a
wrapped Python using $PYTHONHOME (python.buildEnv)[5].

Why does it have different behaviour? Currently Python applications
wrapped with our magical sed expression behave similarly to PYTHONPATH
(i.e. can't import this package). I hope to fix this but I'm not sure
what's going on.

1: https://gnome15.org/
2:
https://github.com/abbradar/nixpkgs/commit/04da99f3a83d59d70d0965a72d761856b823554e
3: https://www.archlinux.org/packages/extra/i686/python2-gconf/files/
4: https://github.com/abbradar/nixpkgs/commits/gnome15
5:
https://github.com/abbradar/nixpkgs/commit/becf31fc204d023639edbbce0feb6076c4810d6c

-- 
Nikolay.