Here is my attempt to write the Python build system.  Even though it
mostly mimics the Perl build system, I failed to finalize it.  Could
you help me?

The main problem is that 'set-path-environment-variable' doesn't seem
to work.  I used Bazaar (don't forget to apply this patch [1] first)
to test the build system:

# /nix/var/nix/profiles/per-user/root/guix-profile/bin/bzr --help
bzr: warning: unsupported locale setting
  bzr could not set the application locale.
  Although this should be no problem for bzr itself, it might
  cause problems with some plugins. To investigate the issue,
  look at the output of the locale(1p) tool.
bzr: ERROR: Couldn't import bzrlib and dependencies.
Please check the directory containing bzrlib is on your PYTHONPATH.

Traceback (most recent call last):
  File "/nix/var/nix/profiles/per-user/root/guix-profile/bin/bzr", line 102, in <module>
    import bzrlib
ImportError: No module named bzrlib

(I guess that the locale-related error can be ignored.  Because it's
probably connected with my chroot.)

Bazaar seems to work if I export PYTHONPATH manually:

# export PYTHONPATH=/nix/store/gcsq72i07mlnpy912qjxwh052jq9fjxm-bazaar-2.3.1/lib/python2.7/site-packages

So what is the problem?

More questions:

1. How can I get the version of Python in 'python-build-system.scm'?
   (I hardcoded it for now.)

2. Can I remove the 'strip' phase?  Is it useful for Python packages?

There is a problem with Bazaar too.  Actually, it's a 'gnutls' problem.
I packaged Bazaar 2.3.1 because I can't get the hashes of the latest
versions.  For example:

# ./pre-inst-env guix download https://launchpad.net/bzr/2.5/2.5.1/+download/bzr-2.5.1.tar.gz

[...]

ERROR: missing interface for module (gnutls)

I've already seen this error [2], but the context was a bit different.

Is it possible (and necessary) to rewrite 'guix/build/download.scm' to
avoid similar problems in the future?

[1] https://lists.gnu.org/archive/html/bug-guix/2013-04/msg00011.html
[2] https://lists.gnu.org/archive/html/bug-guix/2013-02/msg00189.html