gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH.

Using PYTHONPATH as a mean to discover the Python packages had the following
issues:

        1. It is not versioned, so different versions of Python would clash if
        installed in a shared profile.

        2. It would interfere with the host Python site on foreign
        distributions, sometimes preventing a a user to login their GDM
        session (!).

        3. It would take precedence over user installed Python packages
        installed through pip.

        4. It would leak into Python virtualenvs, which are supposed to create
        isolated Python environments.

This changes fixes the above issues by making use of a sitecustomize.py
module.  The newly introduced GUIX_PYTHONPATH environment variable is read
from the environment, filtered for the current Python version of the
interpreter, and spliced in 'sys.path' just before Python's own site location,
which provides the expected behavior.

* gnu/packages/aux-files/python/sitecustomize.py: New file.
* Makefile.am: Register it.
* gnu/packages/python.scm (customize-site)
(guix-pythonpath-search-path): New procedures.
(python-2.7)[phases]{install-sitecustomize.py}: New phase.
[native-inputs]{sitecustomize.py}: New input.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
(python-3.9)[native-search-paths]: Likewise.
[phases]{install-sitecustomize}: Override with correct version.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
* gnu/packages/commencement.scm (python-boot0):
[phases]{install-sitecustomize}: Likewise.
[native-inputs]{sitecustomize.py}: New input.
[native-search-paths]: Replace PYTHONPATH with GUIX_PYTHONPATH.
* guix/build/python-build-system.scm (site-packages): Do not add a trailing
'/'.

squash! gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH.

1 files changed, 41 insertions, 0 deletions

diff --git a/gnu/packages/aux-files/python/sitecustomize.py b/gnu/packages/aux-files/python/sitecustomize.py
new file mode 100644
index 0000000000..65d3c7d554
--- /dev/null
+++ b/gnu/packages/aux-files/python/sitecustomize.py
@@ -0,0 +1,41 @@
+# -*- coding: utf-8 -*-
+# GNU Guix --- Functional package management for GNU
+# Copyright © 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+#
+# This file is part of GNU Guix.
+#
+# GNU Guix is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or (at
+# your option) any later version.
+#
+# GNU Guix is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import sys
+
+python_root = os.path.realpath(sys.executable).split('/bin/')[0]
+major_minor = '{}.{}'.format(sys.version_info[0], sys.version_info[1])
+site_packages_prefix = 'lib/python' + major_minor + '/site-packages'
+python_site = python_root + '/' + site_packages_prefix
+
+try:
+    all_sites_raw = os.environ['GUIX_PYTHONPATH'].split(':')
+except KeyError:
+    all_sites_raw = []
+# Normalize paths, otherwise a trailing slash would cause it to not match.
+all_sites_norm = [os.path.normpath(p) for p in all_sites_raw]
+matching_sites = [p for p in all_sites_norm
+                  if p.endswith(site_packages_prefix)]
+
+# Insert sites matching the current version into sys.path, right before
+# Python's own site.
+sys_path_absolute = [os.path.realpath(p) for p in sys.path]
+index = sys_path_absolute.index(python_site)
+sys.path = sys.path[:index] + matching_sites + sys.path[index:]