[bug#46028] [PATCH core-updates 2/2] gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH_X_Y.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

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_X_Y environment variable, where
X is the major and Y the minor version numbers, is read from the environment
and spliced in 'sys.path' just before Python's own site location, which
provides the expected behavior.

* gnu/packages/python.scm (python-2.7)[phases]{customize-site}: Add phase.
[native-search-paths]: Modify so that GUIX_PYTHONPATH_X_Y is used.
* gnu/packages/python.scm (python-3.9)[native-search-paths]: Likewise.
---
 gnu/packages/python.scm | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/gnu/packages/python.scm b/gnu/packages/python.scm
index d3df1a4dab..79cea3d43a 100644
--- a/gnu/packages/python.scm
+++ b/gnu/packages/python.scm
@@ -313,7 +313,30 @@
                                     "/site-packages")))
                       (install-file tkinter.so target)
                       (delete-file tkinter.so)))))
-               #t))))))
+               #t)))
+         (add-after 'install 'customize-site
+           (lambda* (#:key outputs #:allow-other-keys)
+             (let* ((out (assoc-ref outputs "out"))
+                    (site-packages (string-append
+                                    out "/lib/python"
+                                    ,(version-major+minor version)
+                                    "/site-packages"))
+                    (pythonpath (format #f "GUIX_PYTHONPATH_~a_~a"
+                                        ,(version-major version)
+                                        ,(version-minor version)))
+                    (sitecustomize.py (string-append site-packages
+                                                     "/sitecustomize.py")))
+               (with-output-to-file sitecustomize.py
+                 (lambda _
+                   (format #t "\
+import os
+import sys
+
+# Insert the GUIX_PYTHONPATH_X_Y values just before Python's own site.
+sys_path_absolute = [os.path.realpath(p) for p in sys.path]
+index = sys_path_absolute.index(~s)
+sys.path = sys.path[:index] + os.environ[~s].split(':') + sys.path[index:]
+" site-packages pythonpath)))))))))
     (inputs
      `(("bzip2" ,bzip2)
        ("expat" ,expat)
@@ -334,8 +357,12 @@
              '())))
     (native-search-paths
      (list (search-path-specification
-            (variable "PYTHONPATH")
-            (files '("lib/python2.7/site-packages")))))
+            (variable (format #f "GUIX_PYTHONPATH_~a_~a"
+                              (version-major version)
+                              (version-minor version)))
+            (files (list (string-append "lib/python"
+                                        (version-major+minor version)
+                                        "/site-packages"))))))
     (home-page "https://www.python.org")
     (synopsis "High-level, dynamically-typed programming language")
     (description
@@ -472,7 +499,9 @@ data types.")
        ,@(package-native-inputs python-2)))
     (native-search-paths
      (list (search-path-specification
-            (variable "PYTHONPATH")
+            (variable (format #f "GUIX_PYTHONPATH_~a_~a"
+                              (version-major version)
+                              (version-minor version)))
             (files (list (string-append "lib/python"
                                         (version-major+minor version)
                                         "/site-packages"))))
-- 
2.29.2