[bug#46028] [PATCH core-updates v2] gnu: python: Replace PYTHONPATH by GUIX_PYTHONPATH_X_Y.

[Maxim Cournoyer]

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 (customize-site)
(guix-pythonpath-search-path): New procedures.
* gnu/packages/python.scm (python-2.7)[phases]{customize-site}: New phase.
[native-search-paths]: Adjust.
* gnu/packages/python.scm (python-3.9)[native-search-paths]: Likewise.
[phases]{customize-site}: Override the inherited phase, so that it uses the
correct version.
* gnu/packages/commencement.scm (python-boot0)
[phases]{customize-site}: Likewise.
[native-search-paths]: Override the inherited one.
---
 gnu/packages/commencement.scm |  4 ++-
 gnu/packages/python.scm       | 57 +++++++++++++++++++++++++++--------
 2 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/gnu/packages/commencement.scm b/gnu/packages/commencement.scm
index 0713bcb843..6d7593e223 100644
--- a/gnu/packages/commencement.scm
+++ b/gnu/packages/commencement.scm
@@ -3176,6 +3176,7 @@ memoized as a function of '%current-system'."
                      ;; built, since it requires Linux headers.
                      (("'linux', ") ""))))
                (delete 'set-TZDIR)
+               (replace 'customize-site ,(customize-site version))
                ,@(if (hurd-system?)
                      `((add-before 'build 'fix-regen
                          (lambda* (#:key inputs #:allow-other-keys)
@@ -3184,7 +3185,8 @@ memoized as a function of '%current-system'."
                                (("/usr/include/")
                                 (string-append libc "/include/")))))))
                      '())))
-           ((#:tests? _ #f) #f))))))
+           ((#:tests? _ #f) #f))))
+    (native-search-paths (list (guix-pythonpath-search-path version)))))
 
 (define/system-dependent ld-wrapper-boot0
   ;; The first 'ld' wrapper, defined with 'define/system-dependent' because
diff --git a/gnu/packages/python.scm b/gnu/packages/python.scm
index d3df1a4dab..c3453d63a9 100644
--- a/gnu/packages/python.scm
+++ b/gnu/packages/python.scm
@@ -102,7 +102,44 @@
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system trivial)
   #:use-module (srfi srfi-1)
-  #:use-module (srfi srfi-26))
+  #:use-module (srfi srfi-26)
+  #:export (customize-site
+            guix-pythonpath-search-path))
+
+(define* (customize-site version)
+  "Generate a sitecustomize.py phase, using VERSION."
+  `(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))))))
+
+(define (guix-pythonpath-search-path version)
+  "Generate a GUIX_PYTHONPATH_X_Y search path specification, using VERSION."
+  (search-path-specification
+   (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")))))
 
 (define-public python-2.7
   (package
@@ -313,7 +350,9 @@
                                     "/site-packages")))
                       (install-file tkinter.so target)
                       (delete-file tkinter.so)))))
-               #t))))))
+               #t)))
+         (add-after 'install 'customize-site
+           ,(customize-site version)))))
     (inputs
      `(("bzip2" ,bzip2)
        ("expat" ,expat)
@@ -332,10 +371,7 @@
              `(("python2" ,this-package)
                ("which" ,which))
              '())))
-    (native-search-paths
-     (list (search-path-specification
-            (variable "PYTHONPATH")
-            (files '("lib/python2.7/site-packages")))))
+    (native-search-paths (list (guix-pythonpath-search-path version)))
     (home-page "https://www.python.org";)
     (synopsis "High-level, dynamically-typed programming language")
     (description
@@ -463,7 +499,8 @@ data types.")
                                "-x" "lib2to3/.*"
                                ,out))))
                   (list "none" "-O" "-OO"))
-                 #t)))))))
+                 #t)))
+           (replace 'customize-site ,(customize-site version))))))
     (native-inputs
      `(("tzdata" ,tzdata-for-tests)
        ,@(if (%current-target-system)
@@ -471,11 +508,7 @@ data types.")
              '())
        ,@(package-native-inputs python-2)))
     (native-search-paths
-     (list (search-path-specification
-            (variable "PYTHONPATH")
-            (files (list (string-append "lib/python"
-                                        (version-major+minor version)
-                                        "/site-packages"))))
+     (list (guix-pythonpath-search-path version)
            ;; Used to locate tzdata by the zoneinfo module introduced in
            ;; Python 3.9.
            (search-path-specification
-- 
2.29.2