Python: fix virtualenv with Python 2

pkgs/development/interpreters/python/sitecustomize.py

···
 if paths:
     functools.reduce(lambda k, p: site.addsitedir(p, k), paths.split(':'), site._init_pathinfo())
 
-# Check whether we are in a venv. 
-# Note Python 2 does not support base_prefix so we assume we are not in a venv.
-in_venv = sys.version_info.major == 3 and sys.prefix != sys.base_prefix
+# Check whether we are in a venv or virtualenv. 
+# For Python 3 we check whether our `base_prefix` is different from our current `prefix`.
+# For Python 2 we check whether the non-standard `real_prefix` is set.
+# https://stackoverflow.com/questions/1871549/determine-if-python-is-running-inside-virtualenv
+in_venv = (sys.version_info.major == 3 and sys.prefix != sys.base_prefix) or (sys.version_info.major == 2 and hasattr(sys, "real_prefix"))
 
 if not in_venv:
     executable = os.environ.pop('NIX_PYTHONEXECUTABLE', None)
···
     if 'PYTHONEXECUTABLE' not in os.environ and executable is not None:
         sys.executable = executable
     if prefix is not None:
-        # Because we cannot check with Python 2 whether we are in a venv, 
-        # creating a venv from a Nix env won't work as well with Python 2.
-        # Also, note that sysconfig does not like it when sys.prefix is set to None
+        # Sysconfig does not like it when sys.prefix is set to None
         sys.prefix = sys.exec_prefix = prefix
         site.PREFIXES.insert(0, prefix)

pkgs/development/interpreters/python/tests.nix

···
       is_nixenv = "False";
       is_virtualenv = "False";
     };
-  } // lib.optionalAttrs (python.isPy3k && !python.isPyPy) {
+  } // lib.optionalAttrs (!python.isPyPy) {
     # Use virtualenv from a Nix env.
-    # Does not function with Python 2
-    # ValueError: source and destination is the same /nix/store/38kz3j1a87cq5y59k5w7k9yk4cqgc5b2-python-2.7.18/lib/python2.7/os.py
     nixenv-virtualenv = rec {
       env = runCommand "${python.name}-virtualenv" {} ''
         ${pythonVirtualEnv.interpreter} -m virtualenv $out

pkgs/development/interpreters/python/tests/test_python.py

···
         else:
             self.assertEqual(sys.prefix, sys.base_prefix)
 
+    @unittest.skipIf(sys.version_info.major==3, "sys.real_prefix is only set by virtualenv in case of Python 2.")
+    def test_real_prefix(self):
+        self.assertTrue(hasattr(sys, "real_prefix") == IS_VIRTUALENV)
+
     def test_python_version(self):
         self.assertTrue(platform.python_version().startswith(PYTHON_VERSION))
 

pkgs/development/python-modules/virtualenv/0001-Check-base_prefix-and-base_exec_prefix-for-Python-2.patch

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+From 21563405d6e2348ee457187f7fb61beb102bb367 Mon Sep 17 00:00:00 2001
+From: Frederik Rietdijk <[e-mail]>
+Date: Sun, 24 May 2020 09:33:13 +0200
+Subject: [PATCH] Check base_prefix and base_exec_prefix for Python 2
+
+This is a Nixpkgs-specific change so it can support virtualenvs from Nix envs.
+---
+ src/virtualenv/discovery/py_info.py | 8 ++++++--
+ 1 file changed, 6 insertions(+), 2 deletions(-)
+
+diff --git a/src/virtualenv/discovery/py_info.py b/src/virtualenv/discovery/py_info.py
+index 6f12128..74e9218 100644
+--- a/src/virtualenv/discovery/py_info.py
++++ b/src/virtualenv/discovery/py_info.py
+@@ -51,13 +51,17 @@ class PythonInfo(object):
+         self.version = u(sys.version)
+         self.os = u(os.name)
+ 
++        config_vars = {} if sys.version_info.major is not 2 else sysconfig._CONFIG_VARS
++        base_prefix = config_vars.get("prefix")
++        base_exec_prefix = config_vars.get("exec_prefix")
++
+         # information about the prefix - determines python home
+         self.prefix = u(abs_path(getattr(sys, "prefix", None)))  # prefix we think
+-        self.base_prefix = u(abs_path(getattr(sys, "base_prefix", None)))  # venv
++        self.base_prefix = u(abs_path(getattr(sys, "base_prefix", base_prefix)))  # venv
+         self.real_prefix = u(abs_path(getattr(sys, "real_prefix", None)))  # old virtualenv
+ 
+         # information about the exec prefix - dynamic stdlib modules
+-        self.base_exec_prefix = u(abs_path(getattr(sys, "base_exec_prefix", None)))
++        self.base_exec_prefix = u(abs_path(getattr(sys, "base_exec_prefix", base_exec_prefix)))
+         self.exec_prefix = u(abs_path(getattr(sys, "exec_prefix", None)))
+ 
+         self.executable = u(abs_path(sys.executable))  # the executable we were invoked via
+-- 
+2.25.1
+

pkgs/development/python-modules/virtualenv/default.nix

···
     importlib-metadata
   ];
 
+  patches = lib.optionals (isPy27) [
+    ./0001-Check-base_prefix-and-base_exec_prefix-for-Python-2.patch
+  ];
+
   meta = {
     description = "A tool to create isolated Python environments";
     homepage = "http://www.virtualenv.org";