Fixes #24875: pip can now be installed in a venv with --system-site-packages.

Lib/test/test_venv.py

@@ -328,13 +328,7 @@ class EnsurePipTest(BaseTest):
         with open(os.devnull, "rb") as f:
             self.assertEqual(f.read(), b"")
 
-    # Requesting pip fails without SSL (http://bugs.python.org/issue19744)
-    @unittest.skipIf(ssl is None, ensurepip._MISSING_SSL_MESSAGE)
-    @unittest.skipUnless(threading, 'some dependencies of pip import threading'
-                                    ' module unconditionally')
-    # Issue #26610: pip/pep425tags.py requires ctypes
-    @unittest.skipUnless(ctypes, 'pip requires ctypes')
-    def test_with_pip(self):
+    def do_test_with_pip(self, system_site_packages):
         rmtree(self.env_dir)
         with EnvironmentVarGuard() as envvars:
             # pip's cross-version compatibility may trigger deprecation
@@ -368,6 +362,7 @@ class EnsurePipTest(BaseTest):
                 # config in place to ensure we ignore it
                 try:
                     self.run_with_capture(venv.create, self.env_dir,
+                                          system_site_packages=system_site_packages,
                                           with_pip=True)
                 except subprocess.CalledProcessError as exc:
                     # The output this produces can be a little hard to read,
@@ -417,9 +412,21 @@ class EnsurePipTest(BaseTest):
         out = out.decode("latin-1") # Force to text, prevent decoding errors
         self.assertIn("Successfully uninstalled pip", out)
         self.assertIn("Successfully uninstalled setuptools", out)
-        # Check pip is now gone from the virtual environment
-        self.assert_pip_not_installed()
+        # Check pip is now gone from the virtual environment. This only
+        # applies in the system_site_packages=False case, because in the
+        # other case, pip may still be available in the system site-packages
+        if not system_site_packages:
+            self.assert_pip_not_installed()
 
+    # Requesting pip fails without SSL (http://bugs.python.org/issue19744)
+    @unittest.skipIf(ssl is None, ensurepip._MISSING_SSL_MESSAGE)
+    @unittest.skipUnless(threading, 'some dependencies of pip import threading'
+                                    ' module unconditionally')
+    # Issue #26610: pip/pep425tags.py requires ctypes
+    @unittest.skipUnless(ctypes, 'pip requires ctypes')
+    def test_with_pip(self):
+        self.do_test_with_pip(False)
+        self.do_test_with_pip(True)
 
 if __name__ == "__main__":
     unittest.main()

Lib/venv/__init__.py

@@ -77,6 +77,10 @@ class EnvBuilder:
         """
         env_dir = os.path.abspath(env_dir)
         context = self.ensure_directories(env_dir)
+        # See issue 24875. We need system_site_packages to be False
+        # until after pip is installed.
+        true_system_site_packages = self.system_site_packages
+        self.system_site_packages = False
         self.create_configuration(context)
         self.setup_python(context)
         if self.with_pip:
@@ -84,6 +88,11 @@ class EnvBuilder:
         if not self.upgrade:
             self.setup_scripts(context)
             self.post_setup(context)
+        if true_system_site_packages:
+            # We had set it to False before, now
+            # restore it and rewrite the configuration
+            self.system_site_packages = True
+            self.create_configuration(context)
 
     def clear_directory(self, path):
         for fn in os.listdir(path):