Close #19694: venv now runs ensurepip in isolated mode

Lib/test/test_venv.py

@@ -12,7 +12,7 @@ import subprocess
 import sys
 import tempfile
 from test.support import (captured_stdout, captured_stderr, run_unittest,
-                          can_symlink)
+                          can_symlink, EnvironmentVarGuard)
 import unittest
 import venv
 
@@ -280,6 +280,11 @@ class EnsurePipTest(BaseTest):
 
     def test_with_pip(self):
         shutil.rmtree(self.env_dir)
+        with EnvironmentVarGuard() as envvars:
+            # pip's cross-version compatibility may trigger deprecation
+            # warnings in current versions of Python. Ensure related
+            # environment settings don't cause venv to fail.
+            envvars["PYTHONWARNINGS"] = "e"
             self.run_with_capture(venv.create, self.env_dir, with_pip=True)
         envpy = os.path.join(os.path.realpath(self.env_dir), self.bindir, self.exe)
         cmd = [envpy, '-m', 'pip', '--version']

Lib/venv/__init__.py

@@ -234,7 +234,10 @@ class EnvBuilder:
 
     def _setup_pip(self, context):
         """Installs or upgrades pip in a virtual environment"""
-        cmd = [context.env_exe, '-m', 'ensurepip', '--upgrade',
+        # We run ensurepip in isolated mode to avoid side effects from
+        # environment vars, the current directory and anything else
+        # intended for the global Python environment
+        cmd = [context.env_exe, '-Im', 'ensurepip', '--upgrade',
                                                     '--default-pip']
         subprocess.check_output(cmd)