gh-89830: Show a better error message on failing to create a venv due to failing ensurepip

Lib/test/test_venv.py

@@ -1008,6 +1008,23 @@ def test_devnull(self):
 
         self.assertTrue(os.path.exists(os.devnull))
 
+    def test_ensurepip_failure_shows_stderr(self):
+        """Test that ensurepip failure output is shown to the user (gh-89830)"""
+        with tempfile.TemporaryDirectory() as env_dir:
+            builder = venv.EnvBuilder(with_pip=True)
+            # Simulate ensurepip failing with a traceback on stderr
+            def fake_call_new_python(context, *args, **kwargs):
+                raise subprocess.CalledProcessError(
+                    returncode=1,
+                    cmd=args,
+                    stderr=b"AttributeError: simulated ensurepip failure"
+                )
+            with patch.object(venv.EnvBuilder, '_call_new_python', fake_call_new_python):
+                with captured_stderr() as err:
+                    with self.assertRaises(subprocess.CalledProcessError):
+                        builder.create(env_dir)
+                self.assertIn("simulated ensurepip failure", err.getvalue())
+
     def do_test_with_pip(self, system_site_packages):
         rmtree(self.env_dir)
         with EnvironmentVarGuard() as envvars:

Lib/venv/__init__.py

@@ -459,8 +459,13 @@ def _call_new_python(self, context, *py_args, **kwargs):
 
     def _setup_pip(self, context):
         """Installs or upgrades pip in a virtual environment"""
-        self._call_new_python(context, '-m', 'ensurepip', '--upgrade',
-                              '--default-pip', stderr=subprocess.STDOUT)
+        try:
+            self._call_new_python(context, '-m', 'ensurepip', '--upgrade',
+                                '--default-pip', stderr=subprocess.PIPE)
+        except subprocess.CalledProcessError as e:
+            if e.stderr:
+                print(e.stderr.decode(errors='replace'), file=sys.stderr)
+            raise
 
     def setup_scripts(self, context):
         """

Misc/NEWS.d/next/Library/2026-06-10-19-00-53.gh-issue-89830.YGHExb.rst

@@ -0,0 +1,2 @@
+Fix unhelpful error message when :mod:`venv` fails to install pip via
+:mod:`ensurepip` by displaying the subprocess stderr output.