gh-103186: Fix or catch 'extra' stderr output from unittests (#103196)

Reduce test noise by fixing or catching and testing stderr messages from individual tests.

test_cmd_line_script.test_script_as_dev_fd calls spawn_python and hence subprocess.Popen with incompatible arguments. On POSIX, pass_fds forces close_fds to be True (subprocess.py line 848). Correct the call.

test_uuid.test_cli_namespace_required_for_uuid3: when the namespace is omitted, uuid.main calls argparse.Argument_Parser.error, which prints to stderr before calling sys.exit, which raises SystemExit. Unittest assertRaises catches the exception but not the previous output. Catch the output and test it.

test_warnings.test_catchwarnings_with_simplefilter_error similarly prints before raising. Catch the output and test it.
---------

Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/Lib/test/test_uuid.py b/Lib/test/test_uuid.py
index a178e94..9cec1e8 100755
--- a/Lib/test/test_uuid.py
+++ b/Lib/test/test_uuid.py
@@ -706,20 +706,23 @@ class BaseTestUUID:
         self.assertIs(strong, weak())
 
     @mock.patch.object(sys, "argv", ["", "-u", "uuid3", "-n", "@dns"])
-    def test_cli_namespace_required_for_uuid3(self):
+    @mock.patch('sys.stderr', new_callable=io.StringIO)
+    def test_cli_namespace_required_for_uuid3(self, mock_err):
         with self.assertRaises(SystemExit) as cm:
             self.uuid.main()
 
         # Check that exception code is the same as argparse.ArgumentParser.error
         self.assertEqual(cm.exception.code, 2)
+        self.assertIn("error: Incorrect number of arguments", mock_err.getvalue())
 
     @mock.patch.object(sys, "argv", ["", "-u", "uuid3", "-N", "python.org"])
-    def test_cli_name_required_for_uuid3(self):
+    @mock.patch('sys.stderr', new_callable=io.StringIO)
+    def test_cli_name_required_for_uuid3(self, mock_err):
         with self.assertRaises(SystemExit) as cm:
             self.uuid.main()
-
         # Check that exception code is the same as argparse.ArgumentParser.error
         self.assertEqual(cm.exception.code, 2)
+        self.assertIn("error: Incorrect number of arguments", mock_err.getvalue())
 
     @mock.patch.object(sys, "argv", [""])
     def test_cli_uuid4_outputted_with_no_args(self):