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>

Lib/test/test_cmd_line_script.py

@@ -777,7 +777,7 @@ class CmdLineTest(unittest.TestCase):
         with os_helper.temp_dir() as work_dir:
             script_name = _make_test_script(work_dir, 'script.py', script)
             with open(script_name, "r") as fp:
-                p = spawn_python(f"/dev/fd/{fp.fileno()}", close_fds=False, pass_fds=(0,1,2,fp.fileno()))
+                p = spawn_python(f"/dev/fd/{fp.fileno()}", close_fds=True, pass_fds=(0,1,2,fp.fileno()))
                 out, err = p.communicate()
                 self.assertEqual(out, b"12345678912345678912345\n")
 

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):

Lib/test/test_warnings/__init__.py

@@ -387,9 +387,13 @@ class FilterTests(BaseTest):
             with self.module.catch_warnings(
                 module=self.module, action="error", category=FutureWarning
             ):
-                self.module.warn("Other types of warnings are not errors")
-                self.assertRaises(FutureWarning,
-                                  self.module.warn, FutureWarning("msg"))
+                with support.captured_stderr() as stderr:
+                    error_msg = "Other types of warnings are not errors"
+                    self.module.warn(error_msg)
+                    self.assertRaises(FutureWarning,
+                                      self.module.warn, FutureWarning("msg"))
+                    stderr = stderr.getvalue()
+                    self.assertIn(error_msg, stderr)
 
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings