GH-139946: Colorize error and warning messages in argparse (#140695)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Lib/test/test_argparse.py

@@ -2283,6 +2283,7 @@ class TestNegativeNumber(ParserTestCase):
         ('--complex -1e-3j', NS(int=None, float=None, complex=-0.001j)),
     ]
 
+@force_not_colorized_test_class
 class TestArgumentAndSubparserSuggestions(TestCase):
     """Test error handling and suggestion when a user makes a typo"""
 
@@ -6147,6 +6148,7 @@ def spam(string_to_convert):
 # Check that deprecated arguments output warning
 # ==============================================
 
+@force_not_colorized_test_class
 class TestDeprecatedArguments(TestCase):
 
     def test_deprecated_option(self):
@@ -7370,6 +7372,45 @@ def test_subparser_prog_is_stored_without_color(self):
         help_text = demo_parser.format_help()
         self.assertNotIn('\x1b[', help_text)
 
+    def test_error_and_warning_keywords_colorized(self):
+        parser = argparse.ArgumentParser(prog='PROG')
+        parser.add_argument('foo')
+
+        with self.assertRaises(SystemExit):
+            with captured_stderr() as stderr:
+                parser.parse_args([])
+
+        err = stderr.getvalue()
+        error_color = self.theme.error
+        reset = self.theme.reset
+        self.assertIn(f'{error_color}error:{reset}', err)
+
+        with captured_stderr() as stderr:
+            parser._warning('test warning')
+
+        warn = stderr.getvalue()
+        warning_color = self.theme.warning
+        self.assertIn(f'{warning_color}warning:{reset}', warn)
+
+    def test_error_and_warning_not_colorized_when_disabled(self):
+        parser = argparse.ArgumentParser(prog='PROG', color=False)
+        parser.add_argument('foo')
+
+        with self.assertRaises(SystemExit):
+            with captured_stderr() as stderr:
+                parser.parse_args([])
+
+        err = stderr.getvalue()
+        self.assertNotIn('\x1b[', err)
+        self.assertIn('error:', err)
+
+        with captured_stderr() as stderr:
+            parser._warning('test warning')
+
+        warn = stderr.getvalue()
+        self.assertNotIn('\x1b[', warn)
+        self.assertIn('warning:', warn)
+
 
 class TestModule(unittest.TestCase):
     def test_deprecated__version__(self):