[3.13] gh-63143: Fix parsing mutually exclusive arguments in argparse (GH-124307)

Lib/argparse.py

@@ -1978,9 +1978,8 @@ def take_action(action, argument_strings, option_string=None):
             argument_values = self._get_values(action, argument_strings)
 
             # error if this argument is not allowed with other previously
-            # seen arguments, assuming that actions that use the default
-            # value don't really count as "present"
-            if argument_values is not action.default:
+            # seen arguments
+            if action.option_strings or argument_strings:
                 seen_non_default_actions.add(action)
                 for conflict_action in action_conflicts.get(action, []):
                     if conflict_action in seen_non_default_actions:

Lib/test/test_argparse.py

@@ -2900,26 +2900,30 @@ def test_failures_when_not_required(self):
         parse_args = self.get_parser(required=False).parse_args
         error = ArgumentParserError
         for args_string in self.failures:
-            self.assertRaises(error, parse_args, args_string.split())
+            with self.subTest(args=args_string):
+                self.assertRaises(error, parse_args, args_string.split())
 
     def test_failures_when_required(self):
         parse_args = self.get_parser(required=True).parse_args
         error = ArgumentParserError
         for args_string in self.failures + ['']:
-            self.assertRaises(error, parse_args, args_string.split())
+            with self.subTest(args=args_string):
+                self.assertRaises(error, parse_args, args_string.split())
 
     def test_successes_when_not_required(self):
         parse_args = self.get_parser(required=False).parse_args
         successes = self.successes + self.successes_when_not_required
         for args_string, expected_ns in successes:
-            actual_ns = parse_args(args_string.split())
-            self.assertEqual(actual_ns, expected_ns)
+            with self.subTest(args=args_string):
+                actual_ns = parse_args(args_string.split())
+                self.assertEqual(actual_ns, expected_ns)
 
     def test_successes_when_required(self):
         parse_args = self.get_parser(required=True).parse_args
         for args_string, expected_ns in self.successes:
-            actual_ns = parse_args(args_string.split())
-            self.assertEqual(actual_ns, expected_ns)
+            with self.subTest(args=args_string):
+                actual_ns = parse_args(args_string.split())
+                self.assertEqual(actual_ns, expected_ns)
 
     def test_usage_when_not_required(self):
         format_usage = self.get_parser(required=False).format_usage
@@ -3306,6 +3310,111 @@ def get_parser(self, required):
     test_successes_when_not_required = None
     test_successes_when_required = None
 
+
+class TestMutuallyExclusiveOptionalOptional(MEMixin, TestCase):
+    def get_parser(self, required=None):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        group = parser.add_mutually_exclusive_group(required=required)
+        group.add_argument('--foo')
+        group.add_argument('--bar', nargs='?')
+        return parser
+
+    failures = [
+        '--foo X --bar Y',
+        '--foo X --bar',
+    ]
+    successes = [
+        ('--foo X', NS(foo='X', bar=None)),
+        ('--bar X', NS(foo=None, bar='X')),
+        ('--bar', NS(foo=None, bar=None)),
+    ]
+    successes_when_not_required = [
+        ('', NS(foo=None, bar=None)),
+    ]
+    usage_when_required = '''\
+        usage: PROG [-h] (--foo FOO | --bar [BAR])
+        '''
+    usage_when_not_required = '''\
+        usage: PROG [-h] [--foo FOO | --bar [BAR]]
+        '''
+    help = '''\
+
+        options:
+          -h, --help   show this help message and exit
+          --foo FOO
+          --bar [BAR]
+        '''
+
+
+class TestMutuallyExclusiveOptionalWithDefault(MEMixin, TestCase):
+    def get_parser(self, required=None):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        group = parser.add_mutually_exclusive_group(required=required)
+        group.add_argument('--foo')
+        group.add_argument('--bar', type=bool, default=True)
+        return parser
+
+    failures = [
+        '--foo X --bar Y',
+        '--foo X --bar=',
+    ]
+    successes = [
+        ('--foo X', NS(foo='X', bar=True)),
+        ('--bar X', NS(foo=None, bar=True)),
+        ('--bar=', NS(foo=None, bar=False)),
+    ]
+    successes_when_not_required = [
+        ('', NS(foo=None, bar=True)),
+    ]
+    usage_when_required = '''\
+        usage: PROG [-h] (--foo FOO | --bar BAR)
+        '''
+    usage_when_not_required = '''\
+        usage: PROG [-h] [--foo FOO | --bar BAR]
+        '''
+    help = '''\
+
+        options:
+          -h, --help  show this help message and exit
+          --foo FOO
+          --bar BAR
+        '''
+
+
+class TestMutuallyExclusivePositionalWithDefault(MEMixin, TestCase):
+    def get_parser(self, required=None):
+        parser = ErrorRaisingArgumentParser(prog='PROG')
+        group = parser.add_mutually_exclusive_group(required=required)
+        group.add_argument('--foo')
+        group.add_argument('bar', nargs='?', type=bool, default=True)
+        return parser
+
+    failures = [
+        '--foo X Y',
+    ]
+    successes = [
+        ('--foo X', NS(foo='X', bar=True)),
+        ('X', NS(foo=None, bar=True)),
+    ]
+    successes_when_not_required = [
+        ('', NS(foo=None, bar=True)),
+    ]
+    usage_when_required = '''\
+        usage: PROG [-h] (--foo FOO | bar)
+        '''
+    usage_when_not_required = '''\
+        usage: PROG [-h] [--foo FOO | bar]
+        '''
+    help = '''\
+
+        positional arguments:
+          bar
+
+        options:
+          -h, --help  show this help message and exit
+          --foo FOO
+        '''
+
 # =================================================
 # Mutually exclusive group in parent parser tests
 # =================================================

Misc/NEWS.d/next/Library/2024-09-21-23-56-41.gh-issue-63143.YKu-LQ.rst

@@ -0,0 +1,3 @@
+Fix parsing mutually exclusive arguments in :mod:`argparse`. Arguments with
+the value identical to the default value (e.g. booleans, small integers,
+empty or 1-character strings) are no longer considered "not present".