bpo-43220: Accept explicit default args in required groups

Lib/argparse.py

@@ -1981,12 +1981,12 @@ def _parse_known_args(self, arg_strings, namespace):
 
         def take_action(action, argument_strings, option_string=None):
             seen_actions.add(action)
-            argument_values = self._get_values(action, argument_strings)
+            argument_values, explicit = 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:
+            if argument_values is not action.default or explicit:
                 seen_non_default_actions.add(action)
                 for conflict_action in action_conflicts.get(action, []):
                     if conflict_action in seen_non_default_actions:
@@ -2488,6 +2488,7 @@ def parse_known_intermixed_args(self, args=None, namespace=None):
     # ========================
     def _get_values(self, action, arg_strings):
         # for everything but PARSER, REMAINDER args, strip out first '--'
+        explicit = True
         if action.nargs not in [PARSER, REMAINDER]:
             try:
                 arg_strings.remove('--')
@@ -2500,6 +2501,7 @@ def _get_values(self, action, arg_strings):
                 value = action.const
             else:
                 value = action.default
+                explicit = False
             if isinstance(value, str):
                 value = self._get_value(action, value)
                 self._check_value(action, value)
@@ -2510,6 +2512,7 @@ def _get_values(self, action, arg_strings):
               not action.option_strings):
             if action.default is not None:
                 value = action.default
+                explicit = False
                 self._check_value(action, value)
             else:
                 # since arg_strings is always [] at this point
@@ -2542,7 +2545,7 @@ def _get_values(self, action, arg_strings):
                 self._check_value(action, v)
 
         # return the converted value
-        return value
+        return value, explicit
 
     def _get_value(self, action, arg_string):
         type_func = self._registry_get('type', action.type, action.type)

Lib/test/test_argparse.py

@@ -3255,6 +3255,23 @@ def get_parser(self, required):
     test_successes_when_not_required = None
     test_successes_when_required = None
 
+class TestMutuallyExclusiveRequiredDefault(TestCase):
+
+    def test_default_arg_passed(self):
+        parser = ErrorRaisingArgumentParser()
+        group = parser.add_mutually_exclusive_group(required=True)
+        group.add_argument('--foo', default='1')
+        group.add_argument('--bar')
+        self.assertEqual(NS(foo='1', bar=None),
+                         parser.parse_args(['--foo', '1']))
+
+    def test_nothing_passed(self):
+        parser = ErrorRaisingArgumentParser()
+        group = parser.add_mutually_exclusive_group(required=True)
+        group.add_argument('--foo', default='1')
+        group.add_argument('--bar')
+        self.assertRaises(ArgumentParserError, parser.parse_args, [])
+
 # =================================================
 # Mutually exclusive group in parent parser tests
 # =================================================

Misc/NEWS.d/next/Library/2021-02-16-18-13-00.bpo-43220.XfDZu0.rst

@@ -0,0 +1 @@
+Accept explicitly passed default arguments in required argparse groups. Previously short strings that were used as default values in argparse groups would be rejected if they were passed with the same value. Now they are accepted.