gh-138525: raise error when BooleanOptionalAction used with prefix_chars without '-'

Lib/argparse.py

@@ -922,6 +922,10 @@ def format_usage(self):
     def __call__(self, parser, namespace, values, option_string=None):
         raise NotImplementedError('.__call__() not defined')
 
+    def _check_compatibility(self, container):
+        """Check whether the action is compatible with the container."""
+        pass
+
 
 class BooleanOptionalAction(Action):
     def __init__(self,
@@ -960,6 +964,10 @@ def __call__(self, parser, namespace, values, option_string=None):
     def format_usage(self):
         return ' | '.join(self.option_strings)
 
+    def _check_compatibility(self, container):
+        if '-' not in container.prefix_chars:
+            raise ValueError(f"BooleanOptionalAction requires '-' in prefix_chars.")
+
 
 class _StoreAction(Action):
 
@@ -1542,6 +1550,9 @@ def add_argument(self, *args, **kwargs):
         if not action.option_strings and action.nargs == 0:
             raise ValueError(f'action {action_name!r} is not valid for positional arguments')
 
+        # raise an error if the action is not compatible with the parser
+        action._check_compatibility(self)
+
         # raise an error if the action type is not callable
         type_func = self._registry_get('type', action.type, action.type)
         if not callable(type_func):

Lib/test/test_argparse.py

@@ -795,6 +795,13 @@ def test_invalid_name(self):
         self.assertEqual(str(cm.exception),
                          "invalid option name '--no-foo' for BooleanOptionalAction")
 
+    def test_prefix_chars_incompatibility(self):
+        parser = argparse.ArgumentParser(prefix_chars='+')
+        with self.assertRaises(ValueError) as cm:
+            parser.add_argument('++foo', action=argparse.BooleanOptionalAction)
+        self.assertEqual(str(cm.exception),
+                         "BooleanOptionalAction requires '-' in prefix_chars.")
+
 class TestBooleanOptionalActionRequired(ParserTestCase):
     """Tests BooleanOptionalAction required"""
 

Misc/NEWS.d/next/Library/2025-09-05-15-42-16.gh-issue-138525.ngZ0qT.rst

@@ -0,0 +1,2 @@
+Raise error when :class:`argparse.BooleanOptionalAction` is used with
+:class:`argparse.ArgumentParser` instance whose ``prefix_chars`` has no '-'.