gh-85935: Check for nargs=0 for positional arguments in argparse

Lib/argparse.py

@@ -1433,11 +1433,17 @@ def add_argument(self, *args, **kwargs):
                 kwargs['default'] = self.argument_default
 
         # create the action object, and add it to the parser
+        action_name = kwargs.get('action')
         action_class = self._pop_action_class(kwargs)
         if not callable(action_class):
             raise ValueError('unknown action "%s"' % (action_class,))
         action = action_class(**kwargs)
 
+        # raise an error if action for positional argument does not
+        # consume arguments
+        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 type is not callable
         type_func = self._registry_get('type', action.type, action.type)
         if not callable(type_func):
@@ -1541,7 +1547,9 @@ def _get_positional_kwargs(self, dest, **kwargs):
         # mark positional arguments as required if at least one is
         # always required
         nargs = kwargs.get('nargs')
-        if nargs not in [OPTIONAL, ZERO_OR_MORE, REMAINDER, SUPPRESS, 0]:
+        if nargs == 0:
+            raise ValueError('nargs for positionals must be != 0')
+        if nargs not in [OPTIONAL, ZERO_OR_MORE, REMAINDER, SUPPRESS]:
             kwargs['required'] = True
 
         # return the keyword arguments with no option strings

Lib/test/test_argparse.py

@@ -5350,8 +5350,11 @@ def test_no_argument_actions(self):
                     with self.subTest(attrs=attrs):
                         self.assertTypeError('-x', action=action, **attrs)
                         self.assertTypeError('x', action=action, **attrs)
+                self.assertValueError('x', action=action,
+                    errmsg=f"action '{action}' is not valid for positional arguments")
                 self.assertTypeError('-x', action=action, nargs=0)
-                self.assertTypeError('x', action=action, nargs=0)
+                self.assertValueError('x', action=action, nargs=0,
+                    errmsg='nargs for positionals must be != 0')
 
     def test_no_argument_no_const_actions(self):
         # options with zero arguments
@@ -5371,7 +5374,7 @@ def test_more_than_one_argument_actions(self):
                 self.assertValueError('-x', nargs=0, action=action,
                     errmsg=f'nargs for {action_name} actions must be != 0')
                 self.assertValueError('spam', nargs=0, action=action,
-                    errmsg=f'nargs for {action_name} actions must be != 0')
+                    errmsg='nargs for positionals must be != 0')
 
                 # const is disallowed with non-optional arguments
                 for nargs in [1, '*', '+']:

Misc/NEWS.d/next/Library/2024-10-01-13-11-53.gh-issue-85935.CTwJUy.rst

@@ -0,0 +1,4 @@
+:meth:`argparse.ArgumentParser.add_argument` now raises an exception if
+an :ref:`action` that does not consume arguments (like 'store_const' or
+'store_true') or explicit ``nargs=0`` are specified for positional
+arguments.