[3.9] bpo-45235: Revert an argparse bugfix that caused a regression (GH-29525)

Lib/argparse.py

@@ -1209,8 +1209,7 @@ def __call__(self, parser, namespace, values, option_string=None):
         # namespace for the relevant parts.
         subnamespace, arg_strings = parser.parse_known_args(arg_strings, None)
         for key, value in vars(subnamespace).items():
-            if not hasattr(namespace, key):
-                setattr(namespace, key, value)
+            setattr(namespace, key, value)
 
         if arg_strings:
             vars(namespace).setdefault(_UNRECOGNIZED_ARGS_ATTR, [])
@@ -1844,6 +1843,11 @@ def parse_known_args(self, args=None, namespace=None):
                     if action.default is not SUPPRESS:
                         setattr(namespace, action.dest, action.default)
 
+        # add any parser defaults that aren't present
+        for dest in self._defaults:
+            if not hasattr(namespace, dest):
+                setattr(namespace, dest, self._defaults[dest])
+
         # parse the arguments and exit if there are any errors
         if self.exit_on_error:
             try:
@@ -1854,11 +1858,6 @@ def parse_known_args(self, args=None, namespace=None):
         else:
             namespace, args = self._parse_known_args(args, namespace)
 
-        # add any parser defaults that aren't present
-        for dest in self._defaults:
-            if not hasattr(namespace, dest):
-                setattr(namespace, dest, self._defaults[dest])
-
         if hasattr(namespace, _UNRECOGNIZED_ARGS_ATTR):
             args.extend(getattr(namespace, _UNRECOGNIZED_ARGS_ATTR))
             delattr(namespace, _UNRECOGNIZED_ARGS_ATTR)

Lib/test/test_argparse.py

@@ -3095,12 +3095,6 @@ def test_set_defaults_on_parent_and_subparser(self):
         xparser.set_defaults(foo=2)
         self.assertEqual(NS(foo=2), parser.parse_args(['X']))
 
-    def test_set_defaults_on_subparser_with_namespace(self):
-        parser = argparse.ArgumentParser()
-        xparser = parser.add_subparsers().add_parser('X')
-        xparser.set_defaults(foo=1)
-        self.assertEqual(NS(foo=2), parser.parse_args(['X'], NS(foo=2)))
-
     def test_set_defaults_same_as_add_argument(self):
         parser = ErrorRaisingArgumentParser()
         parser.set_defaults(w='W', x='X', y='Y', z='Z')

Misc/NEWS.d/next/Library/2021-11-11-13-03-17.bpo-45235.8ZbkHa.rst

@@ -0,0 +1,3 @@
+Reverted an argparse bugfix that caused regression in the handling of
+default arguments for subparsers.  This prevented leaf level arguments from
+taking precedence over root level arguments.