gh-85427: Prevent exits if ArgumentParser.exit_on_error is False

Lib/argparse.py

@@ -1869,6 +1869,11 @@ def parse_args(self, args=None, namespace=None):
         args, argv = self.parse_known_args(args, namespace)
         if argv:
             msg = _('unrecognized arguments: %s')
+            if not self.exit_on_error:
+                raise ArgumentError(
+                    None,
+                    msg % ' '.join(argv)
+                )
             self.error(msg % ' '.join(argv))
         return args
 
@@ -2139,8 +2144,11 @@ def consume_positionals(start_index):
                                 self._get_value(action, action.default))
 
         if required_actions:
-            self.error(_('the following arguments are required: %s') %
-                       ', '.join(required_actions))
+            raise ArgumentError(
+                None,
+                _('the following arguments are required: %s') %
+                ', '.join(required_actions)
+            )
 
         # make sure all required groups had one option present
         for group in self._mutually_exclusive_groups:
@@ -2155,7 +2163,10 @@ def consume_positionals(start_index):
                              for action in group._group_actions
                              if action.help is not SUPPRESS]
                     msg = _('one of the arguments %s is required')
-                    self.error(msg % ' '.join(names))
+                    raise ArgumentError(
+                        None,
+                        msg % ' '.join(names)
+                    )
 
         # return the updated namespace and the extra arguments
         return namespace, extras
@@ -2384,7 +2395,10 @@ def parse_intermixed_args(self, args=None, namespace=None):
         args, argv = self.parse_known_intermixed_args(args, namespace)
         if argv:
             msg = _('unrecognized arguments: %s')
-            self.error(msg % ' '.join(argv))
+            formatted_msg = msg % ' '.join(argv)
+            if self.exit_on_error:
+                self.error(formatted_msg)
+            raise ArgumentError(None, formatted_msg)
         return args
 
     def parse_known_intermixed_args(self, args=None, namespace=None):

Lib/test/test_argparse.py

@@ -5594,7 +5594,8 @@ class TestExitOnError(TestCase):
 
     def setUp(self):
         self.parser = argparse.ArgumentParser(exit_on_error=False)
-        self.parser.add_argument('--integers', metavar='N', type=int)
+        self.parser.add_argument(
+            '--integers', metavar='N', type=int, required=True)
 
     def test_exit_on_error_with_good_args(self):
         ns = self.parser.parse_args('--integers 4'.split())
@@ -5604,6 +5605,28 @@ def test_exit_on_error_with_bad_args(self):
         with self.assertRaises(argparse.ArgumentError):
             self.parser.parse_args('--integers a'.split())
 
+    def test_exit_on_error_missing_required_arg(self):
+        msg = 'the following arguments are required: --integers'
+        with self.assertRaisesRegex(argparse.ArgumentError, msg):
+            self.parser.parse_args([])
+
+    def test_exit_on_error_unknown_arg(self):
+        msg = 'unrecognized arguments: --unknown'
+        with self.assertRaisesRegex(argparse.ArgumentError, msg):
+            self.parser.parse_args('--integers 4 --unknown'.split())
+        with self.assertRaisesRegex(argparse.ArgumentError, msg.replace('--', '')):
+            self.parser.parse_intermixed_args('--integers 4 unknown'.split())
+
+    def test_exit_on_error_mutually_exclusive_group(self):
+        other_parser = argparse.ArgumentParser(exit_on_error=False)
+        group = other_parser.add_mutually_exclusive_group(required=True)
+        group.add_argument('--up', action='store_true')
+        group.add_argument('--down', action='store_true')
+
+        msg = 'one of the arguments --up --down is required'
+        with self.assertRaisesRegex(argparse.ArgumentError, msg):
+            other_parser.parse_args([])
+
 
 def tearDownModule():
     # Remove global references to avoid looking like we have refleaks.

Misc/NEWS.d/next/Library/2022-01-23-11-58-40.bpo-46440.tMts6U.rst

@@ -0,0 +1,2 @@
+The :attr:`exit_on_error` attribute of :class:`argparse.ArgumentParser` did
+not always avoid exiting when set to False.