bpo-41255: handle argparse errors with exit_on_error=False consistently

Doc/library/argparse.rst

@@ -2090,8 +2090,9 @@ Exiting methods
 
 .. method:: ArgumentParser.error(message)
 
-   This method prints a usage message including the *message* to the
-   standard error and terminates the program with a status code of 2.
+   When exit on error is ``True`` this method prints a usage message including
+   the *message* to the standard error and terminates the program with a status
+   code of 2, otherwise raises an ArgumentError.
 
 
 Intermixed parsing

Lib/argparse.py

@@ -1872,13 +1872,11 @@ def parse_known_args(self, args=None, namespace=None):
                 setattr(namespace, dest, self._defaults[dest])
 
         # parse the arguments and exit if there are any errors
-        if self.exit_on_error:
-            try:
-                namespace, args = self._parse_known_args(args, namespace)
-            except ArgumentError as err:
-                self.error(str(err))
-        else:
+        try:
             namespace, args = self._parse_known_args(args, namespace)
+        except ArgumentError:
+            err = _sys.exc_info()[1]
+            self.error(str(err))
 
         if hasattr(namespace, _UNRECOGNIZED_ARGS_ATTR):
             args.extend(getattr(namespace, _UNRECOGNIZED_ARGS_ATTR))
@@ -2588,12 +2586,16 @@ def exit(self, status=0, message=None):
     def error(self, message):
         """error(message: string)
 
-        Prints a usage message incorporating the message to stderr and
-        exits.
+        When exit_on_error is true prints a usage message
+        incorporating the message to stderr and exits, otherwise raises
+        an ArgumentError.
 
         If you override this in a subclass, it should not return -- it
         should either exit or raise an exception.
         """
+        if not self.exit_on_error:
+            raise ArgumentError(None, message)
+
         self.print_usage(_sys.stderr)
         args = {'prog': self.prog, 'message': message}
         self.exit(2, _('%(prog)s: error: %(message)s\n') % args)

Lib/test/test_argparse.py

@@ -5541,7 +5541,7 @@ 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())
@@ -5551,6 +5551,14 @@ 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_args(self):
+        with self.assertRaises(argparse.ArgumentError):
+            self.parser.parse_args([])
+
+    def test_exit_on_error_unknown_args(self):
+        with self.assertRaises(argparse.ArgumentError):
+            self.parser.parse_args("--integers 1 --unknown-arg".split())
+
 
 def tearDownModule():
     # Remove global references to avoid looking like we have refleaks.

Misc/NEWS.d/next/Library/2021-07-22-21-56-34.bpo-41255.tbSAVo.rst

@@ -0,0 +1 @@
+fixed errors with argparse exit_on_error causing unexpected behavior