bpo-22433: do not consider "--foo='bar baz'" to be a positional argument

Lib/argparse.py

@@ -2223,8 +2223,8 @@ def _parse_optional(self, arg_string):
             if not self._has_negative_number_optionals:
                 return None
 
-        # if it contains a space, it was meant to be a positional
-        if ' ' in arg_string:
+        # if it contains a space (before any equal sign), it was meant to be a positional
+        if ' ' in arg_string.split("=", 1)[0]:
             return None
 
         # it was meant to be an optional but there is no such option

Lib/test/test_argparse.py

@@ -2018,6 +2018,15 @@ def test_parse_known_args(self):
             self.parser.parse_known_args('0.5 -W 1 b -X Y -w 7 Z'.split()),
             (NS(foo=False, bar=0.5, w=7, x='b'), ['-W', '-X', 'Y', 'Z']),
         )
+        self.assertEqual(
+            self.parser.parse_known_args(['0.5', '--opt="with space"']),
+            (NS(foo=False, bar=0.5), ['--opt="with space"']),
+        )
+        self.assertRaisesRegexp(
+            ArgumentParserError,
+            "invalid choice: '--opt with space'",
+            self.parser.parse_known_args,
+            ['0.5', '--opt with space'])
 
     def test_dest(self):
         parser = ErrorRaisingArgumentParser()