diff -r 1c48171fc8da Lib/argparse.py
--- a/Lib/argparse.py	Sat Aug 23 14:48:22 2014 +1000
+++ b/Lib/argparse.py	Tue Sep 23 15:53:25 2014 -0700
@@ -2121,6 +2121,19 @@
             if not self._has_negative_number_optionals:
                 return None
 
+        if '=' in arg_string:
+            # added test to allow space in the argument part of a '=' string
+            # such a string is treated as positional only if the space is in the 1st part
+            # otherwise it is an 'unknown'
+            # is there any backward compatibility issue with adding this test?
+            # issue 22433
+            option_prefix, explicit_arg = arg_string.split('=', 1)
+            if ' ' in option_prefix:
+                return None
+            else:
+                return None, arg_string, None
+
+
         # if it contains a space, it was meant to be a positional
         if ' ' in arg_string:
             return None
diff -r 1c48171fc8da Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Sat Aug 23 14:48:22 2014 +1000
+++ b/Lib/test/test_argparse.py	Tue Sep 23 15:53:25 2014 -0700
@@ -4737,6 +4737,29 @@
         self.assertEqual(NS(v=3, spam=True, badger="B"), args)
         self.assertEqual(["C", "--foo", "4"], extras)
 
+    def test_space(self):
+        # test handling of spaces in argument of unknown optional; issue 22433
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--foo')
+        parser.add_argument('pos')
+        args, extras = parser.parse_known_args(['xxx', '--foo=one_two', '--bar=one_two'])
+        self.assertEqual(NS(foo='one_two', pos='xxx'), args)
+        self.assertEqual(['--bar=one_two'], extras)
+
+        args, extras = parser.parse_known_args(['xxx', '--foo=one two', '--bar=one two'])
+        if True:
+            # with 22433 patch; space ok in argument of unknown optional
+            self.assertEqual(NS(foo='one two', pos='xxx'), args)
+            self.assertEqual(['--bar=one two'], extras)
+        else:
+            # for existing code; all strings with space assumed to be positional
+            self.assertEqual(NS(foo='one two', pos='--bar=one two'), args)
+            self.assertEqual(['xxx'], extras)
+        # space in flag slot makes it a positional argument
+        args, extras = parser.parse_known_args(['--foo bar=one_two', 'xxx'])
+        self.assertEqual(NS(foo=None, pos='--foo bar=one_two'), args)
+        self.assertEqual(['xxx'], extras)
+
 # ==========================
 # add_argument metavar tests
 # ==========================