diff -r 44f455e6163d Lib/argparse.py
--- a/Lib/argparse.py	Thu Jun 27 12:23:29 2013 +0200
+++ b/Lib/argparse.py	Fri Feb 21 13:33:45 2014 -0800
@@ -1551,6 +1551,9 @@
         super(_MutuallyExclusiveGroup, self).__init__(container)
         self.required = required
         self._container = container
+        # register the test for this type of group
+        mytest = _MutuallyExclusiveGroup.test_mut_ex_groups
+        self.register('cross_tests', 'mxg', mytest)
 
     def _add_action(self, action):
         if action.required:
@@ -1564,6 +1567,28 @@
         self._container._remove_action(action)
         self._group_actions.remove(action)
 
+    @staticmethod
+    def test_mut_ex_groups(parser, seen_non_default_actions, *vargs):
+        # alternative mutually_exclusive_groups test
+        # performed once at end of parse_args rather than with each entry
+        # the arguments listed in the error message may differ
+        # this gives a small speed improvement
+        # more importantly it is easier to customize and expand
+
+        for group in parser._mutually_exclusive_groups:
+            group_seen = seen_non_default_actions.intersection(group._group_actions)
+            cnt = len(group_seen)
+            if cnt > 1:
+                msg = 'only one the arguments %s is allowed'
+            elif cnt == 0 and group.required:
+                msg = 'one of the arguments %s is required'
+            else:
+                msg = None
+            if msg:
+                names = [_get_action_name(action)
+                            for action in group._group_actions
+                            if action.help is not SUPPRESS]
+                parser.error(msg % ' '.join(names))
 
 class ArgumentParser(_AttributeHolder, _ActionsContainer):
     """Object for parsing command line strings into Python objects.
@@ -1623,6 +1648,10 @@
             return string
         self.register('type', None, identity)
 
+        # initialize cross_tests
+        # self.register('cross_tests', ?,?)
+        self._registries['cross_tests'] = {}
+
         # add help argument if necessary
         # (using explicit default to override global argument_default)
         default_prefix = '-' if '-' in prefix_chars else prefix_chars[0]
@@ -1757,15 +1786,9 @@
         if self.fromfile_prefix_chars is not None:
             arg_strings = self._read_args_from_files(arg_strings)
 
-        # map all mutually exclusive arguments to the other arguments
-        # they can't occur with
-        action_conflicts = {}
-        for mutex_group in self._mutually_exclusive_groups:
-            group_actions = mutex_group._group_actions
-            for i, mutex_action in enumerate(mutex_group._group_actions):
-                conflicts = action_conflicts.setdefault(mutex_action, [])
-                conflicts.extend(group_actions[:i])
-                conflicts.extend(group_actions[i + 1:])
+        """
+        remove action_conflicts collection
+        """
 
         # find all option indices, and determine the arg_string_pattern
         # which has an 'O' if there is an option at an index,
@@ -1808,11 +1831,9 @@
             # value don't really count as "present"
             if argument_values is not action.default:
                 seen_non_default_actions.add(action)
-                for conflict_action in action_conflicts.get(action, []):
-                    if conflict_action in seen_non_default_actions:
-                        msg = _('not allowed with argument %s')
-                        action_name = _get_action_name(conflict_action)
-                        raise ArgumentError(action, msg % action_name)
+                """
+                remove action_conflicts use
+                """
 
             # take the action if we didn't receive a SUPPRESS value
             # (e.g. from a default)
@@ -1980,20 +2001,13 @@
             self.error(_('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:
-            if group.required:
-                for action in group._group_actions:
-                    if action in seen_non_default_actions:
-                        break
-
-                # if no actions were used, report the error
-                else:
-                    names = [_get_action_name(action)
-                             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))
+        """
+        remove required mutually_exclusive_groups test
+        """
+        # give user a hook to run more general tests on arguments
+        # its primary purpose is to give the user access to seen(_non_default)_actions
+        for testfn in self._get_cross_tests():
+            testfn(self, seen_non_default_actions, seen_actions, namespace, extras)
 
         # return the updated namespace and the extra arguments
         return namespace, extras
@@ -2207,6 +2221,21 @@
         # return the pattern
         return nargs_pattern
 
+    def _get_cross_tests(self):
+        # fetch a list (possibly empty) of tests to be run at the end of parsing
+        # for example, the mutually_exclusive_group tests
+        # or user supplied tests
+
+        # this could be in a 'parser.cross_tests' attribute
+        # tests = getattr(self, 'cross_tests', [])
+        # but here I am looking in the _registries
+        # _registries is already shared among groups
+        # allowing me to define the group tests in the group class itself
+        # This use of _registries is slight non_standard since I am
+        # ignoring the 2nd level keys
+        tests = self._registries['cross_tests'].values()
+        return tests
+
     # ========================
     # Value conversion methods
     # ========================