diff -r a26df2d03989 Lib/argparse.py
--- a/Lib/argparse.py	Mon Apr 22 22:54:16 2013 +0300
+++ b/Lib/argparse.py	Wed Apr 24 18:39:23 2013 -0700
@@ -582,6 +582,9 @@
         elif action.nargs == PARSER:
             result = '%s ...' % get_metavar(1)
         else:
+            if not isinstance(action.nargs, int):
+                valid_nargs = [None,OPTIONAL,ZERO_OR_MORE,ONE_OR_MORE,REMAINDER,PARSER]
+                raise ValueError('nargs %r not integer or %s'%(action.nargs, valid_nargs))
             formats = ['%s' for _ in range(action.nargs)]
             result = ' '.join(formats) % get_metavar(action.nargs)
         return result
@@ -1325,6 +1328,9 @@
         if not callable(type_func):
             raise ValueError('%r is not callable' % (type_func,))
 
+        if hasattr(self, "_check_argument"):
+            self._check_argument(action)
+
         # raise an error if the metavar does not match the type
         if hasattr(self, "_get_formatter"):
             try:
@@ -1534,6 +1540,7 @@
         self._has_negative_number_optionals = \
             container._has_negative_number_optionals
         self._mutually_exclusive_groups = container._mutually_exclusive_groups
+        self._check_argument = container._check_argument
 
     def _add_action(self, action):
         action = super(_ArgumentGroup, self)._add_action(action)
@@ -1707,6 +1714,18 @@
                 for action in self._actions
                 if not action.option_strings]
 
+    def _check_argument(self, action):
+        # check action arguments
+        # focus on the arguments that the parent container does not know about
+        # check nargs and metavar tuple
+        try:
+            self._get_formatter()._format_args(action, None)
+        except ValueError as e:
+            raise ArgumentError(action, str(e))
+        except TypeError:
+            #raise ValueError("length of metavar tuple does not match nargs")
+            raise ArgumentError(action, "length of metavar tuple does not match nargs")
+
     # =====================================
     # Command line argument parsing methods
     # =====================================
@@ -2197,6 +2216,8 @@
 
         # all others should be integers
         else:
+            if not isinstance(action.nargs, int):
+                raise ValueError('nargs %r not integer or valid string'%(action.nargs))
             nargs_pattern = '(-*%s-*)' % '-*'.join('A' * nargs)
 
         # if this is an optional action, -- is not allowed
diff -r a26df2d03989 Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py	Mon Apr 22 22:54:16 2013 +0300
+++ b/Lib/test/test_argparse.py	Wed Apr 24 18:39:23 2013 -0700
@@ -4251,6 +4251,40 @@
         self.assertRaises(Success, parser.add_argument, 'spam',
                           action=Action, default=Success, const=Success)
 
+    def test_nargs_value(self):
+        """test for invalid values of nargs, not integer or accepted string
+        tests parser and groups"""
+        error_msg = r"nargs (.*) not integer or"
+        error_msg = r"argument --foo: nargs (.*) not integer or \[(.*)\]"
+        error_type = argparse.ArgumentError
+        parser = ErrorRaisingArgumentParser()
+        group = parser.add_argument_group('g')
+        m = parser.add_mutually_exclusive_group()
+
+        with self.assertRaisesRegexp(error_type, error_msg):
+            parser.add_argument('--foo', nargs='1')
+        with self.assertRaisesRegexp(error_type, error_msg):
+            group.add_argument('--foo', nargs='**')
+        with self.assertRaisesRegexp(error_type, error_msg):
+            m.add_argument('--foo', nargs='1')
+
+    def test_nargs_metavar_tuple(self):
+        "test that metavar tuple matches with nargs; test parser and groups"
+        error_msg = r'length of metavar tuple does not match nargs'
+        error_type = ValueError
+        error_msg = r'argument (.*): length of metavar tuple does not match nargs'
+        error_type = argparse.ArgumentError
+        parser = ErrorRaisingArgumentParser()
+        group = parser.add_argument_group('g')
+        m = parser.add_mutually_exclusive_group()
+
+        with self.assertRaisesRegexp(error_type, error_msg):
+            parser.add_argument('-w', help='w', nargs='+', metavar=('W1',))
+        with self.assertRaisesRegexp(error_type, error_msg):
+            group.add_argument('-x', help='x', nargs='*', metavar=('X1', 'X2', 'x3'))
+        with self.assertRaisesRegexp(error_type, error_msg):
+            m.add_argument('-y', help='y', nargs=3, metavar=('Y1', 'Y2'))
+
 # ================================
 # Actions returned by add_argument
 # ================================
@@ -4720,19 +4754,23 @@
 
 class TestAddArgumentMetavar(TestCase):
 
-    EXPECTED_MESSAGE = "length of metavar tuple does not match nargs"
+    EXPECTED_MESSAGE = "argument --foo: length of metavar tuple does not match nargs"
+    EXPECTED_ERROR = argparse.ArgumentError
 
     def do_test_no_exception(self, nargs, metavar):
         parser = argparse.ArgumentParser()
         parser.add_argument("--foo", nargs=nargs, metavar=metavar)
 
+    #def do_test_exception(self, nargs, metavar):
+    #    parser = argparse.ArgumentParser()
+    #    with self.assertRaises(ValueError) as cm:
+    #        parser.add_argument("--foo", nargs=nargs, metavar=metavar)
+    #    self.assertEqual(cm.exception.args[0], self.EXPECTED_MESSAGE)
+
     def do_test_exception(self, nargs, metavar):
         parser = argparse.ArgumentParser()
-        with self.assertRaises(ValueError) as cm:
+        with self.assertRaisesRegex(self.EXPECTED_ERROR, self.EXPECTED_MESSAGE):
             parser.add_argument("--foo", nargs=nargs, metavar=metavar)
-        self.assertEqual(cm.exception.args[0], self.EXPECTED_MESSAGE)
-
-    # Unit tests for different values of metavar when nargs=None
 
     def test_nargs_None_metavar_string(self):
         self.do_test_no_exception(nargs=None, metavar="1")