diff -r ac27d979078a Lib/enum.py
--- a/Lib/enum.py	Thu Sep 05 00:23:08 2013 +0200
+++ b/Lib/enum.py	Wed Sep 04 22:23:28 2013 -0700
@@ -256,20 +256,33 @@ class EnumMeta(type):
 
     def __iter__(cls):
         return (cls._member_map_[name] for name in cls._member_names_)
 
     def __len__(cls):
         return len(cls._member_names_)
 
     def __repr__(cls):
         return "<enum %r>" % cls.__name__
 
+    def __setattr__(cls, name, value):
+        """Block attempts to reassign Enum members.
+
+        A simple assignment to the class namespace only changes one of the
+        several possible ways to get an Enum member from the Enum class,
+        resulting in an inconsistent Enumeration.
+
+        """
+        member_map = cls.__dict__.get('_member_map_', ())
+        if name in member_map:
+            raise AttributeError('Cannot reassign members.')
+        super().__setattr__(name, value)
+
     def _create_(cls, class_name, names=None, *, module=None, type=None):
         """Convenience method to create a new Enum class.
 
         `names` can be:
 
         * A string containing member names, separated either with spaces or
           commas.  Values are auto-numbered from 1.
         * An iterable of member names.  Values are auto-numbered from 1.
         * An iterable of (member name, value) pairs.
         * A mapping of member name -> value.
diff -r ac27d979078a Lib/test/test_enum.py
--- a/Lib/test/test_enum.py	Thu Sep 05 00:23:08 2013 +0200
+++ b/Lib/test/test_enum.py	Wed Sep 04 22:23:28 2013 -0700
@@ -145,20 +145,25 @@ class TestEnum(unittest.TestCase):
 
     def test_value_name(self):
         Season = self.Season
         self.assertEqual(Season.SPRING.name, 'SPRING')
         self.assertEqual(Season.SPRING.value, 1)
         with self.assertRaises(AttributeError):
             Season.SPRING.name = 'invierno'
         with self.assertRaises(AttributeError):
             Season.SPRING.value = 2
 
+    def test_changing_member(self):
+        Season = self.Season
+        with self.assertRaises(AttributeError):
+            Season.WINTER = 'really cold'
+
     def test_invalid_names(self):
         with self.assertRaises(ValueError):
             class Wrong(Enum):
                 mro = 9
         with self.assertRaises(ValueError):
             class Wrong(Enum):
                 _create_= 11
         with self.assertRaises(ValueError):
             class Wrong(Enum):
                 _get_mixins_ = 9