diff -r 97ce83671ec1 Lib/_abcoll.py
--- a/Lib/_abcoll.py	Fri Jul 23 10:17:15 2010 +0300
+++ b/Lib/_abcoll.py	Mon Jul 26 07:22:41 2010 +0300
@@ -404,6 +404,11 @@
     def __iter__(self):
         for key in self._mapping:
             yield key
+    
+    @classmethod
+    def _from_iterable(self, it):
+        return set(it)
+
 
 KeysView.register(dict_keys)
 
@@ -422,6 +427,10 @@
     def __iter__(self):
         for key in self._mapping:
             yield (key, self._mapping[key])
+            
+    @classmethod
+    def _from_iterable(self, it):
+        return set(it)
 
 ItemsView.register(dict_items)
 
diff -r 97ce83671ec1 Lib/test/test_collections.py
--- a/Lib/test/test_collections.py	Fri Jul 23 10:17:15 2010 +0300
+++ b/Lib/test/test_collections.py	Mon Jul 26 07:22:41 2010 +0300
@@ -531,13 +531,41 @@
                 return iter(())
         self.validate_comparison(MyMapping())
 
-    def test_MutableMapping(self):
+    def test_MutableMapping_types(self):
         for sample in [dict]:
             self.assertIsInstance(sample(), MutableMapping)
             self.assertTrue(issubclass(sample, MutableMapping))
         self.validate_abstract_methods(MutableMapping, '__contains__', '__iter__', '__len__',
             '__getitem__', '__setitem__', '__delitem__')
 
+    class MySimpleMapping(MutableMapping):
+        def __init__(self, *args, **kw):
+            self.inner = dict(*args, **kw)
+        def __contains__(self, item):
+            return item in self.inner
+        def __iter__(self):
+            return iter(self.inner)
+        def __len__(self):
+            return len(self.inner)
+        def __getitem__(self, index):
+            return self.inner[index]
+        def __setitem__(self, key, value):
+            self.inner[key] = value
+        def __delitem__(self, key):
+            del self.inner[key]
+            
+    def test_MutableMapping_subclass(self):
+        map = self.MySimpleMapping()
+        map['red'] = 5
+        y = map.keys()
+        self.assertIsInstance(y, Set)
+        z = map.keys() | {'orange'}
+        self.assertIsInstance(z, set)
+        
+        # Shouldn't affect 'z'
+        map['blue'] = 7
+        self.assertEqual(list(z), ['orange', 'red'])
+
     def test_Sequence(self):
         for sample in [tuple, list, bytes, str]:
             self.assertIsInstance(sample(), Sequence)