Anti-registration of various ABC methods.

- Issue #25958: Support "anti-registration" of special methods from
  various ABCs, like __hash__, __iter__ or __len__.  All these (and
  several more) can be set to None in an implementation class and the
  behavior will be as if the method is not defined at all.
  (Previously, this mechanism existed only for __hash__, to make
  mutable classes unhashable.)  Code contributed by Andrew Barnert and
  Ivan Levkivskyi.

Lib/test/test_contains.py

@@ -84,6 +84,31 @@ class TestContains(unittest.TestCase):
             self.assertTrue(container == constructor(values))
             self.assertTrue(container == container)
 
+    def test_block_fallback(self):
+        # blocking fallback with __contains__ = None
+        class ByContains(object):
+            def __contains__(self, other):
+                return False
+        c = ByContains()
+        class BlockContains(ByContains):
+            """Is not a container
+
+            This class is a perfectly good iterable (as tested by
+            list(bc)), as well as inheriting from a perfectly good
+            container, but __contains__ = None prevents the usual
+            fallback to iteration in the container protocol. That
+            is, normally, 0 in bc would fall back to the equivalent
+            of any(x==0 for x in bc), but here it's blocked from
+            doing so.
+            """
+            def __iter__(self):
+                while False:
+                    yield None
+            __contains__ = None
+        bc = BlockContains()
+        self.assertFalse(0 in c)
+        self.assertFalse(0 in list(bc))
+        self.assertRaises(TypeError, lambda: 0 in bc)
 
 if __name__ == '__main__':
     unittest.main()