Index: Lib/abc.py
===================================================================
--- Lib/abc.py	(revision 79538)
+++ Lib/abc.py	(working copy)
@@ -3,6 +3,7 @@
 
 """Abstract Base Classes (ABCs) according to PEP 3119."""
 
+from _weakrefset import WeakSet
 
 # Instance of old-style class
 class _C: pass
@@ -93,9 +94,9 @@
                     abstracts.add(name)
         cls.__abstractmethods__ = frozenset(abstracts)
         # Set up inheritance registry
-        cls._abc_registry = set()
-        cls._abc_cache = set()
-        cls._abc_negative_cache = set()
+        cls._abc_registry = WeakSet()
+        cls._abc_cache = WeakSet()
+        cls._abc_negative_cache = WeakSet()
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls
 
@@ -126,7 +127,7 @@
         """Override for isinstance(instance, cls)."""
         # Inline the cache checking when it's simple.
         subclass = getattr(instance, '__class__', None)
-        if subclass in cls._abc_cache:
+        if subclass is not None and subclass in cls._abc_cache:
             return True
         subtype = type(instance)
         # Old-style instances
@@ -150,7 +151,7 @@
         # Check negative cache; may have to invalidate
         if cls._abc_negative_cache_version < ABCMeta._abc_invalidation_counter:
             # Invalidate the negative cache
-            cls._abc_negative_cache = set()
+            cls._abc_negative_cache = WeakSet()
             cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         elif subclass in cls._abc_negative_cache:
             return False
Index: Modules/Setup.dist
===================================================================
--- Modules/Setup.dist	(revision 79538)
+++ Modules/Setup.dist	(working copy)
@@ -118,6 +118,7 @@
 				# if $HOME is not set
 _sre _sre.c			# Fredrik Lundh's new regular expressions
 _codecs _codecsmodule.c		# access to the builtin codecs and codec registry
+_weakref _weakref.c             # weak references
 
 # The zipimport module is always imported at startup. Having it as a
 # builtin module avoids some bootstrapping problems and reduces overhead.