Make isinstance(OldstyleClass, NewstyleClass) return False instead of raising

an exception. Issue reported by Joseph Armbruster.

Lib/abc.py

@@ -116,18 +116,18 @@ class ABCMeta(type):
 
     def __instancecheck__(cls, instance):
         """Override for isinstance(instance, cls)."""
-        # Inline the cache checking for new-style classes.
-        subclass = instance.__class__
+        # Inline the cache checking when it's simple.
+        subclass = getattr(instance, '__class__', None)
         if subclass in cls._abc_cache:
             return True
         subtype = type(instance)
-        if subtype is subclass:
+        if subtype is subclass or subclass is None:
             if (cls._abc_negative_cache_version ==
                 ABCMeta._abc_invalidation_counter and
-                subclass in cls._abc_negative_cache):
+                subtype in cls._abc_negative_cache):
                 return False
             # Fall back to the subclass check.
-            return cls.__subclasscheck__(subclass)
+            return cls.__subclasscheck__(subtype)
         return (cls.__subclasscheck__(subclass) or
                 cls.__subclasscheck__(subtype))
 

Lib/test/test_abc.py

@@ -70,6 +70,17 @@ class TestABC(unittest.TestCase):
         self.assertFalse(issubclass(OldstyleClass, A))
         self.assertFalse(issubclass(A, OldstyleClass))
 
+    def test_isinstance_class(self):
+        class A:
+            __metaclass__ = abc.ABCMeta
+        class OldstyleClass:
+            pass
+        self.assertFalse(isinstance(OldstyleClass, A))
+        self.assertTrue(isinstance(OldstyleClass, type(OldstyleClass)))
+        self.assertFalse(isinstance(A, OldstyleClass))
+        # This raises a recursion depth error, but is low-priority:
+        # self.assertTrue(isinstance(A, abc.ABCMeta))
+
     def test_registration_basics(self):
         class A:
             __metaclass__ = abc.ABCMeta