Fix for bug #1109

Warning required when calling register() on an ABCMeta subclass.

Lib/abc.py

@@ -137,8 +137,11 @@ class ABCMeta(type):
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls
 
-    def register(cls, subclass):
+    def register(cls, subclass=None):
         """Register a virtual subclass of an ABC."""
+        if subclass is None:
+            raise TypeError("register() cannot be called on an ABCMeta "
+                "subclass, use class Example(metaclass=abc.ABCMeta) instead.")
         if not isinstance(cls, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):

Lib/test/test_abc.py

@@ -146,6 +146,13 @@ class TestABC(unittest.TestCase):
         C()
         self.assertEqual(B.counter, 1)
 
+    def test_error_on_subclass(self):
+        class A(abc.ABCMeta):
+            pass
+        class B:
+            pass
+        self.assertRaises(TypeError, A.register, B)
+
 
 def test_main():
     test_support.run_unittest(TestABC)