raise a clear TypeError when trying to register a non-class

Lib/abc.py

@@ -96,7 +96,7 @@ class ABCMeta(type):
 
     def register(cls, subclass):
         """Register a virtual subclass of an ABC."""
-        if not isinstance(cls, type):
+        if not isinstance(subclass, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):
             return  # Already a subclass

Lib/test/test_abc.py

@@ -149,6 +149,12 @@ class TestABC(unittest.TestCase):
         self.assertRaises(RuntimeError, C.register, A)  # cycles not allowed
         C.register(B)  # ok
 
+    def test_register_non_class(self):
+        class A(object):
+            __metaclass__ = abc.ABCMeta
+        self.assertRaisesRegexp(TypeError, "Can only register classes",
+                                A.register, 4)
+
     def test_registration_transitiveness(self):
         class A:
             __metaclass__ = abc.ABCMeta

Misc/NEWS

@@ -47,6 +47,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #7792: Registering non-classes to ABCs raised an obscure error.
+
 - Removed the functions 'verify' and 'vereq' from Lib/test/test_support.py.
 
 - Issue #7773: Fix an UnboundLocalError in platform.linux_distribution() when