bpo-30463: Add an empty __slots__ to abc.ABC.

Lib/abc.py

@@ -235,7 +235,7 @@ class ABC(metaclass=ABCMeta):
     """Helper class that provides a standard way to create an ABC using
     inheritance.
     """
-    pass
+    __slots__ = ()
 
 
 def get_cache_token():

Lib/test/test_abc.py

@@ -402,6 +402,9 @@ class TestABC(unittest.TestCase):
         C()
         self.assertEqual(B.counter, 1)
 
+    def test_ABC_has___slots__(self):
+        self.assertTrue(hasattr(abc.ABC, '__slots__'))
+
 
 class TestABCWithInitSubclass(unittest.TestCase):
     def test_works_with_init_subclass(self):

Misc/NEWS

@@ -345,6 +345,9 @@ Extension Modules
 Library
 -------
 
+- bpo-30463: Addded empty __slots__ to abc.ABC.  This allows subclassers
+  to deny __dict__ and __weakref__ creation.  Patch by Aaron Hall.
+
 - bpo-30520: Loggers are now pickleable.
 
 - bpo-30557: faulthandler now correctly filters and displays exception codes