[3.6] bpo-29581: Make __init_subclass__ work on ABCMeta type classes

Lib/abc.py

@@ -129,8 +129,8 @@ class ABCMeta(type):
     #       external code.
     _abc_invalidation_counter = 0
 
-    def __new__(mcls, name, bases, namespace):
-        cls = super().__new__(mcls, name, bases, namespace)
+    def __new__(mcls, name, bases, namespace, **kwargs):
+        cls = super().__new__(mcls, name, bases, namespace, **kwargs)
         # Compute set of abstract method names
         abstracts = {name
                      for name, value in namespace.items()

Lib/test/test_abc.py

@@ -404,5 +404,17 @@ class C(A, B):
         self.assertEqual(B.counter, 1)
 
 
+class TestABCWithInitSubclass(unittest.TestCase):
+    def test_works_with_init_subclass(self):
+        saved_kwargs = {}
+        class ReceivesClassKwargs:
+            def __init_subclass__(cls, **kwargs):
+                super().__init_subclass__()
+                saved_kwargs.update(kwargs)
+        class Receiver(ReceivesClassKwargs, abc.ABC, x=1, y=2, z=3):
+            pass
+        self.assertEqual(saved_kwargs, dict(x=1, y=2, z=3))
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS

@@ -45,6 +45,9 @@ Core and Builtins
 Library
 -------
 
+- bpo-29581: ABCMeta.__new__ now accepts ``**kwargs``, allowing abstract base
+  classes to use keyword parameters in __init_subclass__. Patch by Nate Soares.
+
 - bpo-30557: faulthandler now correctly filters and displays exception codes
   on Windows