gh-73055: Set up inheritance registry in ABCMeta.__init__.

Doc/whatsnew/3.9.rst

@@ -645,6 +645,12 @@ Changes in the Python API
   since the *buffering* parameter has been removed.
   (Contributed by Victor Stinner in :issue:`39357`.)
 
+* The inheritance registry for an :class:`~abc.ABCMeta` instance is now
+  set up in the ``__init__()`` method instead of the ``__new__()`` method.
+  Be aware of this if you implement a subclass of ``ABCMeta`` and override
+  its ``__new__()`` or ``__init__()`` methods.
+  (Contributed by Serhiy Storchaka in :issue:`28869`.)
+
 
 CPython bytecode changes
 ------------------------

Lib/_py_abc.py

@@ -32,8 +32,7 @@ class ABCMeta(type):
     #       external code.
     _abc_invalidation_counter = 0
 
-    def __new__(mcls, name, bases, namespace, /, **kwargs):
-        cls = super().__new__(mcls, name, bases, namespace, **kwargs)
+    def __init__(cls, name, bases, namespace, /, **kwargs):
         # Compute set of abstract method names
         abstracts = {name
                      for name, value in namespace.items()
@@ -49,7 +48,6 @@ def __new__(mcls, name, bases, namespace, /, **kwargs):
         cls._abc_cache = WeakSet()
         cls._abc_negative_cache = WeakSet()
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
-        return cls
 
     def register(cls, subclass):
         """Register a virtual subclass of an ABC.

Lib/abc.py

@@ -81,10 +81,8 @@ class ABCMeta(type):
         implementations defined by the registering ABC be callable (not
         even via super()).
         """
-        def __new__(mcls, name, bases, namespace, **kwargs):
-            cls = super().__new__(mcls, name, bases, namespace, **kwargs)
+        def __init__(cls, name, bases, namespace, /, **kwargs):
             _abc_init(cls)
-            return cls
 
         def register(cls, subclass):
             """Register a virtual subclass of an ABC.

Lib/test/test_abc.py

@@ -488,6 +488,13 @@ class C(with_metaclass(abc_ABCMeta, A, B)):
                 pass
             self.assertEqual(C.__class__, abc_ABCMeta)
 
+        def test_type_module(self):
+            class A(metaclass=abc_ABCMeta):
+                pass
+            self.assertEqual(A.__module__, __name__)
+            B = abc_ABCMeta('B', (), {})
+            self.assertEqual(B.__module__, __name__)
+
 
     class TestABCWithInitSubclass(unittest.TestCase):
         def test_works_with_init_subclass(self):

Misc/NEWS.d/next/Library/2019-10-07-13-10-09.bpo-28869.6epshM.rst

@@ -0,0 +1,4 @@
+Types created by calling :func:`~abc.ABCMeta` have now a proper
+:attr:`__module__` attribute.
+The inheritance registry for an ``ABCMeta`` instance is now
+set up in the ``__init__()`` method instead of the ``__new__()`` method.