Allow usage of SomeABC.register as a class decorator. Patch by Edoardo Spadolini (#10868).

Doc/library/abc.rst

@@ -55,6 +55,9 @@ This module provides the following class:
         assert issubclass(tuple, MyABC)
         assert isinstance((), MyABC)
 
+      .. versionchanged:: 3.3
+         Returns the registered subclass, to allow usage as a class decorator.
+
    You can also override this method in an abstract base class:
 
    .. method:: __subclasshook__(subclass)

Lib/abc.py

@@ -133,11 +133,14 @@ class ABCMeta(type):
         return cls
 
     def register(cls, subclass):
-        """Register a virtual subclass of an ABC."""
+        """Register a virtual subclass of an ABC.
+
+        Returns the subclass, to allow usage as a class decorator.
+        """
         if not isinstance(subclass, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):
-            return  # Already a subclass
+            return subclass  # Already a subclass
         # Subtle: test for cycles *after* testing for "already a subclass";
         # this means we allow X.register(X) and interpret it as a no-op.
         if issubclass(cls, subclass):
@@ -145,6 +148,7 @@ class ABCMeta(type):
             raise RuntimeError("Refusing to create an inheritance cycle")
         cls._abc_registry.add(subclass)
         ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
+        return subclass
 
     def _dump_registry(cls, file=None):
         """Debug helper to print the ABC registry."""

Lib/test/test_abc.py

@@ -121,11 +121,12 @@ class TestABC(unittest.TestCase):
         self.assertFalse(issubclass(B, (A,)))
         self.assertNotIsInstance(b, A)
         self.assertNotIsInstance(b, (A,))
-        A.register(B)
+        B1 = A.register(B)
         self.assertTrue(issubclass(B, A))
         self.assertTrue(issubclass(B, (A,)))
         self.assertIsInstance(b, A)
         self.assertIsInstance(b, (A,))
+        self.assertIs(B1, B)
         class C(B):
             pass
         c = C()
@@ -134,6 +135,27 @@ class TestABC(unittest.TestCase):
         self.assertIsInstance(c, A)
         self.assertIsInstance(c, (A,))
 
+    def test_register_as_class_deco(self):
+        class A(metaclass=abc.ABCMeta):
+            pass
+        @A.register
+        class B(object):
+            pass
+        b = B()
+        self.assertTrue(issubclass(B, A))
+        self.assertTrue(issubclass(B, (A,)))
+        self.assertIsInstance(b, A)
+        self.assertIsInstance(b, (A,))
+        @A.register
+        class C(B):
+            pass
+        c = C()
+        self.assertTrue(issubclass(C, A))
+        self.assertTrue(issubclass(C, (A,)))
+        self.assertIsInstance(c, A)
+        self.assertIsInstance(c, (A,))
+        self.assertIs(C, A.register(C))
+
     def test_isinstance_invalidation(self):
         class A(metaclass=abc.ABCMeta):
             pass

Misc/ACKS

@@ -798,6 +798,7 @@ Rafal Smotrzyk
 Dirk Soede
 Paul Sokolovsky
 Cody Somerville
+Edoardo Spadolini
 Clay Spence
 Per Spilling
 Joshua Spoerri

Misc/NEWS

@@ -30,6 +30,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #10868: Allow usage of the register method of an ABC as a class
+  decorator.
+
 - Issue #11224: Fixed a regression in tarfile that affected the file-like
   objects returned by TarFile.extractfile() regarding performance, memory
   consumption and failures with the stream interface.