Anti-registration of various ABC methods.

- Issue #25958: Support "anti-registration" of special methods from
  various ABCs, like __hash__, __iter__ or __len__.  All these (and
  several more) can be set to None in an implementation class and the
  behavior will be as if the method is not defined at all.
  (Previously, this mechanism existed only for __hash__, to make
  mutable classes unhashable.)  Code contributed by Andrew Barnert and
  Ivan Levkivskyi.

1 files changed, 29 insertions, 49 deletions

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index ffee385..077bde4 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -62,6 +62,18 @@ del _coro
 
 ### ONE-TRICK PONIES ###
 
+def _check_methods(C, *methods):
+    mro = C.__mro__
+    for method in methods:
+        for B in mro:
+            if method in B.__dict__:
+                if B.__dict__[method] is None:
+                    return NotImplemented
+                break
+        else:
+            return NotImplemented
+    return True
+
 class Hashable(metaclass=ABCMeta):
 
     __slots__ = ()
@@ -73,11 +85,7 @@ class Hashable(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Hashable:
-            for B in C.__mro__:
-                if "__hash__" in B.__dict__:
-                    if B.__dict__["__hash__"]:
-                        return True
-                    break
+            return _check_methods(C, "__hash__")
         return NotImplemented
 
 
@@ -92,11 +100,7 @@ class Awaitable(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Awaitable:
-            for B in C.__mro__:
-                if "__await__" in B.__dict__:
-                    if B.__dict__["__await__"]:
-                        return True
-                    break
+            return _check_methods(C, "__await__")
         return NotImplemented
 
 
@@ -137,14 +141,7 @@ class Coroutine(Awaitable):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Coroutine:
-            mro = C.__mro__
-            for method in ('__await__', 'send', 'throw', 'close'):
-                for base in mro:
-                    if method in base.__dict__:
-                        break
-                else:
-                    return NotImplemented
-            return True
+            return _check_methods(C, '__await__', 'send', 'throw', 'close')
         return NotImplemented
 
 
@@ -162,8 +159,7 @@ class AsyncIterable(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is AsyncIterable:
-            if any("__aiter__" in B.__dict__ for B in C.__mro__):
-                return True
+            return _check_methods(C, "__aiter__")
         return NotImplemented
 
 
@@ -182,9 +178,7 @@ class AsyncIterator(AsyncIterable):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is AsyncIterator:
-            if (any("__anext__" in B.__dict__ for B in C.__mro__) and
-                any("__aiter__" in B.__dict__ for B in C.__mro__)):
-                return True
+            return _check_methods(C, "__anext__", "__aiter__")
         return NotImplemented
 
 
@@ -200,8 +194,7 @@ class Iterable(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Iterable:
-            if any("__iter__" in B.__dict__ for B in C.__mro__):
-                return True
+            return _check_methods(C, "__iter__")
         return NotImplemented
 
 
@@ -220,9 +213,7 @@ class Iterator(Iterable):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Iterator:
-            if (any("__next__" in B.__dict__ for B in C.__mro__) and
-                any("__iter__" in B.__dict__ for B in C.__mro__)):
-                return True
+            return _check_methods(C, '__iter__', '__next__')
         return NotImplemented
 
 Iterator.register(bytes_iterator)
@@ -246,16 +237,13 @@ class Reversible(Iterable):
 
     @abstractmethod
     def __reversed__(self):
-        return NotImplemented
+        while False:
+            yield None
 
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Reversible:
-            for B in C.__mro__:
-                if "__reversed__" in B.__dict__:
-                    if B.__dict__["__reversed__"] is not None:
-                        return True
-                    break
+            return _check_methods(C, "__reversed__", "__iter__")
         return NotImplemented
 
 
@@ -302,17 +290,10 @@ class Generator(Iterator):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Generator:
-            mro = C.__mro__
-            for method in ('__iter__', '__next__', 'send', 'throw', 'close'):
-                for base in mro:
-                    if method in base.__dict__:
-                        break
-                else:
-                    return NotImplemented
-            return True
+            return _check_methods(C, '__iter__', '__next__',
+                                  'send', 'throw', 'close')
         return NotImplemented
 
-
 Generator.register(generator)
 
 
@@ -327,8 +308,7 @@ class Sized(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Sized:
-            if any("__len__" in B.__dict__ for B in C.__mro__):
-                return True
+            return _check_methods(C, "__len__")
         return NotImplemented
 
 
@@ -343,8 +323,7 @@ class Container(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Container:
-            if any("__contains__" in B.__dict__ for B in C.__mro__):
-                return True
+            return _check_methods(C, "__contains__")
         return NotImplemented
 
 
@@ -359,8 +338,7 @@ class Callable(metaclass=ABCMeta):
     @classmethod
     def __subclasshook__(cls, C):
         if cls is Callable:
-            if any("__call__" in B.__dict__ for B in C.__mro__):
-                return True
+            return _check_methods(C, "__call__")
         return NotImplemented
 
 
@@ -640,6 +618,8 @@ class Mapping(Sized, Iterable, Container):
             return NotImplemented
         return dict(self.items()) == dict(other.items())
 
+    __reversed__ = None
+
 Mapping.register(mappingproxy)