gh-85588: Restore the __ne__() method in Set and Mapping

Lib/_collections_abc.py

@@ -594,6 +594,8 @@ def __eq__(self, other):
             return NotImplemented
         return len(self) == len(other) and self.__le__(other)
 
+    __ne__ = object.__ne__
+
     @classmethod
     def _from_iterable(cls, it):
         '''Construct an instance of the class from any iterable input.
@@ -821,6 +823,8 @@ def __eq__(self, other):
             return NotImplemented
         return dict(self.items()) == dict(other.items())
 
+    __ne__ = object.__ne__
+
     __reversed__ = None
 
 Mapping.register(mappingproxy)

Lib/test/test_collections.py

@@ -751,7 +751,8 @@ def validate_isinstance(self, abc, name):
         self.assertNotIsInstance(C(), abc)
         self.assertNotIsSubclass(C, abc)
 
-    def validate_comparison(self, instance):
+    def validate_comparison(self, klass):
+        instance = klass()
         ops = ['lt', 'gt', 'le', 'ge', 'ne', 'or', 'and', 'xor', 'sub']
         operators = {}
         for op in ops:
@@ -782,6 +783,21 @@ def __eq__(self, other):
             self.assertTrue(other.right_side,'Right side not called for %s.%s'
                             % (type(instance), name))
 
+        # gh-85588: Inherited __ne__ should be overriddent together with __eq__
+        # in Set and Mapping.
+        class Mixin:
+            def __eq__(self, other):
+                raise AssertionError('should not be called')
+            __ne__ = __eq__
+        class C(klass, Mixin):
+            pass
+        instance = C()
+        other = object()
+        self.assertIs(instance == other, False)
+        self.assertIs(instance != other, True)
+        self.assertIs(instance.__eq__(other), NotImplemented)
+        self.assertIs(instance.__ne__(other), NotImplemented)
+
 def _test_gen():
     yield
 
@@ -1430,7 +1446,7 @@ def __len__(self):
                 return 0
             def __iter__(self):
                 return iter([])
-        self.validate_comparison(MySet())
+        self.validate_comparison(MySet)
 
     def test_hash_Set(self):
         class OneTwoThreeSet(Set):
@@ -1852,7 +1868,7 @@ def __getitem__(self, i):
                 raise IndexError
             def __iter__(self):
                 return iter(())
-        self.validate_comparison(MyMapping())
+        self.validate_comparison(MyMapping)
         self.assertRaises(TypeError, reversed, MyMapping())
 
     def test_MutableMapping(self):

Misc/NEWS.d/next/Library/2025-11-10-18-38-58.gh-issue-85588.562vvi.rst

@@ -0,0 +1,5 @@
+Restore the :meth:`!__ne__` method (identical to :meth:`object.__ne__`) in
+:class:`collections.abc.Set` and :class:`collections.abc.Mapping` classes.
+This guarantees that the ``!=`` operator is consistent with the ``==``
+operator, even if the :meth:`!__ne__` method was defined in other parent
+class.