gh-142740: Fix ABC registration for SetProxy in multiprocessing.managers

Lib/multiprocessing/managers.py

@@ -1224,7 +1224,7 @@ def __isub__(self, value):
 
     __class_getitem__ = classmethod(types.GenericAlias)
 
-collections.abc.MutableMapping.register(_BaseSetProxy)
+collections.abc.MutableSet.register(_BaseSetProxy)
 
 
 ArrayProxy = MakeProxyType('ArrayProxy', (

Lib/test/_test_multiprocessing.py

@@ -2786,6 +2786,29 @@ def test_dict_proxy_nested(self):
         self.assertIsInstance(outer[0], list)  # Not a ListProxy
         self.assertEqual(outer[-1][-1]['feed'], 3)
 
+    def test_set_isinstance(self):
+        s = self.set()
+        self.assertIsInstance(s, collections.abc.MutableSet)
+        self.assertNotIsInstance(s, collections.abc.MutableMapping)
+
+        mutable_set_methods = (
+            '__contains__', '__iter__', '__len__',
+            'add', 'discard', 'remove', 'pop', 'clear',
+            'update', 'difference_update', 'intersection_update',
+            'symmetric_difference_update',
+        )
+        for name in mutable_set_methods:
+            with self.subTest(name=name):
+                self.assertTrue(callable(getattr(s, name)))
+
+        mapping_only_methods = (
+            '__getitem__', '__setitem__', 'setdefault',
+            'keys', 'items', 'values', 'get',
+        )
+        for name in mapping_only_methods:
+            with self.subTest(name=name):
+                self.assertFalse(hasattr(s, name))
+
     def test_nested_queue(self):
         a = self.list() # Test queue inside list
         a.append(self.Queue())
@@ -7131,6 +7154,7 @@ class ManagerMixin(BaseMixin):
     Array = property(operator.attrgetter('manager.Array'))
     list = property(operator.attrgetter('manager.list'))
     dict = property(operator.attrgetter('manager.dict'))
+    set = property(operator.attrgetter('manager.set'))
     Namespace = property(operator.attrgetter('manager.Namespace'))
 
     @classmethod

Misc/NEWS.d/next/Library/2025-12-18-10-37-01.gh-issue-142740.Di48Rf.rst

@@ -0,0 +1 @@
+Register ``multiprocessing.managers.SetProxy`` as ``collections.abc.MutableSet`` rather than ``MutableMapping``.