bpo-38761: Register WeakSet as a MutableSet (GH-17104)

Lib/test/test_weakset.py

@@ -2,6 +2,7 @@ import unittest
 from weakref import WeakSet
 import string
 from collections import UserString as ustr
+from collections.abc import Set, MutableSet
 import gc
 import contextlib
 
@@ -437,6 +438,10 @@ class TestWeakSet(unittest.TestCase):
     def test_repr(self):
         assert repr(self.s) == repr(self.s.data)
 
+    def test_abc(self):
+        self.assertIsInstance(self.s, Set)
+        self.assertIsInstance(self.s, MutableSet)
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/weakref.py

@@ -33,6 +33,9 @@ __all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
            "WeakSet", "WeakMethod", "finalize"]
 
 
+_collections_abc.Set.register(WeakSet)
+_collections_abc.MutableSet.register(WeakSet)
+
 class WeakMethod(ref):
     """
     A custom `weakref.ref` subclass which simulates a weak reference to

Misc/NEWS.d/next/Library/2019-11-10-13-40-33.bpo-38761.P1UUIZ.rst

@@ -0,0 +1 @@
+WeakSet is now registered as a collections.abc.MutableSet.