From 64ecfe9a6db069d5b3ab937f9774de35894db05b Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Sun, 10 Nov 2019 13:41:04 -0800
Subject: [PATCH 1/4] bpo-38761: Register WeakSet as a MutableSet

---
 Lib/_collections_abc.py                                       | 4 +++-
 Lib/test/test_weakset.py                                      | 4 ++++
 .../next/Library/2019-11-10-13-40-33.bpo-38761.P1UUIZ.rst     | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-11-10-13-40-33.bpo-38761.P1UUIZ.rst

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index 2b2ddba170e1b1..ce6356cc148ce3 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -551,7 +551,9 @@ def _hash(self):
         return h
 
 Set.register(frozenset)
-
+import _weakrefset
+Set.register(_weakrefset.WeakSet)
+del _weakrefset
 
 class MutableSet(Set):
     """A mutable set is a finite, iterable container.
diff --git a/Lib/test/test_weakset.py b/Lib/test/test_weakset.py
index 569facdd30c11c..2bcc0a3f9bc3da 100644
--- a/Lib/test/test_weakset.py
+++ b/Lib/test/test_weakset.py
@@ -2,6 +2,7 @@
 from weakref import WeakSet
 import string
 from collections import UserString as ustr
+from collections.abc import MutableSet
 import gc
 import contextlib
 
@@ -437,6 +438,9 @@ def test_len_race(self):
     def test_repr(self):
         assert repr(self.s) == repr(self.s.data)
 
+    def test_abc(self):
+        self.assertIsInstance(self.s, MutableSet)
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2019-11-10-13-40-33.bpo-38761.P1UUIZ.rst b/Misc/NEWS.d/next/Library/2019-11-10-13-40-33.bpo-38761.P1UUIZ.rst
new file mode 100644
index 00000000000000..4dde5ebc3208ca
--- /dev/null
+++ b/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.

From d5fc2925be72c65f6dfa953cb62e48a38d6ec1de Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Sun, 10 Nov 2019 14:41:05 -0800
Subject: [PATCH 2/4] Register to both Set and MutableSet

---
 Lib/_collections_abc.py  | 3 +++
 Lib/test/test_weakset.py | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index ce6356cc148ce3..852032b68a9079 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -635,6 +635,9 @@ def __isub__(self, it):
         return self
 
 MutableSet.register(set)
+import _weakrefset
+MutableSet.register(_weakrefset.WeakSet)
+del _weakrefset
 
 
 ### MAPPINGS ###
diff --git a/Lib/test/test_weakset.py b/Lib/test/test_weakset.py
index 2bcc0a3f9bc3da..49a9b5c3c658a8 100644
--- a/Lib/test/test_weakset.py
+++ b/Lib/test/test_weakset.py
@@ -2,7 +2,7 @@
 from weakref import WeakSet
 import string
 from collections import UserString as ustr
-from collections.abc import MutableSet
+from collections.abc import Set, MutableSet
 import gc
 import contextlib
 
@@ -439,6 +439,7 @@ 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)
 
 

From 116881843b433c9df431b5bcf1f648e21a36b468 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Sun, 10 Nov 2019 18:07:43 -0800
Subject: [PATCH 3/4] Move registration to the weakref module

---
 Lib/_collections_abc.py | 6 ------
 Lib/weakref.py          | 3 +++
 2 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index 852032b68a9079..ed3b0778984044 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -551,9 +551,6 @@ def _hash(self):
         return h
 
 Set.register(frozenset)
-import _weakrefset
-Set.register(_weakrefset.WeakSet)
-del _weakrefset
 
 class MutableSet(Set):
     """A mutable set is a finite, iterable container.
@@ -635,9 +632,6 @@ def __isub__(self, it):
         return self
 
 MutableSet.register(set)
-import _weakrefset
-MutableSet.register(_weakrefset.WeakSet)
-del _weakrefset
 
 
 ### MAPPINGS ###
diff --git a/Lib/weakref.py b/Lib/weakref.py
index d17b3ed38eacb7..e3c2ce2d9b8b86 100644
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -33,6 +33,9 @@
            "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

From ce40418a6454efba8d2d16ead22aeee55a0375f5 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <raymond.hettinger@gmail.com>
Date: Sun, 10 Nov 2019 18:10:09 -0800
Subject: [PATCH 4/4] Restore line spacing

---
 Lib/_collections_abc.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index ed3b0778984044..2b2ddba170e1b1 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -552,6 +552,7 @@ def _hash(self):
 
 Set.register(frozenset)
 
+
 class MutableSet(Set):
     """A mutable set is a finite, iterable container.