Instances of weakref.WeakSet are not instances of Set and therefore not of MutableSet but they are instances of Collection.
They however implement all required methods for a MutableSet and Weak(Key|Value)Dictionary are correctly identified.
Is this just an oversight or am I missing something?


from weakref import WeakKeyDictionary, WeakValueDictionary, WeakSet
from collections.abc import MutableMapping, Collection, Set, MutableSet

wkdict = WeakKeyDictionary()
wvdict = WeakValueDictionary()
ws = WeakSet()

assert isinstance(wkdict, MutableMapping)
assert isinstance(wvdict, MutableMapping)
assert isinstance(ws, Collection)
assert not isinstance(ws, Set)
assert not isinstance(ws, MutableSet)

We could register the WeakSet in _collections_abc, right after MutableSet is defined.  That module only registered builtins so that it can avoid imports; however, since we know that WeakSet is already loaded, it is reasonable to add an import for registration purposes.

This was likely omitted because WeakSet is used in the implementation of the ABC.

Marking this as a proposed enhancement rather than as a bug because the collections ABCs are under no obligation to register anything more than the builtins.

As I wrote between the lines in the PR, this would be a bug in the weakref module, not a bug in collections.abc.

New changeset 84ac4376587e35d16b4d0977c4f330d9d04b690a by Raymond Hettinger in branch 'master':
bpo-38761: Register WeakSet as a MutableSet (GH-17104)
https://github.com/python/cpython/commit/84ac4376587e35d16b4d0977c4f330d9d04b690a

Nils, thanks for the report.

Guido, thanks for the clarification.