gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (#91499)
Co-authored-by: Pieter Eendebak <P.T.eendebak@tudelft.nl>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index 73e7b21..1406b66 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -172,6 +172,30 @@
application without adding attributes to those objects. This can be especially
useful with objects that override attribute accesses.
+ Note that when a key with equal value to an existing key (but not equal identity)
+ is inserted into the dictionary, it replaces the value but does not replace the
+ existing key. Due to this, when the reference to the original key is deleted, it
+ also deletes the entry in the dictionary::
+
+ >>> class T(str): pass
+ ...
+ >>> k1, k2 = T(), T()
+ >>> d = weakref.WeakKeyDictionary()
+ >>> d[k1] = 1 # d = {k1: 1}
+ >>> d[k2] = 2 # d = {k1: 2}
+ >>> del k1 # d = {}
+
+ A workaround would be to remove the key prior to reassignment::
+
+ >>> class T(str): pass
+ ...
+ >>> k1, k2 = T(), T()
+ >>> d = weakref.WeakKeyDictionary()
+ >>> d[k1] = 1 # d = {k1: 1}
+ >>> del d[k1]
+ >>> d[k2] = 2 # d = {k2: 2}
+ >>> del k1 # d = {k2: 2}
+
.. versionchanged:: 3.9
Added support for ``|`` and ``|=`` operators, specified in :pep:`584`.
