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gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (GH-91499) 

(cherry picked from commit c615286e85)

Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>
Co-authored-by: Pieter Eendebak <P.T.eendebak@tudelft.nl>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
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Miss Islington (bot) 2022-12-20 19:34:17 -08:00 committed by GitHub
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Doc/library/weakref.rst
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@ -172,6 +172,30 @@ See :ref:`__slots__ documentation <slots>` for details.
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`.