From c615286e8576f2555d4380f38a966c300805b1a5 Mon Sep 17 00:00:00 2001 From: Stanley <46876382+slateny@users.noreply.github.com> Date: Tue, 20 Dec 2022 19:27:02 -0800 Subject: [PATCH] gh-91081: Add note on WeakKeyDictionary behavior when deleting a replaced entry (#91499) Co-authored-by: Pieter Eendebak Co-authored-by: Jelle Zijlstra --- Doc/library/weakref.rst | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst index 73e7b21ae405d2..1406b663c6a8e2 100644 --- a/Doc/library/weakref.rst +++ b/Doc/library/weakref.rst @@ -172,6 +172,30 @@ See :ref:`__slots__ documentation ` 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`.