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| -rw-r--r-- | Doc/library/weakref.rst | 24 |
1 files changed, 24 insertions, 0 deletions
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 @@ 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`. |