[issue31254] WeakKeyDictionary/Mapping doesn't call __missing__
Raymond Hettinger
report at bugs.python.org
Thu Aug 24 20:31:23 EDT 2017
Raymond Hettinger added the comment:
[Anthony Lee]
> The use case is to generate a mapping of weakly-held objects
> to unique ids, with something like
>
> id_map = WeakKeyDictionaryWithMissing(lambda *, _counter=itertools.count(): > next(_counter))
Where are you keeping hard references to the keys? ISTM, you only have a weak reference, so the object has no hard references. Entries in the dictionary are discarded when there is no longer a strong reference to the key.
Why did you decide to use a dictionary? AFAICT, nothing external to the dictionary knows about the keys so there is no way to do lookups.
Overall, it doesn't seem like a WeakKeyDictionary with a __missing__() method is the right approach for this problem. It makes me question where it makes any sense at all to auto-generate missing keys for a WeakKeyDictionary.
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<http://bugs.python.org/issue31254>
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