[Python-Dev] Weak Dictionary Iteration Behavior in Python 3

[Georg Brandl]

Greg Ewing schrieb:
> Georg Brandl wrote:
> 
>> In this case, what WeakKeyDictionary.keys() returns is not a view anyway,
>> so using list(self.data.keys()) in the method seems like a good compromise.
> 
> I don't think that helps, because the implementation has
> to iterate over that to dereference the weak refs, and if
> that's done in Python code there's a chance for a weak
> ref to disappear in the process.

Yes, but in that case the ref will simply not be there anymore. It can't be
guaranteed anyway which objects will be alive at the precise moment you
call keys().  In contrast, iterating over the keys() view of the dictionary
one-by-one leads to an exception when one of the refs vanishes while still
iterating.

See Armin's test case for an example that demonstrates this.

Georg

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