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Mon Aug 25 13:22:10 CEST 2008 · https://mail.python.org/pipermail/python-dev/2008-August/081971.html

Hello,

2008/8/25 Robert Schuppenies <robert.schuppenies at gmail.com>:
> Hi.
>
> Could you please explain to me why some iterators have a tp_traverse
> implementation and others do not? For example tupleiterator has one,
> but none of the dict iterators. Same for set iterators (and possibly
> others). It shows in Python when you use the get_referents function.
>
>>>> t = (1,2,3)
>>>> gc.get_referents(iter(t))
> [(1, 2, 3)]
>>>> s = set([1,2,3])
>>>> gc.get_referents(iter(s))
> []
>>>> d = {1:1, 2:2}
>>>> gc.get_referents(iter(d))
> []
>
> And is it correct that you can rephrase the question to 'why some
> iterators are seen as container objects and others are not'?

Yes, this can lead to some object cycle that are not collected.
See the attached script: a cycle involving a list iterator is
collected by the garbage collector, but a cycle with a dict iterator
is not.
This is worth a bug report IMO.

-- 
Amaury Forgeot d'Arc
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[Python-Dev] no tp_traverse for dict iters