[Python-ideas] Deterministic iterator cleanup

[Nathaniel Smith]

On Wed, Oct 19, 2016 at 1:33 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> On 2016-10-19 3:33 PM, Nathaniel Smith wrote:
>
>>>>>>> lst = [1,2,3,4]
>>>>>>> >>>>>it = iter(lst)
>>>>>>> >>>>>for i in it:
>>>>
>>>> >>...   if i == 2: break
>>>> >>
>>>>>>>
>>>>>>> >>>>>for i in it:
>>>>
>>>> >>...   print(i)
>>>> >>3
>>>> >>4
>>>>>>>
>>>>>>> >>>>>
>>>>
>>>> >>
>>>> >>With the proposed behaviour, if I understand it, "it" would be closed
>>>> >>after the first loop, so resuming "it" for the second loop wouldn't
>>>> >>work. Am I right in that? I know there's a proposed itertools function
>>>> >>to bring back the old behaviour, but it's still a compatibility break.
>>>> >>And code like this, that partially consumes an iterator, is not
>>>> >>uncommon.
>>>
>>> >
>>> >Right -- did you reach the "transition plan" section? (I know it's
>>> >wayyy down there.) The proposal is to hide this behind a __future__ at
>>> >first + a mechanism during the transition period to catch code that
>>> >depends on the old behavior and issue deprecation warnings. But it is
>>> >a compatibility break, yes.
>>
>> I should also say, regarding your specific example, I guess it's an
>> open question whether we would want list_iterator.__iterclose__ to
>> actually do anything. It could flip the iterator to a state where it
>> always raises StopIteration, or RuntimeError, or it could just be a
>> no-op that allows iteration to continue normally afterwards.
>
>
> Making 'for' loop to behave differently for built-in containers (i.e. make
> __iterclose__ a no-op for them) will only make this whole thing even more
> confusing.
>
> It has to be consistent: if you partially iterate over *anything* without
> wrapping it with `preserve()`, it should always close the iterator.

You're probably right. My gut is leaning the same way, I'm just
hesitant to commit because I haven't thought about it for long. But I
do stand by the claim that this is probably not *that* important
either way :-).

-n

-- 
Nathaniel J. Smith -- https://vorpus.org