[Python-ideas] Deterministic iterator cleanup

[Nathaniel Smith]

On Wed, Oct 19, 2016 at 11:38 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> On 19 October 2016 at 19:13, Chris Angelico <rosuav at gmail.com> wrote:
>> Now it *won't* correctly call the end-of-iteration function, because
>> there's no 'for' loop. This is going to either (a) require that EVERY
>> consumer of an iterator follow this new protocol, or (b) introduce a
>> ton of edge cases.
>
> Also, unless I'm misunderstanding the proposal, there's a fairly major
> compatibility break. At present we have:
>
>>>> 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.

-n

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