[Python-ideas] Add lookahead iterator (peeker) to itertools

[Serhiy Storchaka]

On 25.02.13 22:27, Antoine Pitrou wrote:
> On Mon, 25 Feb 2013 22:21:03 +0200
> Serhiy Storchaka <storchaka at gmail.com>
> wrote:
>> On 25.02.13 20:53, Antoine Pitrou wrote:
>>>>> def test_lookahead():
>>>>>        it = iter('abc')
>>>>>        while True:
>>>>>            it, peeking = itertools.tee(it)
>>>>
>>>> This should be outside a loop.
>>>
>>> Only if you restrict yourself to access peeking each time you access it.
>>> (which, I suppose, is not the general use case for the lookahead
>>> proposal)
>>
>> Only if your do not want to consume O(N) memory and spend O(N**2) time.
>
> No, that's beside the point. If you don't consume "peeking" in
> lock-step with "it", then "peeking" and "it" become desynchronized and
> therefore the semantics are wrong w.r.t to the original feature
> request (where "peeking" is supposed to be some proxy to "it", not an
> independently-running iterator).

Yes, of course, you should consume "peeking" in lock-step with "it".

My note is that if you create tee every iteration, this will lead to an 
linear increase in memory consumption and degradation of speed.