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

[Antoine Pitrou]

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).

Regards

Antoine.