Peek inside iterator (is there a PEP about this?)

Aaron "Castironpi" Brady castironpi at gmail.com
Wed Oct 1 21:04:58 CEST 2008


On Oct 1, 9:46 am, Luis Zarrabeitia <ky... at uh.cu> wrote:
> Hi there.
>
> For most use cases I think about, the iterator protocol is more than enough.
> However, on a few cases, I've needed some ugly hacks.
>
> Ex 1:
>
> a = iter([1,2,3,4,5]) # assume you got the iterator from a function and
> b = iter([1,2,3])     # these two are just examples.
>
> then,
>
> zip(a,b)
>
> has a different side effect from
>
> zip(b,a)
>
> After the excecution, in the first case, iterator a contains just [5], on the
> second, it contains [4,5]. I think the second one is correct (the 5 was never
> used, after all). I tried to implement my 'own' zip, but there is no way to
> know the length of the iterator (obviously), and there is also no way
> to 'rewind' a value after calling 'next'.
>
> Ex 2:
>
> Will this iterator yield any value? Like with most iterables, a construct
>
> if iterator:
>    # do something
>
> would be a very convenient thing to have, instead of wrapping a 'next' call on
> a try...except and consuming the first item.
>
> Ex 3:
>
> if any(iterator):
>    # do something ... but the first true value was already consumed and
>    # cannot be reused. "Any" cannot peek inside the iterator without
>    # consuming the value.
>
> Instead,
>
> i1, i2 = tee(iterator)
> if any(i1):
>    # do something with i2
>
> Question/Proposal:
>
> Has there been any PEP regarding the problem of 'peeking' inside an iterator?
> Knowing if the iteration will end or not, and/or accessing the next value,
> without consuming it? Is there any (simple, elegant) way around it?
>
> Cheers,
>
> --
> Luis Zarrabeitia (aka Kyrie)
> Fac. de Matemática y Computación, UH.http://profesores.matcom.uh.cu/~kyrie

It wouldn't be that hard to make your own.

a = peekingiter([1,2,3,4,5])
b = peekingiter([1,2,3])

Just don't cross it with typing and get peking duck.



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