slicing return iter?

[Arnaud Delobelle]

On Oct 17, 3:40 pm, StarWing <weasley... at sina.com> wrote:
> hello everyone, I'm new here :-)
>
> sometimes I want to iterate a part of a sequence. but don't want to
> copy it. i.e.
>
> a = list(...)
> # now a is a list, and a[:] is another list, and so a[m:n]
> # now we do something with the 0~len(a)-3 elements of a
> for val in a[:-2]:
>     #do something....
>
> but, this will cause a copy on a, has some convenience way to get a
> iter to iterate on a part of list?
>
> i made this:
> class iterslice:
>     def __init__(self, list):
>         self.list = list
>
>     def __len__(self):
>         return len(self.list)
>
>     def __getitem__(self, slice):
>         import itertools
>         listlen = len(self.list)
>         range = (((slice.start + listlen) % listlen) if slice.start
> else 0,
>                 ((slice.stop + listlen) % listlen) if slice.stop else
> listlen,
>                 slice.step)
>         return itertools.islice(self.list, *range)
>
> a = [1,2,3,4]
> for i in iterslice(a)[:-1:2]:
>     print i
>
> my question is:
>   - are there any *Standard* way to do this? (a buit-in function? a
> module?)
>   - are there any better implements?
>
> thanks for attention :-)

Check the itertools module.

HTH

--
Arnaud