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Re: Good use for itertools.dropwhile and itertools.takewhile

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From Ian Kelly <ian.g.kelly@gmail.com>
Date Wed, 5 Dec 2012 09:16:09 -0700
Subject Re: Good use for itertools.dropwhile and itertools.takewhile
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On Wed, Dec 5, 2012 at 6:45 AM, Chris Angelico <rosuav@gmail.com> wrote:
> On Wed, Dec 5, 2012 at 12:17 PM, Nick Mellor <thebalancepro@gmail.com> wrote:
>>
>> takewhile mines for gold at the start of a sequence, dropwhile drops the dross at the start of a sequence.
>
> When you're using both over the same sequence and with the same
> condition, it seems odd that you need to iterate over it twice.
> Perhaps a partitioning iterator would be cleaner - something like
> this:
>
> def partitionwhile(predicate, iterable):
>     iterable = iter(iterable)
>     while True:
>         val = next(iterable)
>         if not predicate(val): break
>         yield val
>     raise StopIteration # Signal the end of Phase 1
>     for val in iterable: yield val # or just "yield from iterable", I think
>
> Only the cold hard boot of reality just stomped out the spark of an
> idea. Once StopIteration has been raised, that's it, there's no
> "resuming" the iterator. Is there a way around that? Is there a clean
> way to say "Done for now, but next time you ask, there'll be more"?

Return two separate iterators, with the contract that the second
iterator can't be used until the first has completed.  Combined with
Neil's groupby suggestion, we end up with something like this:

def partitionwhile(predicate, iterable):
    it = itertools.groupby(iterable, lambda x: bool(predicate(x)))
    pushback = missing = object()
    def first():
        nonlocal pushback
        pred, subit = next(it)
        if pred:
            yield from subit
            pushback = None
        else:
            pushback = subit
    def second():
        if pushback is missing:
            raise TypeError("can't yield from second iterator before
first iterator completes")
        elif pushback is not None:
            yield from pushback
        yield from itertools.chain.from_iterable(subit for key, subit in it)
    return first(), second()

>>> list(map(' '.join, partitionwhile(lambda x: x.upper() == x, "CAPSICUM RED fresh from QLD".split())))
['CAPSICUM RED', 'fresh from QLD']

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Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 05:57 -0800
  Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-04 14:23 +0000
    Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 06:47 -0800
      Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-04 15:17 +0000
  Re: Good use for itertools.dropwhile and itertools.takewhile Vlastimil Brom <vlastimil.brom@gmail.com> - 2012-12-04 15:31 +0100
    Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 07:24 -0800
      Re: Good use for itertools.dropwhile and itertools.takewhile Vlastimil Brom <vlastimil.brom@gmail.com> - 2012-12-04 22:08 +0100
    Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 07:24 -0800
      Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-04 18:26 +0000
  Re: Good use for itertools.dropwhile and itertools.takewhile Alexander Blinne <news@blinne.net> - 2012-12-04 18:18 +0100
    Re: Good use for itertools.dropwhile and itertools.takewhile DJC <djc@news.invalid> - 2012-12-04 18:28 +0000
      Re: Good use for itertools.dropwhile and itertools.takewhile Alexander Blinne <news@blinne.net> - 2012-12-04 19:48 +0100
        Re: Good use for itertools.dropwhile and itertools.takewhile Ian Kelly <ian.g.kelly@gmail.com> - 2012-12-04 12:37 -0700
          Re: Good use for itertools.dropwhile and itertools.takewhile Alexander Blinne <news@blinne.net> - 2012-12-04 21:33 +0100
          Re: Good use for itertools.dropwhile and itertools.takewhile Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-12-04 21:13 +0000
        Re: Good use for itertools.dropwhile and itertools.takewhile MRAB <python@mrabarnett.plus.com> - 2012-12-04 20:17 +0000
  Re: Good use for itertools.dropwhile and itertools.takewhile Terry Reedy <tjreedy@udel.edu> - 2012-12-04 15:44 -0500
    Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 17:17 -0800
      Re: Good use for itertools.dropwhile and itertools.takewhile Chris Angelico <rosuav@gmail.com> - 2012-12-06 00:45 +1100
        Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-05 14:34 +0000
          Re: Good use for itertools.dropwhile and itertools.takewhile Ian Kelly <ian.g.kelly@gmail.com> - 2012-12-05 08:33 -0700
            Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-05 16:11 +0000
      Re: Good use for itertools.dropwhile and itertools.takewhile Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-12-05 15:32 +0000
      Re: Good use for itertools.dropwhile and itertools.takewhile Ian Kelly <ian.g.kelly@gmail.com> - 2012-12-05 09:16 -0700
      Re: Good use for itertools.dropwhile and itertools.takewhile MRAB <python@mrabarnett.plus.com> - 2012-12-05 17:57 +0000
    Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-04 17:17 -0800
      Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-05 13:29 +0000
        Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-05 09:04 -0800
          Re: Good use for itertools.dropwhile and itertools.takewhile MRAB <python@mrabarnett.plus.com> - 2012-12-05 17:57 +0000
          Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-05 18:16 +0000
            Re: Good use for itertools.dropwhile and itertools.takewhile Nick Mellor <thebalancepro@gmail.com> - 2012-12-05 11:01 -0800
              Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-05 20:13 +0000
              Re: Good use for itertools.dropwhile and itertools.takewhile Vlastimil Brom <vlastimil.brom@gmail.com> - 2012-12-05 22:36 +0100
                Re: Good use for itertools.dropwhile and itertools.takewhile Neil Cerutti <neilc@norwich.edu> - 2012-12-06 13:06 +0000
                Re: Good use for itertools.dropwhile and itertools.takewhile Vlastimil Brom <vlastimil.brom@gmail.com> - 2012-12-06 15:12 +0100
          Re: Good use for itertools.dropwhile and itertools.takewhile Alexander Blinne <news@blinne.net> - 2012-12-06 14:40 +0100
  Re: Good use for itertools.dropwhile and itertools.takewhile Terry Reedy <tjreedy@udel.edu> - 2012-12-04 17:21 -0500
  Re: Good use for itertools.dropwhile and itertools.takewhile Paul Rubin <no.email@nospam.invalid> - 2012-12-06 13:29 -0800

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