Re: Nested iteration?

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From	Oscar Benjamin <oscar.j.benjamin@gmail.com>
Date	2013-04-23 23:42 +0100
Subject	Re: Nested iteration?
Newsgroups	comp.lang.python
Message-ID	<mailman.1004.1366756982.3114.python-list@python.org> (permalink)

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On 23 April 2013 22:41, Joshua Landau <joshua.landau.ws@gmail.com> wrote:
> On 23 April 2013 22:29, Oscar Benjamin <oscar.j.benjamin@gmail.com> wrote:
>>
>> I just thought I'd add that Python 3 has a convenient way to avoid
>> this problem with next() which is to use the starred unpacking syntax:
>>
>> >>> numbers = [1, 2, 3, 4]
>> >>> first, *numbers = numbers
>
>
> That creates a new list every time. You'll not want that over
> try-next-except if you're doing this in a loop, and on addition (if you were
> talking in context) your method will exhaust the iterator in the outer loop.

Oh, you're right. I'm not using Python 3 yet and I assumed without
checking that it would be giving me an iterator rather than unpacking
everything into a list.

Then the best I can think of is a helper function:

>>> def unpack(iterable, count):
...   iterator = iter(iterable)
...   for n in range(count):
...     yield next(iterator)
...   yield iterator
...
>>> numbers = [1, 2, 3, 4]
>>> first, numbers = unpack(numbers, 1)
>>> first
1
>>> numbers
<list_iterator object at 0x24e1590>
>>> list(numbers)
[2, 3, 4]
>>> first, numbers = unpack([], 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 0 values to unpack


Oscar

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Thread

Nested iteration? roy@panix.com (Roy Smith) - 2013-04-23 11:40 -0400
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 17:05 +0100
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:05 -0600
  Re: Nested iteration? Peter Otten <__peter__@web.de> - 2013-04-23 18:15 +0200
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:21 +1000
  Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:35 +0000
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:30 -0600
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:39 -0600
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:42 +1000
    Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:53 +0000
  Re: Nested iteration? Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:49 -0400
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:14 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 22:29 +0100
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:41 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 23:42 +0100

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