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To: python-list@python.org
From: Mark Lawrence <breamoreboy@yahoo.co.uk>
Subject: Re: Why chunks is not part of the python standard lib?
Date: Wed, 01 May 2013 14:06:15 +0100
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On 01/05/2013 10:00, Oscar Benjamin wrote:
> On 1 May 2013 08:10, Mark Lawrence <breamoreboy@yahoo.co.uk> wrote:
>> On 01/05/2013 07:26, Ricardo Azpeitia Pimentel wrote:
>>>
>>> After reading How do you split a list into evenly sized chunks in
>>> Python?
>>>
>>> <http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python>
>>>
>>> and seeing this kind of mistakes happening
>>> https://code.djangoproject.com/ticket/18972 all the time.
>>>
>>> Why is not a |chunks| function in itertools?
>>>
>>> |grouper| from
>>> http://docs.python.org/2/library/itertools.html#recipes doesn't have the
>>> same behavior as |chunks |
>>>
>>> Example:
>>> |
>>>
>>> |chunks([1,  2,  3,  4,  5],  3)
>>> # Should return [[1, 2, 3], [4, 5]] or the iterator equivalent.|
>>>
>>> |Original Post on StackOverflow:
>>>
>>> http://stackoverflow.com/questions/16313008/why-chunks-is-not-part-of-the-python-standard-lib
>>>
>>
>> Asked and answered a trillion times.  There's no concensus on how chucks
>> should behave.
>
> I'm not sure that's a valid argument against it since a chunks
> function could just do a different thing depending on the arguments
> given.
>
> The issue is around how to deal with the last chunk if it isn't the
> same length as the others and I can only think of 4 reasonable
> responses:
>
> 1) Yield a shorter chunk
> 2) Extend the chunk with fill values
> 3) Raise an error
> 4) Ignore the last chunk
>
> Cases 2 and 4 can be achieved with current itertools primitives e.g.:
> 2) izip_longest(fillvalue=fillvalue, *[iter(iterable)] * n)
> 4) zip(*[iter(iterable)] * n)
>
> However I have only ever had use cases for 1 and 3 and these are not
> currently possible without something additional (e.g. a generator
> function).
>
> In any case a chunks function can simply take arguments to give all 4
> behaviours:
>
> def chunks(iterable, chunksize, uneven='return_short', fillvalue=None):
>     # loop through yielding all even chunks
>     # and then
>     if uneven == 'return_short:
>        yield chunk
>     elif uneven == 'raise':
>        raise ValueError('No items left')
>     elif uneven == 'fill':
>        yield chunk + [fillvalue] * (chunksize - len(chunk))
>     elif uneven == 'ignore':
>        pass
>
>
> Oscar
>

All that's needed to get this into the standard library is to overcome 
this quote "This has been rejected before." from 
http://bugs.python.org/issue6021#msg87745

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Mark Lawrence