I have a Python script which takes as input a list of integers, which I need to work with four integers at a time. Unfortunately, I don’t have control of the input, or I’d have it passed in as a list of four-element tuples. Currently, I’m iterating over it this way:

for i in xrange(0, len(ints), 4):
# dummy op for example code
foo += ints[i] * ints[i + 1] + ints[i + 2] * ints[i + 3]

It looks a lot like “C-think”, though, which makes me suspect there’s a more pythonic way of dealing with this situation. The list is discarded after iterating, so it needn’t be preserved. Perhaps something like this would be better?

while ints:
foo += ints[0] * ints[1] + ints[2] * ints[3]
ints[0:4] = []

Still doesn’t quite “feel” right, though. :-/

Related question: How do you split a list into evenly sized chunks in Python?

Answer

Modified from the recipes section of Python’s itertools docs:

from itertools import zip_longest

def grouper(iterable, n, fillvalue=None):
    args = [iter(iterable)] * n
    return zip_longest(*args, fillvalue=fillvalue)

Example
In pseudocode to keep the example terse.

grouper('ABCDEFG', 3, 'x') --> 'ABC' 'DEF' 'Gxx'

Note: on Python 2 use izip_longest instead of zip_longest.

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