Alternative way to split a list into groups of n [duplicate]

by

Here you go:

list_of_groups = zip(*(iter(the_list),) * group_size)

Example:

print zip(*(iter(range(10)),) * 3)
[(0, 1, 2), (3, 4, 5), (6, 7, 8)]

If the number of elements is not divisible by N but you still want to include them you can use izip_longest but it is only available since python 2.6

izip_longest(*(iter(range(10)),) * 3)

The result is a generator so you need to convert it into a list if you want to print it.

Finally, if you don’t have python 2.6 and stuck with an older version but you still want to have the same result you can use map:

print map(None, *(iter(range(10)),) * 3)
[(0, 1, 2), (3, 4, 5), (6, 7, 8), (9, None, None)]

I’d like to add some speed comparison between the different methods presented so far:

python -m timeit -s 'from itertools import izip_longest; L = range(1000)' 'list(izip_longest(*(iter(L),) * 3))'
10000 loops, best of 3: 47.1 usec per loop

python -m timeit -s 'L = range(1000)' 'zip(*(iter(L),) * 3)'
10000 loops, best of 3: 50.1 usec per loop

python -m timeit -s 'L = range(1000)' 'map(None, *(iter(L),) * 3)'
10000 loops, best of 3: 50.7 usec per loop

python -m timeit -s 'L = range(1000)' '[L[i:i+3] for i in range(0, len(L), 3)]'
10000 loops, best of 3: 157 usec per loop

python -m timeit -s 'import itertools; L = range(1000)' '[list(group) for key, group in itertools.groupby(L, lambda k: k//3)]'
1000 loops, best of 3: 1.41 msec per loop

The list comprehension and the group by methods are clearly slower than zip, izip_longest and map

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