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Asked: 2026-05-13T06:17:13+00:00

I’m trying to write the Haskell function ‘splitEvery’ in Python. Here is it’s definition:

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I’m trying to write the Haskell function ‘splitEvery’ in Python. Here is it’s definition:

splitEvery :: Int -> [e] -> [[e]]
    @'splitEvery' n@ splits a list into length-n pieces.  The last
    piece will be shorter if @n@ does not evenly divide the length of
    the list.

The basic version of this works fine, but I want a version that works with generator expressions, lists, and iterators. And, if there is a generator as an input it should return a generator as an output!

Tests

# should not enter infinite loop with generators or lists
splitEvery(itertools.count(), 10)
splitEvery(range(1000), 10)

# last piece must be shorter if n does not evenly divide
assert splitEvery(5, range(9)) == [[0, 1, 2, 3, 4], [5, 6, 7, 8]]

# should give same correct results with generators
tmp = itertools.islice(itertools.count(), 10)
assert list(splitEvery(5, tmp)) == [[0, 1, 2, 3, 4], [5, 6, 7, 8]]

Current Implementation

Here is the code I currently have but it doesn’t work with a simple list.

def splitEvery_1(n, iterable):
    res = list(itertools.islice(iterable, n))
    while len(res) != 0:
        yield res
        res = list(itertools.islice(iterable, n))

This one doesn’t work with a generator expression (thanks to jellybean for fixing it):

def splitEvery_2(n, iterable): 
    return [iterable[i:i+n] for i in range(0, len(iterable), n)]

There has to be a simple piece of code that does the splitting. I know I could just have different functions but it seems like it should be and easy thing to do. I’m probably getting stuck on an unimportant problem but it’s really bugging me.

It is similar to grouper from http://docs.python.org/library/itertools.html#itertools.groupby but I don’t want it to fill extra values.

def grouper(n, iterable, fillvalue=None):
    "grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx"
    args = [iter(iterable)] * n
    return izip_longest(fillvalue=fillvalue, *args)

It does mention a method that truncates the last value. This isn’t what I want either.

The left-to-right evaluation order of the iterables is guaranteed. This makes possible an idiom for clustering a data series into n-length groups using izip(*[iter(s)]*n).

list(izip(*[iter(range(9))]*5)) == [[0, 1, 2, 3, 4]]
# should be [[0, 1, 2, 3, 4], [5, 6, 7, 8]]
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1 Answer

  1. Editorial Team
    from itertools import islice

def split_every(n, iterable):
    i = iter(iterable)
    piece = list(islice(i, n))
    while piece:
        yield piece
        piece = list(islice(i, n))

    Some tests:

    >>> list(split_every(5, range(9)))
[[0, 1, 2, 3, 4], [5, 6, 7, 8]]

>>> list(split_every(3, (x**2 for x in range(20))))
[[0, 1, 4], [9, 16, 25], [36, 49, 64], [81, 100, 121], [144, 169, 196], [225, 256, 289], [324, 361]]

>>> [''.join(s) for s in split_every(6, 'Hello world')]
['Hello ', 'world']

>>> list(split_every(100, []))
[]
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