Python generator that groups another iterable into groups of N

How about this one? It doesn't have a fill value though.

>>> def partition(itr, n):
...     i = iter(itr)
...     res = None
...     while True:
...             res = list(itertools.islice(i, 0, n))
...             if res == []:
...                     break
...             yield res
...
>>> list(partition([1, 2, 3, 4, 5, 6, 7, 8, 9], 3))
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>>

It utilizes a copy of the original iterable, which it exhausts for each successive splice. The only other way my tired brain could come up with was generating splice end-points with range.

Maybe I should change list() to tuple() so it better corresponds to your output.

When you want to group an iterator in chunks of n without padding the final group with a fill value, use iter(lambda: list(IT.islice(iterable, n)), []):

import itertools as IT

def grouper(n, iterable):
    """
    >>> list(grouper(3, 'ABCDEFG'))
    [['A', 'B', 'C'], ['D', 'E', 'F'], ['G']]
    """
    iterable = iter(iterable)
    return iter(lambda: list(IT.islice(iterable, n)), [])

seq = [1,2,3,4,5,6,7]
print(list(grouper(3, seq)))

yields

[[1, 2, 3], [4, 5, 6], [7]]

There is an explanation of how it works in the second half of this answer.

---

When you want to group an iterator in chunks of n and pad the final group with a fill value, use the grouper recipe zip_longest(*[iterator]*n):

For example, in Python2:

>>> list(IT.izip_longest(*[iter(seq)]*3, fillvalue='x'))
[(1, 2, 3), (4, 5, 6), (7, 'x', 'x')]

In Python3, what was izip_longest is now renamed zip_longest:

>>> list(IT.zip_longest(*[iter(seq)]*3, fillvalue='x'))
[(1, 2, 3), (4, 5, 6), (7, 'x', 'x')]

---

When you want to group a sequence in chunks of n you can use the chunks recipe:

def chunks(seq, n):
    # https://stackoverflow.com/a/312464/190597 (Ned Batchelder)
    """ Yield successive n-sized chunks from seq."""
    for i in xrange(0, len(seq), n):
        yield seq[i:i + n]

Note that, unlike iterators in general, sequences by definition have a length (i.e. __len__ is defined).

See the grouper recipe in the docs for the itertools package

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

(However, this is a duplicate of quite a few questions.)