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While working through Project Euler, a fun source of exercises, I
composed the following iterator recipe to yield from multiple
iterators in fixed-length groups:

import itertools

def zip_longest_by(*args, fillvalue=None, n=1, grouper=tuple):
"""Yield n at a time from each of the args, with padding.
It terminates when the longest iterator is exhausted.

>>> for i, j in zip_longest_by("ABCDEFGH", "HIJKL",
... fillvalue="-", n=3, grouper=''.join):
... print(i, j)
ABC HIJ
DEF KL-
GH- ---

>>> for n1, n2 in zip_longest_by(reversed('1234'), reversed('678'),
... fillvalue='0', n=3, grouper=lambda a: ''.join(reversed(a))):
... print(n1, n2)
234 678
001 000
"""
it = itertools.zip_longest(*args, fillvalue=fillvalue)
while True:
accum = list()
try:
for i in range(n):
accum += zip(*next(it))
except StopIteration:
for i in range(n - i):
accum.append(tuple(itertools.repeat(fillvalue, len(args))))
yield tuple(grouper(item) for item in zip(*accum))
break
yield tuple(grouper(item) for item in zip(*accum))

The interface could stand improvement. I find the grouper
argument very convenient, but none of the other grouping
iterators find it needful. Forcing n to be a keyword argument is
unfortunate as well.

--
Neil Cerutti