Interleave multiple lists of variable-length in Python3

Question

This answer https://stackoverflow.com/a/7947190/5517459 gives:

>>> l1=[1,2,3]
>>> l2=[10,20,30]
>>> l3=[101,102,103,104]
>>> [y for x in map(None,l1,l2,l3) for y in x if y is not None]
[1, 10, 101, 2, 20, 102, 3, 30, 103, 104]

This snippet does exactly what I want for my project but it doesn't work in python3.

I've worked around the errors using:

func = lambda *x: x
modules = [y for x in map(func,l1,l2,l3) for y in x]

but it now can't handle variable-length lists, now stopping once it exhausts the shortest list.

That whole question lists multiple other answers. Have you tried any of them? — Carcigenicate, Jul 19 '19 at 12:12
Possible duplicate of [Interleave multiple lists of the same length in Python](https://stackoverflow.com/questions/7946798/interleave-multiple-lists-of-the-same-length-in-python) — John Zwinck, Jul 19 '19 at 12:14
@Mehdi I believe zip also stops when the shortest list is exhausted — Goods, Jul 19 '19 at 12:14
@JohnZwinck I believe this is a superset of that question as I need to handle variable-length lists — Goods, Jul 19 '19 at 12:17

score 1 · Accepted Answer · answered Jul 19 '19 at 12:13

Looks like you need itertools.zip_longest

from itertools import zip_longest
l1=[1,2,3]
l2=[10,20,30]
l3=[101,102,103,104]

print([y for x in zip_longest(l1,l2,l3, fillvalue=None) for y in x if y is not None])

Output:

[1, 10, 101, 2, 20, 102, 3, 30, 103, 104]

score 1 · Answer 2 · answered Jul 19 '19 at 12:25

If they are lists you can also pad all lists with None:

longest_len = max(len(l1), len(l2), len(l3))
zipped_lists = zip(
    l1+[None]*(longest_len-len(l1)),
    l2+[None]*(longest_len-len(l2)),
    l3+[None]*(longest_len-len(l3)))
modules = [y for x in zipped_lists for y in x if y is not None]

Interleave multiple lists of variable-length in Python3

2 Answers2