Concentrate two lists alternating python

Question

I want to combine two lists in an alternating way in Python.

list1 = ['A']
list2 = [1, 2, 3, 4]

What I've tried:

combination = [x for y in zip(list1, list2) for x in y]
# Output: 
['A', 1]

# Expected output:
['A', 1, 'A', 2, 'A', 3, 'A', 4]

How can I combine these two lists and get the expected output? (Preferably list comprehension)

To clarify... What should the output be if *list1 = ['A', 'B']* ? — DarkKnight, Jan 25 '23 at 10:38
@cinnamon then don't you think that would be a better example to use in your question? Currently, 2 answers work for the question but fail with that detail... — Tomerikoo, Jan 25 '23 at 10:54

score 2 · Answer 1 · answered Jan 25 '23 at 10:49

2

One way is to use itertools cycle function:

from itertools import cycle

list1 = ['A']
list2 = [1, 2, 3, 4]

combination = [x for y in zip(cycle(list1), list2) for x in y]

This also works with

list1 = ['A', 'B']

answered Jan 25 '23 at 10:49

loocars

Could also use `itertools.chain.from_iterable` which is basically `flatten`, rather than a list comprehension. Also OP if the input doesn't have to be a list, you can just use `itertools.repeat` to create an infinite iterator of a single item. – Masklinn Jan 25 '23 at 10:50
@Masklinn True but OP specifically wants to use a list comprehension – DarkKnight Jan 25 '23 at 10:52

Jamiu S. · Answer 2 · 2023-01-25T20:03:04.430

1

list1*len(list2) will repeat the first list elements as many times as the length of the second list.

combination = [x for y in zip(list1*len(list2), list2) for x in y]

['A', 1, 'A', 2, 'A', 3, 'A', 4]

edited Jan 25 '23 at 20:03

answered Jan 25 '23 at 10:39

Jamiu S.

2 Answers2