All possible replacements of two lists?

Question

(I am aware that the title of the question might be misleading, but I could not find any other way to formulate it - feel free to edit it)

I have two lists, both of

Answer 1

Creating 3 lists of two elements would not over-complicate the code at all. zip can "flip the axes" of multiple lists trivially (making X sequences of Y elements into Y sequences of X elements), making it easy to use itertools.product:

import itertools

a = [1,2,3]
b = [4,5,6]

# Unpacking result of zip(a, b) means you automatically pass
# (1, 4), (2, 5), (3, 6)
# as the arguments to itertools.product
output = list(itertools.product(*zip(a, b)))

print(*output, sep="\n")

Which outputs:

(1, 2, 3)
(1, 2, 6)
(1, 5, 3)
(1, 5, 6)
(4, 2, 3)
(4, 2, 6)
(4, 5, 3)
(4, 5, 6)

Different ordering than your example output, but it's the same set of possible replacements.