python - 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 the same length:

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

And I want to obtain all possible replacements of the first list with the second list.

output[0] = [1,2,3] # no replacements
output[1] = [4,2,3] # first item was replaced
output[2] = [1,5,3] # second item was replaced
output[3] = [1,2,6] # third item was replaced
output[4] = [4,5,3] # first and second items were replaced
output[5] = [4,2,6] # first and third items were replaced
output[6] = [1,5,6] # second and third items were replaced
output[7] = [4,5,6] # all items were replaced

Please note that this question is NOT answered by the following questions:

• How to get all possible combinations of a list’s elements?
• combinations between two lists?
• Python merging two lists with all possible permutations
• All combinations of a list of lists

A possible solution involving the previously linked answers would be to create several lists and then use the itertools.product method on them. For example, instead of having 2 lists of 3 elements, I could create 3 lists of 2 elements; however, that would over-complicate the code and I'd prefer to avoid that if I could.

Is there an easy and quick way to do it?

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="
")

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.