converting a normal list into a nested one using a condition

I want to convert a normal list(main_list) into a nested one(ans_list) but I don’t know how.

main_list = [1, 2, 3, 4, 5, 6, 7, 8, 9]
ans_list = [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

>Solution :

Use a comprehension over appropriate slices:

n = 3
ans_list = [main_list[i::n] for i in range(n)]
# [[1, 4, 7], [2, 5, 8], [3, 6, 9]]