Merging/adding lists in Python

Question

I'm pretty sure there should be a more Pythonic way of doing this - but I can't think of one: How can I merge a two-dimensional list into a one-dimensional list? Sort of like zip/map but with more than two iterators.

Example - I have the following list:

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

I want to have

result = [12, 15, 18] # [1+4+7, 2+5+8, 3+6+9]

So far what I've come up with is:

def add_list(array):
    number_items = len(array[0])
    result = [0] * number_items
    for index in range(number_items):
        for line in array:
            result[index] += line[index]
    return result

But that doesn't look very elegant/Pythonic to me. Aside from not checking if all the "lines" in the 2D array are of the same length, can be added to each other, etc. What would be a better way to do it?

Answer 1

[sum(a) for a in zip(*array)]