124 – Operate on two lists of numbers

If you combine the module operator with itertools.starmap and itertools.zip_longest, you can write any function that merges two lists by operating on respective pairs of elements.

zip_longest allows you to operate until the end of the longer list and then starmap allows you to unpack the tuples into arguments of the operation you chose. If you don’t need zip_longest to get to the end of the longer iterable, then you can just use the built-in map(add, l1, l2) and starmap ends up not being needed either.

The example below adds two lists, element by element, padding the shorter list with zeroes:

from itertools import starmap, zip_longest
from operator import add

def add_lists(l1, l2):
    """Function to add respective elements from two lists."""
    return list(starmap(add, zip_longest(l1, l2, fillvalue=0)))

add_lists(
    [1, 2, 3],
    [10, 20, 30, 40],
)  # [11, 22, 33, 40]