Merge sort a list of integers

Question

This code is meant to sort a list of integers using merge sort.

I'm doing this to improve my style and to improve my knowledge of fundamental algorithms/data structures for an upcoming coding interview.

def merge_sort(l):
    if len(l) == 0 or len(l) == 1:
        return

    mid = len(l) // 2
    left = l[:mid]
    right = l[mid:]
    merge_sort(left)
    merge_sort(right)
    merge(l, left, right)


def merge(l, left, right):
    l_ind, r_ind, ind = 0, 0, 0
    left_len, right_len, l_len = len(left), len(right), len(l)

    while l_ind < left_len and r_ind < right_len:
        if left[l_ind] < right[r_ind]:
            l[ind] = left[l_ind]
            l_ind += 1
            ind += 1
        else:
            l[ind] = right[r_ind]
            r_ind += 1
            ind += 1

    if l_ind < left_len and r_ind >= right_len:
        while ind < l_len:
            l[ind] = left[l_ind]
            l_ind += 1
            ind += 1
    elif l_ind >= left_len and r_ind < right_len:
        while ind < l_len:
            l[ind] = right[r_ind]
            r_ind += 1
            ind += 1

Answer 1

Get rid of some len

left_len, right_len, l_len = len(left), len(right), len(l)

Get rid of these, they don't do anything in terms of readability. They also are unnecessary, the len call is not O(n) it is simply O(1).

(Maybe) a library?

Also, although Rosetta code isn't always the most stylistic source, if you look at their solution, the use a pre-built merge. As you say you are "reinventing the wheel", maybe you don't want to do that but something to consider in the future.

Naming

l_ind, r_ind ...

I would write those out in full as left_ind and right_ind.