Merge sort a list of integers
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.