The Merge sort algorithm simple and readable implementation in Python

merge_sort.py

from typing import List


def merge_sort(lst: List[int]) -> List[int]:
    """The Merge sort algorithm.

    Wikipedia: https://en.wikipedia.org/wiki/Merge_sort
    """
    if len(lst) <= 1:
        return lst

    middle = len(lst) // 2
    return order_ascend(merge_sort(lst[:middle]), merge_sort(lst[middle:]))


def order_ascend(left: List[int], right: List[int]) -> List[int]:
    """Orders the 2 lists into one ordered list for the merge sort algorithm.

    Because of how the merge sort algorithm works, each list entered to this
    function will already be ordered.
    """
    ordered_list = []
    while left and right:
        if left[0] <= right[0]:
            ordered_list.append(left.pop(0))
        else:
            ordered_list.append(right.pop(0))

    if not left:
        ordered_list.extend(right)
    else:
        ordered_list.extend(left)

    return ordered_list