Python implementation of merge sort

merge_sort.py

def ordered_combine(lhs, rhs):
    lst = []
    l_i = r_i = 0
    while l_i < len(lhs) and r_i < len(rhs):
        if lhs[l_i] < rhs[r_i]:
            lst.append(lhs[l_i])
            l_i += 1
        else:
            lst.append(rhs[r_i])
            r_i += 1

    lst += lhs[l_i:]
    lst += rhs[r_i:]
    return lst

def merge_sort(lst):
    size = len(lst)
    if size == 1:
        return lst

    half = int(size / 2)
    lhs = lst[:half]
    rhs = lst[half:]

    lhs = merge_sort(lhs)
    rhs = merge_sort(rhs)
    return ordered_combine(lhs, rhs)

assert merge_sort([5, 2, 9, 3]) == [2, 3, 5, 9]
assert merge_sort([5, 2, 9, 3, 34]) == [2, 3, 5, 9, 34]
assert merge_sort([12, 2, 3, 51, 1235, 399]) == [2, 3, 12, 51, 399, 1235]