Merge sort algorithm

mergesort.py

def mergesort(array):

    left = array[:len(array)//2]
    right = array[len(array)//2:]

    left = mergesort(left) if len(left) >= 2 else left
    right = mergesort(right) if len(right) >= 2 else right

    res = []
    l = 0
    r = 0
    while l < len(left) or r < len(right):

        if l == len(left):
            res.append(right[r])
            r += 1
        elif r == len(right):
            res.append(left[l])
            l += 1

        elif left[l] < right[r]:
            res.append(left[l])
            l += 1

        else:
            res.append(right[r])
            r += 1

    return res


print(mergesort([2, 8, 5, 3, 9, 4, 1, 7]))