merge sort in python

merge.py

from random import shuffle

def merge(left, right, r_list=[]):
    if not left and not right:
        return r_list
    elif not left:
        r = right[0]
        r_list.append(r)
        return merge(left, right[1:], r_list)
    elif not right:
        l = left[0]
        r_list.append(l)
        return merge(left[1:], right, r_list)
    else:
        l = left[0]
        r = right[0]
        if l < r:
            r_list.append(l)
            return merge(left[1:], right, r_list)
        else:
            r_list.append(r)
            return merge(left, right[1:], r_list)

def mergeSort(arr):
    if len(arr) == 1:
        return arr
    else:
        left    = arr[:int(len(arr)/2)]
        right   = arr[int(len(arr)/2):]
        
        left    = mergeSort(left)
        right   = mergeSort(right)
        arr = merge(left, right, [])
        return arr

a = list(range(10))
shuffle(a)
print(a)

a = mergeSort(a)
print(a)

output:

[6, 2, 7, 5, 8, 0, 9, 1, 3, 4]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]