Merge sort implementation in Python

merge_soft.py

#!/usr/bin/env python
# -*- coding:utf-8 -*-

import random

def merge_sort(nums):
    if len(nums) <= 1:
        return nums
    else:
        b, c = split(nums)
        return merge(merge_sort(b), merge_sort(c))

def split(a):
    if len(a) <= 1:
        return a, []
    else:
        half_index = len(a) / 2
        return a[0:half_index], a[half_index:]

def merge(a, b):
    retval = []

    while len(a) != 0 and len(b) != 0:
        if a[0] < b[0]:
            retval.append(a[0])
            a.pop(0)
        else:
            retval.append(b[0])
            b.pop(0)

    if len(a) > 0:
        retval.extend(a)
    elif len(b) > 0:
        retval.extend(b)

    return retval

if __name__ == "__main__":
    nums = [random.randint(0, 1000) for x in range(30)]
    print merge_sort(nums)