Recursive Merge Sort (Python)

merge_sort_recursive.py

def divide(a):
    return a[:len(a)//2], a[len(a)//2:]


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

    while(True):
        if len(a) == 0:
            result += b
            break
        elif len(b) == 0:
            result += a
            break
        if a[0] < b[0]:
            result.append(a.pop(0))
        else:
            result.append(b.pop(0))
    return result


def merge_sort_recursive(a):
    a, b = divide(a)
    if len(a) == 0 or len(b) == 0:
        return merge(a, b)
    a = merge_sort_recursive(a)
    b = merge_sort_recursive(b)
    return merge(a, b)


if __name__ == '__main__':
    import random
    a = random.sample(range(30), 11)
    print(merge_sort_recursive(a))