类型错误：类型为'NoneType'的对象没有长度()

Question

我正在尝试合并排序算法，但收到以下类型错误：

Input #1: [3, 1, 2, 5, 9, 6, 7]
Traceback (most recent call last):
  File "Lab1.py", line 24, in <module>
    print('Output #1: ' + mergesort(a))
  File "Lab1.py", line 18, in mergesort
    left = mergesort(lst[:middle])
  File "Lab1.py", line 20, in mergesort
    return merge(left, right)
  File "Lab1.py", line 6, in merge
    while i < len(left) and j < len(right):
TypeError: object of type 'NoneType' has no len()

。。。关于此代码：

def merge(left, right):
    result = []
    i, j = 0, 0
    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1

def mergesort(lst):
    if len(lst) <= 1:
        return lst
    middle = int(len(lst) / 2)
    left = mergesort(lst[:middle])
    right = mergesort(lst[middle:])
    return merge(left, right)

a = [3,1,2,5,9,6,7]
print 'Input #1: ' + str(a)
print 'Output #1: ' + mergesort(a)