用Python做归并排序法出错，求高人指点

def merge_sort(array,p,q,r):
n1 = q-p
n2 = r-q+1
l = np.zeros(n1+1)
r = np.zeros(n2+1)
for i in range(0,n1):
l[i] = array[p+i]
for j in range(0,n2):
r[j] = array[q+j]
#return r
l[n1] = float("inf")
r[n2] = float("inf")
#return l[0]
l = sorted(l)
r = sorted(r)
#return r
i = 0
j = 0
for k in range(p,r):
if l[i] <= r[j]:
array[k] = l[i]
i = i+1
else:
array[k] = r[j]
j = j+1
return array
上面的代码是基于MIT的算法导论一书写的，运行后报错如下：

>>> merge_sort([4,8,12,1,7,3,9],0,3,6)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 16, in merge_sort
TypeError: range() integer end argument expected, got list.


请问这是什么原因？代码中加了注释的return r/l用于查错，都没有明显问题，个人怀疑从带k的循环中出现问题，求指导啊求指导！