AttributeError'module'对象没有属性'merge_sort'和另一个错误

Question

我正在尝试使用Merge Sort对链接列表进行排序。当我运行Programm时出现错误

x = list_linked.merge_sort(l)
AttributeError: 'module' object has no attribute 'merge_sort'

这是我在文件中的功能（list_l）：

def merge_sort(a):
        a._front = merge_sort_aux(a._front)
        return
def merge_sort_aux(a)
        if a is not None and a._next:

            left, right = merge_sort_aux(a)
            left = merge_sort_aux(left)
            right = merge_sort_aux(right)
            a = merge_sort_aux(left, right)
        return a

这里是主要的：

from list_l import List
import list_linked
a = [3, 4, 5, 3, 6, 3, 5, 2, 5]
l = List()
for i in a:
    l.append(i)

x = list_l.merge_sort(l)
print(x)

同样在我的函数中我得到错误：

 Method 'merge_sort - sorts_linked' should have self as first 
   parameter
Method 'merge_sort_aux - sorts_linked' should have self as first 
 parameter

感谢您的帮助。