python 双链表 头部插入元素问题

Question

def add(self, item):
node = Node(item)
if self.is_empty():
self._head = node
else:
node.next = self._head
self._head.prev = node
self._head = node

出现下面错误
Traceback (most recent call last):
File "D:/pycharm/PychramProject/mysqlTest/DLinkList.py", line 101, in
ll.add(1)
File "D:/pycharm/PychramProject/mysqlTest/DLinkList.py", line 39, in add
self._head.prev = node
AttributeError: 'NoneType' object has no attribute 'prev'

Answer 1

self._head为空。你看下前面的代码，有没有给它赋值