Python有关链表函数出错

AttributeError: 'NoneType' object has no attribute 'val'
    while cur.val!=None:
Line 11 in removeNthFromEnd (Solution.py)
    ret = Solution().removeNthFromEnd(param_1, param_2)
Line 41 in _driver (Solution.py)
    _driver()
Line 52 in <module> (Solution.py)

为什么会出现这样的错误啊，我print head.val不会报错，现在令cur=head,为什么print cur.head就报错了呢，可以解释一下原因吗！请求指导

while cur.val!=None:
改为：
while cur:

.val!=None 这些字符删掉