"NoneType"对象没有属性"值"

Question

删除整个链表时使用" delete_whole (self)">方法。它抛出no attribute错误． 尽管上面的代码运行良好。它不是打印空白列表，而是给出no attribute错误。

class Node:
def __init__(self,value):
self.value = value
self.next = None

class CircularSinglyLinkedList:
def __init__(self):
self.head = None
self.tail = None


def __iter__(self):
node = self.head

while True:
yield node
if node.next is self.head:
break
node = node.next


def insertion(self, value, location):

new_node = Node(value)

if self.head is None:
self.head = new_node
self.tail = new_node
self.tail.next = self.head

else:

if location == 0:

new_node.next = self.head
self.head = new_node
self.tail.next = new_node

elif location == -1:
self.tail.next = new_node
new_node.next = self.head
self.tail = new_node

else:

temp_node = self.head
index = 0
while index < location -1:

if temp_node.next is self.head:
break

temp_node = temp_node.next
index += 1

next_node = temp_node.next
temp_node.next = new_node
new_node.next = next_node

if new_node.next is self.head:
self.tail = new_node

#---------------------------------------------------------------------------------------------#
# deleting the entire linked list
def delete_entire(self):
self.head = None
self.tail.next = None
self.tail = None 


cll = CircularSinglyLinkedList()
cll.insertion(2,1)
cll.insertion(3,2)
cll.insertion(5,3)
cll.insertion(6,4)
cll.insertion(0,0)
cll.insertion(10,-1)
print([node.value for node in call])

output = [0,2,3,5,6,10]

cll.delete_entire()
print([node.value for node in call])

AttributeError: 'NoneType'对象没有属性'value'

期望输出为

[]

Answer 1

您的__iter__()方法总是产生一次，即使列表为空，因为它在检查node是否为None之前执行yield node。当列表为空时，将产生None，调用者将尝试使用None.value。

当node变为None时，应立即停止循环。

def __iter__(self):
node = self.head

while node:
yield node
node = node.next