python单链表尾部添加节点报错，逻辑貌似没问题啊，求解答！

问题遇到的现象和发生背景

python实现单链表尾部添加节点报错

运行结果及报错内容

class Node:
    def __init__(self,data):
        self.data=data
        self.next=None
        return
    def has_value(self,value):
        if self.data==value:
            return True
        else:
            return False
class singlelink:
    def __init__(self):
        self.head=None
        self.tail=None
        self.length=0
        return
    def isempty(self):
        return self.length==0
    def add_node(self,item):
        if not isinstance(item,Node):
            item=Node(item)
        if self.head is None:
            self.head=item
        else:
            self.tail.next=item
            self.tail=item
        self.length+=1
        return
    def insert_node(self,index,data):
        if self.isempty():
            print("this link is empty!")
            return
        if index<0 or index>=self.length:
            print("error:out of index!")
            return
        item=Node(data)
        if index==0:
            item.next=self.head
            self.haed=item
            self.length+=1
            return
        j=0
        node=self.head
        prev=self.head
        while node.next and jprev=node
            node=node.next
            j+=1
        if j==index:
            item._next=node
            prev._next=item
            self.length+=1
    def delete_node_byid(self,item_id):
        id=1
        current_node=self.head
        previous_node=None
        while current_node is not None:
            if id==item_id:
                if previous_node is not None:
                    previous_node.next=current_node.next
                else:
                    self.head=current_node.next
                    return
            previous_node=current_node
            current_node=current_node.next
            id=id+1
        self.length-=1
        return
    def find_node(self,value):
        current_node=self.head
        node_id=1
        results=[]
        while current_node is not None:
            if current_node.has_value(value):
                results.append(node_id)
            current_node=current_node.next
            node_id=node_id+1
        return
    def print_link(self):
        current_node=self.head
        while current_node is not None:
            print(current_node.data)
            current_node=current_node.next
        return
Node1=Node(1)
Node2=Node(2)
Node3=Node(3)
link=singlelink()
for node in [Node1,Node2,Node3]:
    link.add_node(node)
link.print_link()

runfile('C:/Users/Administrator/.spyder-py3/temp.py', wdir='C:/Users/Administrator/.spyder-py3')
Traceback (most recent call last):

  File ~\.spyder-py3\temp.py:90 in 
    link.add_node(node)

  File ~\.spyder-py3\temp.py:25 in add_node
    self.tail.next=item

AttributeError: 'NoneType' object has no attribute 'next'

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

    def has_value(self, value):
        if self.data == value:
            return True
        else:
            return False


class singlelink:
    def __init__(self):
        self.head = None
        self.tail = None
        self.length = 0
        return

    def isempty(self):
        return self.length == 0

    def add_node(self, item):
        # print(isinstance(item, Node))
        # print('data', item.data)
        # print('next', item.next)
        # print(type(self.head))
        # print(type(self.tail))
        # print(self.tail)
        # print(self.head is None)
        # print(self.tail is None)
        if not isinstance(item, Node):
            item = Node(item)
        if self.head is None:
            self.head = item
            self.tail = item
        else:
            self.tail.next = item
            self.tail = item
        self.length += 1
        return

    def insert_node(self, index, data):
        if self.isempty():
            print("this link is empty!")
            return
        if index < 0 or index >= self.length:
            print("error:out of index!")
            return
        item = Node(data)
        if index == 0:
            item.next = self.head
            self.head = item
            self.length += 1
            return
        j = 0
        node = self.head
        prev = self.head
        while node.next and j < index:
            prev = node
            node = node.next
            j += 1
        if j == index:
            item._next = node
            prev._next = item
            self.length += 1

    def delete_node_byid(self, item_id):
        id = 1
        current_node = self.head
        previous_node = None
        while current_node is not None:
            if id == item_id:
                if previous_node is not None:
                    previous_node.next = current_node.next
                else:
                    self.head = current_node.next
                    return
            previous_node = current_node
            current_node = current_node.next
            id = id + 1
        self.length -= 1
        return

    def find_node(self, value):
        current_node = self.head
        node_id = 1
        results = []
        while current_node is not None:
            if current_node.has_value(value):
                results.append(node_id)
            current_node = current_node.next
            node_id = node_id + 1
        return

    def print_link(self):
        current_node = self.head
        while current_node is not None:
            print(current_node.data)
            current_node = current_node.next


Node1 = Node(1)
Node2 = Node(2)
Node3 = Node(3)
link = singlelink()
for node in [Node1, Node2, Node3]:
    # print(node.data)
    link.add_node(node)
link.print_link()

因为你定义了 self.tail=None， 所以self.tail是None，None没有next属性