在python中构建链接列表

Question

大家好，我正在制作链接列表，但出现此错误

Traceback (most recent call last):
  File "python", line 144, in <module>
  File "python", line 93, in get
AttributeError: 'NoneType' object has no attribute 'next'

这是否意味着我不使用next，或者可能是这里完全缺少的东西？还是我没有正确构建链接列表？

class Node(object):

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


class MyLinkedList(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """

        self.head = None
        self.size = 0

    def get(self, index):
        """
        Get the value of the index-th node in the linked list. If the 
index is invalid, return -1.
        :type index: int
        :rtype: int
        """
        if index <=  0 or index > self.size: #checks if index is less the 0 and chech if the index requested is bigger                                                  #then
            return -1                        #self.size
        else:
            current =  self.head
            for i in range(self.size):
                if i != index:
                current = current.next
                else:
                    return current.data

    def addAtHead(self, val):
        """
        Add a node of value val before the first element of the linked 
list. After the insertion, the new node will be the first node of the 
linked list.
        :type val: int
        :rtype: void
        """
        current = self.head
        node = Node(val)
        if current is None:
            current = node

        else:
            current.next = self.head
            self.head = node
        self.size = self.size + 1

    def addAtTail(self, val):
        """
        Append a node of value val to the last element of the linked list.
        :type val: int
        :rtype: void
        """
        tail = Node(val)
        if self.head is None:
            self.head = tail

        else:
            tmp = self.head
            while tmp is not None:
                tmp.next
            tmp = tail