我究竟做错了什么？这是我正在努力完成的家庭作业。我得到了奇怪的语法错误：
File "C:\Users\Ranbir\Desktop\myList2.py", line 56, in add
myList.prev = myList.cursor
AttributeError: 'MyList' object has no attribute 'prev'
我看不出我做错了什么，代码对我来说看起来很完美。prev 在 ListNode 和 mkListNode 中定义。
class EmptyListNode():
"""
An empty list is an instance of EmptyListNode.
"""
__slots__ = ()
class ListNode():
"""
All true value-containing wheel nodes are represented as
instances of ListNode.
"""
__slots__ = ('data', 'next', 'prev')
class MyList():
"""
The container for a linked wheel class.
It contains a reference to a cursor in the wheel.
Invariant: size==0 iff type(cursor)==EmptyListNode
"""
__slots__ = ('cursor', 'size')
def mkList():
"""
Makes a new list, with size 0, pointing to a Empty Node
"""
mylist = MyList()
mylist.cursor = EmptyListNode
mylist.size = 0
return mylist
def mkListNode(data, next, prev):
"""
Make a new list node
Returns a new node
"""
listNode = ListNode()
listNode.data = data
listNode.next = next
listNode.prev = prev
return ListNode
def add(myList, element):
"""
add: Add a node to the wheel right after the cursor
Effect: A new node is in the wheel just after the cursor
"""
myList.size += 1
if myList.cursor == EmptyListNode:
myList.cursor = mkListNode(EmptyListNode, element, EmptyListNode)
myList.cursor.prev = myList.cursor
myList.cursor.next = myList.cursor
else:
temp = mkListNode(myList.cursor.prev, element, myList.cursor.next)
myList.cursor.prev.next = temp
myList.cursor.prev = temp
def backward(List):
"""
advance: Advance the cursor
Effect: The cursor is now at the next node in the wheel
"""
myList.cursor = List.cursor.prev
def forward(myList):
"""
advance: Advance the cursor
Effect: The cursor is now at the next node in the wheel
"""
myList.cursor = myList.cursor.next
def getNext(List):
"""
getNextElement: Get element at node after cursor position
Effect: The element after cursor position is returned
"""
return List.cursor.next.data
def getPrev(List):
"""
getNextElement: Get element at node after cursor position
Effect: The element after cursor position is returned
"""
return List.cursor.prev.data
def printList(myList):
"""
Prints the all the items in the wheel
"""
string1 = ""
current = myList.cursor
for i in range(mylist.size):
string1 += current.data
current = current.next
def size(myList):
"""
size: wheel -> numl
"""
return mylist.size
def remove(myList):
"""
remove: Remove the node at the cursor from the wheel
Effect: The node after the wheel is removed
Precondition: wheel size > 0
"""
if size(myList) == 1:
myList.cursor = EmptyListNode()
else:
myList.cursor.next.prev= List.cursor.prev
myList.cursor.prev.next = List.cursor.next
myList.cursor.data = List.cursor.prev.data
myList.cursor = List.cursor.prev
myList.cursor.prev.next = List.cursor
myList.size -= 1
有什么帮助吗？谢谢