about doubly linked list error after test

Question

I have the following three classes:

class DLLNode(object):
    def __init__ (self, data, prev_node=None, next_node=None):
        self.data=data
        self.prev_node=prev_node
        self.next_node=next_node

    def __str__ (self):
        return str(self.data)

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

    def add_to_head(self,add_obj):
        newNode=DLLNode(add_obj)
        if self.head==None:
            self.head=self.tail=newNode
            self.head.prev_node=self.tail.next_node=None
        else:
            self.head.prev_node=newNode
            newNode.next_node=self.head
            self.head=newNode
            self.head.prev_node=None

    def add_to_tail(self, add_obj):
        newNode=DLLNode(add_obj)
        if self.head==None:
            self.head=self.tail=newNode
            self.head.prev_node=self.tail.next_node=None
        else:
            self.tail.next_node=newNode
            newNode.prev_node=self.tail
            self.tail=newNode
            self.tail.next_node=None

    def remove_head(self):
        if self.head==self.tail:
            self.prev_node=self.next_node=self.head=self.tail=None
            return
        if self.head != self.tail:
            self.head=self.head.next_node
            self.head.prev_node=None
            return self.head

    def remove_tail(self):
        if self.head==self.tail:
            self.prev_node=self.next_node=self.head=self.tail=None
            return
        if self.head != self.tail:
            self.tail=self.tail.prev_node
            self.tail.next_node=None
            return self.tail

    def search (self, element):
        current=self.head
        if current == None:
            return -1
        else:
            while current != None:
                if current == None:
                    return -1
                else:
                    if current.data==element:
                        return current.position
                    else:
                        current=current.next_node

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

    def add (self, add_obj):
        newNode=DLLNode(add_obj)
        current=self.head
        if current==None:
            self.head=self.tail=newNode
        else:
            while add_obj>current.data:
                current=current.next_node
            newNode.next_node=current
            newNode.prev_node=current.prev_node
            current.prev_node.next_node=newNode
            current.prev_node=newNode

    def remove (self, element):
        current=self.head
        while element != current.data:
            current=current.next_node
        current.next_node.prev_node=current.prev_node
        current.prev_node.next_node=current.next_node
        current=None

    def middle (self):
        length=0
        current=self.head
        while current != None:
            current=current.next_node
            length =+ 1
        headNode=self.head
        tailNode=self.tail
        if length/2%1:
            while headNode != tailNode:
                headNode=headNode.next_node
                tailNode=tailNode.prev_node
            return headNode
        elif length/2%0:
            tailnode=tailNode.prev_node
            while headNode != tailNode:
                headNode=headNode.next_node
                tailNode=tailNode.prev_node
                return headNode            

I tried to add object to DLList and tried to search for it. And it brings me the follow error:

Traceback (most recent call last):
  File "C:\Program Files (x86)\Wing IDE 101 5.1\src\debug\tserver\_sandbox.py", line 1, in <module>
    # Used internally for debug sandbox under external interpreter
  File "C:\Program Files (x86)\Wing IDE 101 5.1\src\debug\tserver\_sandbox.py", line 65, in search
    builtins.AttributeError: 'DLLNode' object has no attribute 'position'

Answer 1

Your DLLNode class has no position attribute. Yet, near the end of your search function, you have the line return current.position. Python doesn't know what to do with that since there is no definition for it. You may wish to add some sort of counter to the function and increment it as you iterate and then return that.