python - about doubly linked list error after test -

i have following 3 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 add object dllist , tried search it. , brings me 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 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' 

your dllnode class has no position attribute. yet, near end of search function, have line return current.position. python doesn't know since there no definition it. may wish add sort of counter function , increment iterate , return that.