doubly Linked list iterator python

Question

I am constructing a doubly linked list and I am struggling on construct a doubly linked list iterator method in PYTHON.

This is my code so far

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

class ListIterator:
    def __init__(self):
        self._current = self.head

    def __iter__(self):
        return self

    def next(self):
        if self.size == 0 :
            raise StopIteration
        else:
            item = self._current.data
            self._current=self._current.next
            return item

class DoublyLinkedList:
    def __init__(self):
        self.head= None
        self.tail= None
        self.size = 0

    def add(self,data):
        newnode= DoubleListNode(data)
        self.size+=1
        if self.head is None:
            self.head = newnode
            self.tail = self.head
        elif data < self.head.data: # before head
            newnode.next = self.head
            self.head.prev= newnode
            self.head= newnode
        elif data > self.tail.data: # at the end
            newnode.prev= self.tail
            self.tail.next= newnode
            self.tail=newnode
        else:
            curNode = self.head
            while curNode is not None and curNode.data < data:
                curNode=curNode.next            
            newnode.next= curNode
            newnode.prev=curNode.prev
            curNode.prev.next= newnode
            curNode.prev=newnode

    def remove(self,data):
        curNode=self.head
        while curNode is not None and curNode.data!= data:
            curNode= curNode.next
        if curNode is not None:
            self.size -= 1
            if curNode is self.head:
                self.head= curNode.next
            else:
                curNode.prev.next=curNode.next
            if curNode is self.tail:
                self.tail=curNode.prev
            else:
                curNode.next.prev=curNode.prev

When I run a test it said TypeError: iteration over non-sequence. Did I do something wrong ?

Answer 1

As posted, the code doesn't initialize (i.e. self.head isn't defined).

But overall, you are on the right track. Take a look at the source for Python's collections.OrderedDict for a worked-out example of traversing a doubly linked list.

Here's a simplified example:

class Link:
    def __init__(self, value, prev=None, next=None):
        self.value = value
        self.prev = prev
        self.next = next

    def __iter__(self):
        here = self
        while here:
            yield here.value
            here = here.next

    def __reversed__(self):
        here = self
        while here:
            yield here.value
            here = here.prev

if __name__ == '__main__':
    a = Link('raymond')
    b = Link('rachel', prev=a);  a.next=b
    c = Link('matthew', prev=b); b.next=c

    print 'Forwards:'
    for name in a:
        print name
    print
    print 'Backwards:'
    for name in reversed(c):
        print name