Python Circular Doubly Linked List

Question

I am working on creating a doubly linked circular list but am having a few problems. Here is my code:

#!/usr/bin/env python

class _DoublyLinkedList:

    class _Node(object):

        __slots__ = '_element', '_prev', '_next'

        def __init__(self, element, prev, next):
            self._element = element
            self._prev = prev
            self._next = next

        def getNext(self):
            return self._next

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

    def __init__(self):
        self._header = self._Node(None, None, None)
        self._trailer = self._Node(None, None, None)
        self._header._next = self._trailer
        self._trailer._prev = self._header
        self._size = 0

    def __len__(self):
        return self._size

    def is_empty(self):
        return self._size == 0

    def _insert_between(self, e, predecessor, successor):
        newest = self._Node(e, predecessor, successor)
        predecessor._next = newest
        successor._prev = newest
        self._size += 1
        return newest

    def _delete_node(self, node):
        predecessor = node._prev
        successor = node._next
        predecessor._next = successor
        successor._prev = predecessor
        self._size -= 1
        element = node._element
        node._prev = node._next = node._element = None
        return element

    def first(self): #return but do not remove the element at front
        if self.is_empty():
            raise Empty("Empty!")
        return self._header._next._element

    def last(self):
        if self.is_empty():
            raise Empty("Empty!")
        return self._trailer._prev._element

    def insert_first(self, e):
        self._insert_between(e, self._header, self._header._next)

    def insert_last(self, e):
        self._insert_between(e, self._trailer._prev, self._trailer)

    def delete_first(self):
        if self.is_empty():
            raise Empty("Empty!")
        return self._delete_node(self._header._next)

    def delete_last(self):
        if self.is_empty():
            raise Empty("Empty!")
        return self._delete_node(self._trailer._prev)

    def __str__(self):
        if self._size == 0:
            return '[]'
        retString = "["
        currentElement = self._header.getNext()
        for i in range(self._size):
            retString += str(currentElement) +", "
            currentElement = self._header.getNext()

        return retString[:-2] + ']'


def main():
    d = _DoublyLinkedList()
    d.insert_first(10)
    print(d.__str__())
    d.insert_last(23)
    d.insert_last(24)
    print(d.__str__())

if __name__ == "__main__":
    main()

For starters, as you can see this isn't circular at all so any pointers would help but what i am trying to figure out is why my output from this is:

[10]
[10, 10, 10]   

The program has the correct number of elements that I try to add but prints them all as the element that I inserted in the front and I don't understand why.

Answer 1

The reason is, each time through the loop, you get the same element.

currentElement = self._header.getNext()
for i in range(self._size):
    retString += str(currentElement) +", "
    currentElement = self._header.getNext()

Change this to:

currentElement = self._header.getNext()
for i in range(self._size):
    retString += str(currentElement) +", "
    currentElement = currentElement.getNext()