How to Implement Doubly Linked List in Python

This is my simple implementation of doubly linked list in Python.

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#!/usr/bin/env python   class Node(object):     def __init__(self):         self.next = None         self.previous = None         self.element = None       class LinkedList(object):     def __init__(self):         self.n = 0         self.last = Node()         self.first = self.last               def append(self, element):         self.last.element = element         self.last.next = Node()         tmp = self.last         self.last = self.last.next         self.last.previous = tmp         self.n += 1           def front(self):         if self.n == 0: return None         e = self.first.element         self.first = self.first.next         self.n -= 1         return e               def back(self):         if self.n == 0: return None         e = self.last.previous.element         self.last = self.last.previous         self.last.next = Node()         self.n -= 1         return e               def size(self):         return self.n           def elements(self):         i = self.first         while i.element:             yield i.element             i = i.next           if __name__ == "__main__":     l = LinkedList()           assert None == l.front()     assert None == l.back()           l.append(1)     l.append(2)     l.append(3)           assert 1 == l.front()     assert 2 == l.front()     assert 3 == l.front()       l.append(1)     l.append(2)     l.append(3)           assert 3 == l.back()     assert 2 == l.back()     assert 1 == l.back()       l.append(1)     assert 1 == l.back()           l.append(1)     assert 1 == l.front()