How to Implement Doubly Linked List in Python

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

#!/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()