A trivial queue data structure, written in Python.

queue.py

#!/usr/bin/env python

class LinkedListNode(object):
    def __init__(self, value, next=None):
        self.value = value
        self.next = None

class Queue(object):
    def __init__(self):
        self.head = None
        self.tail = None

    def enqueue(self, value):
        if self.head is None:
            self.head = LinkedListNode(value)
            self.tail = self.head
        else:
            self.tail.next = LinkedListNode(value)

    def dequeue(self):
        if self.head is None:
            return None
        node = self.head
        self.head = self.head.next
        if self.head is None:
            self.tail = None
        return node.value


def test0():
    q = Queue()
    assert q.dequeue() == None

def test1():
    q = Queue()
    q.enqueue(1)
    assert q.dequeue() == 1
    assert q.dequeue() == None

def test2():
    q = Queue()
    q.enqueue(1)
    q.enqueue(2)
    assert q.dequeue() == 1
    assert q.dequeue() == 2
    assert q.dequeue() == None

def test3():
    q = Queue()
    q.enqueue(1)
    assert q.dequeue() == 1
    q.enqueue(2)
    assert q.dequeue() == 2
    assert q.dequeue() == None

if __name__ == "__main__":
    test0()
    test1()
    test2()
    test3()
    print "all tests passed."