#an exception to be raised by an empty Queue

class Empty(Exception):

    pass

​

class Queue:

    class _Node:

        def __init__(self, e, n):

            self._element = e #Users element

            self._next = n #The next node in the list

​

    def __init__(self):

        self._head = None #Reference the first node

        self._tail = None #refernce the last node

        self._size = 0    #The number of elements in the queue

​

    def __len__(self):

        return self._size 

​

    def is_empty(self):

         return len(self) == 0

​

    def first(self):

        if self.is_empty():

            raise Empty("Queue is empty.")

        return self._head._element

​

    def enqueue(self, e):

        #add a new element at the back

        newnode = self._Node(e, None)

        if self.is_empty():

            self._head = newnode #if the queue is empty, the newnode becomes the head

        else:

            self._tail._next = newnode  #if the queue is not empty, the last node points to the new node

        self._tail = newnode #The new node becomes the tail

        self._size += 1

​

    def dequeue(self):

        if self.is_empty():

            raise Empty("Queue is empty.")

        element = self._head._element

        self._head = self._head._next # Make the next node the new head

        self._size -= 1

        return element

​

# test the stack

Q = Queue()

Q.enqueue(100)

Q.enqueue(200)

Q.enqueue(300)

Q.enqueue(400)

Q.enqueue(500)

print("Size: ", len(Q))

print(Q.first())

while not Q.is_empty():

    print(Q.dequeue())

​

print("Size:", len(Q))