class SingleLinked_Queue:

class _Node:

def __init__(self,e,nextLink):

self._element=e

self._next=nextLink

def __init__(self):

self._head=None

self._tail=None

self._size=0

def __len__(self):

return self._size

def is_empty(self):

return self._size==0

def first(self):

if self.is_empty():

raise Empty('Queue is Empty')

return self._head._element

def dequeue(self):

if self.is_empty():

raise Empty('Queue is Empty')

answer=self._head._element

self._head=self._head._next

self._size-=1

if self.is_empty():

self._tail=None

return answer

def enqueue(self,element):

newest=self._Node(element,None)

if self.is_empty():

self._head=newest

else:

self._tail._next=newest

self._tail=newest

self._size+=1

class Empty(Exception):

pass

if __name__=="__main__":

myQueue=SingleLinked_Queue()

print(myQueue.__len__())

myQueue.enqueue(10)

myQueue.enqueue(20)

myQueue.enqueue(30)

myQueue.enqueue(40)

myQueue.enqueue(60)

print(myQueue.first())

print(myQueue.dequeue())

print(myQueue.__len__())

print(myQueue.dequeue())

print(myQueue.dequeue())

print(myQueue.dequeue())

print(myQueue.first())

print(myQueue.dequeue())

print(myQueue.first())