trjhtr

Problem:Implement a queue class in Python: It should support 3 APIs:

queue.top(): prints current element at front of queue

queue.pop(): takes out an element from front of queue

queue.add(): adds a new element at end of queue

class Queue:

 def __init__(self):
 """initialise a Queue class"""
 self.items = 
 self.index = len(self.items) - 1

 def top(self):
 """returns the current element at front of queue"""
 if self.items:
 return self.items[0]
 else:
 raise Exception("Empty Queue")

 def pop(self):
 """takes out an element from front of queue"""
 if self.items:
 self.items.pop(0)
 else :
 raise Exception("Empty Queue")

 def add(self , item):
 """adds a new element at the end of queue"""
 self.items.append(item)

 def __str__(self):
 """returns the string representation"""
 return str(self.items)

 def __iter__(self):
 """iterates over the sequence"""
 return self


 def __next__(self):
 """returns the next value in sequence"""
 if self.index == 0 :
 raise StopIteration
 self.index -= 1
 return self.items[self.index]




queue_1 = Queue()
queue_1.add(12)
queue_1.add(11)
queue_1.add(55)
queue_1.add(66)
queue_1.add(56)
queue_1.add(43)
queue_1.add(33)
queue_1.add(88)
queue_1.add(56)
queue_1.add(34)
iter(queue_1)
for i in queue_1:
 print i 
print queue_1
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()
print queue_1.top()
queue_1.pop()

The code works as intended and executes the 3 APIs specified in the problem . But it gives ""TypeError: instance has no next() method"" when i try to use next method to iterate over the values in queue. What am i missing in defining the next method?

I would have just inherited from the built-in collections.deque:

from collections import deque

class Queue(deque):
 def add(self, value):
 self.append(value)

 def top(self):
 return self[0]

 def pop(self):
 return self.popleft()