Python - return combination of 2 queues

Question

class Queue:
def __init__(self):
    self.items = []

def isEmpty(self):
    return self.items == []

def enqueue(self, item):
    self.items.append(item)

def dequeue(self):
    if self.items == []:
        raise IndexError('The queue is empty.')
    return self.items.pop()

def size(self):
    return len(self.items)

def __str__(self):
    return "Queue: " + (str(self.items))

def enqueue_list(self, list):
    for i in list:
        self.items.append(i)
    return self.items

def splice(self, second_queue):
    for i in second_queue:
        self.items.enqueue(i)
    return self.items

Hi there,

What I am trying to do is at the bottom in the splice method. I want to iterate through a second queue and add it to the end of the original one. I can't find out how I can iterate through a queue without causing an error however. Should I change second_queue into a list somehow first?

Original exception was:
Traceback (most recent call last):
File "prog.python3", line 74, in <module>
my_queue.splice(another_queue)
File "prog.python3", line 28, in splice
for i in second_queue:
TypeError: 'Queue' object is not iterable

Answer 1

Instances of your class Queue are not iterable.

They hold a list items, but Python does not know that it should iterate over that list when you employ a for loop over a Queue instance.

To delegate the iteration to the wrapped list, simply add a method

def __iter__(self):
    return iter(self.items)

Demo with fixed class:

>>> q = Queue()
>>> q.enqueue(1)
>>> q.enqueue(2)
>>> q.items
[1, 2]
>>>
>>> for item in q:
...     print(item)
... 
1
2