FAQ: Queues: Python - Queues Python Enqueue

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This community-built FAQ covers the “Queues Python Enqueue” exercise from the lesson “Queues: Python”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Computer Science

Linear Data Structures

FAQs on the exercise Queues Python Enqueue

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2

Hello people!

Can someone help me understand the Enqueue method? I think i understand well until de Else statement under the first If. Here:

  else:
    self.tail.set_next_node(item_to_add)
    self.tail = item_to_add
  self.size += 1

I just can’t understand why set the next node of the current tail to the item_to_add. When i think of a queue i think in setting the next node of item_to_add to the current tail, not the otherwise.

Thanks!

3

You’re basically saying. If the Queue is NOT empty. Add John(item_to_add) to the back of the queue and tell Mary(tail), that was previously at the back, that John is now behind her. After that you tell your program, that Adam is now the “tail” of the queue.

Remember that “self.tail.set_next_node(item_to_add)” is telling the current tail, that item_to_add is coming after it.

6 Likes
6

Hello,

as far as I can see set_next_node() is a method of Node class. In the exercise we use this method on a Queue class object q. To put it exactly: We do self.tail.set_next_node(item_to_add) where self is a Queue object.
isinstance(self, Node) yields False, i.e self is not an instance of Node. So my question is: Why can we use a method of a different class (Node) on a class (Queue) which is not inherited from the first class ? Why doesn’t self.tail.set_next_node(item_to_add) throw an error ?

Any help is highly appreciated.
Thank you.

7

Why do I get an AttributeError at line? 43?
from node import Node

class Queue:
def init(self, max_size=None):
self.head = None
self.tail = None
self.max_size = max_size
self.size = 0

Add your enqueue method below:

def enqueue(self, value):
if self.has_space():
item_to_add = Node(value)
print(“Adding " + str(item_to_add.get_value()) + " to the queue!”)
if self.is_empty():
self.head = item_to_add
self.tail = item_to_add
else:
self.tail.set_next_node(item_to_add)
self.tail = item_to_add
self.size += 1
else:
print(“Sorry, no more room!”)

def peek(self):
if self.is_empty():
print(“Nothing to see here!”)
else:
return self.head.get_value()

def get_size(self):
return self.size

def has_space(self):
if self.max_size == None:
return True
else:
return self.max_size > self.get_size()

def is_empty(self):
return self.size == 0
q = Queue(“all the fluffy kitties”)
q.enque(“all the fluffy kitties”)

8

During step 2 I am getting “Did you add self and value as paramenters for enqueque()?” (the misspelling is there). The weird thing is that I defined enqueque() with parameters self and value in step 1 and I passed that step.

When I replace my code (below) with the solution code, it lets me pass.

from node import Node

class Queue:
def init(self, max_size=None):
self.head = None
self.tail = None
self.max_size = max_size
self.size = 0

Add your enqueue method below:

def enqueue(self, value):
if self.has_space():
item_to_add = Node(value)
print("Adding "+ str(item_to_add.get_value() + ’ to the queue!'))
if self.is_empty():
self.tail = item_to_add
self.head = item_to_add
else:
self.tail.set_next_node(item_to_add)
self.tail = item_to_add
self.size +=1
else:
print(“Sorry, no more room!”)

def peek(self):
if self.is_empty():
print(“Nothing to see here!”)
else:
return self.head.get_value()

def get_size(self):
return self.size

def has_space(self):
if self.max_size == None:
return True
else:
return self.max_size > self.get_size()

def is_empty(self):
return self.size == 0

q = Queue()
q.enqueue(“all the fluffy kitties”)
print(q.peek())

9

Ah, I see what I did. I’m missing a parenthesis 3 lines down.