Queue in Python

A Queue is a linear data structure that follows a specific order for all operations. This order is based on the First-In, First-Out (FIFO) principle.

Imagine a ticket line: the first person to join the line is the first person to be served and leave.

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Queue Operations

Queues have two distinct ends where operations occur: the Rear (for insertion) and the Front (for removal).

Operation	Description	Analogy
Enqueue	Adds an element to the Rear.	Joining the back of the line.
Dequeue	Removes an element from the Front.	Leaving the front of the line.
Peek	Returns the front element without removing it.	Looking at the person next in line.

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Queue Operations Using Python List

In the simplest Python implementation, we use a built-in list as the underlying storage. For a true FIFO queue:

• Enqueue is performed using list.append() (at the rear/end).
• Dequeue is performed using list.pop(0) (from the front/start).
• Peek is performed using list indexing list[0] (at the fromt/start).

The Queue Class

class Queue:
    """Implements a Queue using the FIFO principle with a Python list."""
    def __init__(self):
        # The storage container for the queue elements
        self._items = [] 

    def is_empty(self):
        """Check if the queue is empty."""
        return not self._items

    def enqueue(self, data):
        """Adds an element to the rear of the queue (list.append()). (O(1))"""
        self._items.append(data)
        print(f"Enqueued: {data}")

    def dequeue(self):
        """Removes and returns the front element (list.pop(0)). (O(N))"""
        if self.is_empty():
            raise IndexError("Error: Cannot dequeue from an empty queue.")
        
        # pop(0) removes the first item (the front of the queue)
        return self._items.pop(0)

    def peek(self):
        """Returns the front element without removing it."""
        if self.is_empty():
            raise IndexError("Error: Cannot peek at an empty queue.")
        
        # Access the first item (0 index)
        return self._items[0]

Example

my_queue = Queue()
my_queue.enqueue("Task 1") 
my_queue.enqueue("Task 2") 
my_queue.enqueue("Task 3") 

print("\nFront element (Peek):", my_queue.peek())

served_item = my_queue.dequeue()
print("Dequeued element:", served_item)

print("Queue Status:", my_queue._items)

my_queue.dequeue()
my_queue.dequeue()

# my_queue.dequeue() # Uncommenting this line will raise an IndexError

Output:

Enqueued: Task 1
Enqueued: Task 2
Enqueued: Task 3

Front element (Peek): Task 1
Dequeued element: Task 1
Queue Status: ['Task 2', 'Task 3']