> ## Documentation Index
> Fetch the complete documentation index at: https://python4ai.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Queues

> Manage First-In, First-Out (FIFO) structures in Python

## What is a Queue?

A **Queue** is a linear data structure that follows the **FIFO (First-In, First-Out)** principle. The element that enters first is the one that gets removed first.

Think of it like a line of people waiting to buy tickets: the person at the front of the line is served first, and new people join the line at the back.

* **Enqueue**: Adding an element to the back of the queue.
* **Dequeue**: Removing an element from the front of the queue.

***

## Why NOT to use standard Lists for Queues

In Python, you can use a list as a queue by appending to the end and popping from the front (`list.pop(0)`). However, **this is very inefficient**:

```python theme={null}
queue = ["Aarav", "Amit", "Dev"]
queue.append("Priya")  # Enqueue is fast: O(1)
queue.pop(0)           # Dequeue is slow: O(N)!
```

### The Performance Problem:

When you pop the first item from a standard Python list, Python must shift all subsequent items one position to the left in memory. For a list with thousands of items, this operation becomes slow and degrades program performance.

***

## The Correct Way: `collections.deque`

To handle queues efficiently, Python provides the `deque` (double-ended queue) class in the built-in `collections` module.

A `deque` is optimized for fast appends and pops from both ends, achieving **$O(1)$ constant time complexity** for both enqueue and dequeue operations.

```python theme={null}
from collections import deque

# 1. Initialize an empty queue
ticket_counter = deque()

# 2. Enqueue: Add people to the back of the queue
ticket_counter.append("Aarav")
ticket_counter.append("Amit")
ticket_counter.append("Dev")
print("Initial Queue:", ticket_counter) # deque(['Aarav', 'Amit', 'Dev'])

# 3. Dequeue: Serve the person at the front of the queue
served = ticket_counter.popleft()
print("Served:", served)                # Served: Aarav
print("Remaining Queue:", ticket_counter) # deque(['Amit', 'Dev'])
```

## Common mistakes

<AccordionGroup>
  <Accordion title="Using pop(0) on collections.deque">
    To dequeue from a `deque`, you must use `.popleft()`. If you use `.pop(0)` on a `deque`, Python will raise a `TypeError` because `deque` objects do not support indexing in their pop operations.
  </Accordion>

  <Accordion title="Confusing Dequeues with Lists">
    Although `deque` shares many methods with standard lists (like `.append()`), it is a different object type. If your code needs extensive sorting or slicing (like `queue[1:3]`), convert the deque back to a list first using `list(queue)`.
  </Accordion>
</AccordionGroup>

***

## Practice & Exercises

To reinforce what you've learned in this section (FIFO structures and Deques), practice with these interactive notebooks:

<CardGroup cols={2}>
  <Card title="Follow-Along Practice" icon="laptop-code">
    Practice initializing deques, performing enqueues and dequeues, and comparing performance speeds.

    [💻 VS Code](vscode://file/Users/sivaprasad/Downloads/python%20material/python4ai/public/notebooks/basics_exercises/Queues_Practice.ipynb) | [🚀 Colab](https://colab.research.google.com/github/prasad230776/python4ai/blob/master/public/notebooks/basics_exercises/Queues_Practice.ipynb) | <a href="/public/notebooks/basics_exercises/Queues_Practice.ipynb" download>📥 Download</a>
  </Card>

  <Card title="Practice Exercises" icon="pen-to-square">
    Test your knowledge with hands-on exercises on ticketing lines, customer support simulation, and error cases.

    [💻 VS Code](vscode://file/Users/sivaprasad/Downloads/python%20material/python4ai/public/notebooks/basics_exercises/Queues_Exercises.ipynb) | [🚀 Colab](https://colab.research.google.com/github/prasad230776/python4ai/blob/master/public/notebooks/basics_exercises/Queues_Exercises.ipynb) | <a href="/public/notebooks/basics_exercises/Queues_Exercises.ipynb" download>📥 Download</a>
  </Card>
</CardGroup>

***

## What's next?

Compare Arrays, Stacks, Queues, Maps, and Trees in our comprehensive Data Structures Summary.

<Card title="Data Structures Summary" icon="table" href="/data-structures/summary">
  Compare implementations and CRUD operations
</Card>