Python Tutorial



Python Tutorial

Queue in Python


Effective data management involves efficiently organizing, storing, and retrieving data in various applications, from web development to scientific computing. Queue plays a pivotal role in this process as a vital essential data structure.

As a linear data structure, a queue follows the First-In-First-Out (FIFO) principle as the element that enters the queue first is the one to be removed first. For comparison, imagine a queue of people waiting in line at a ticket counter; the person who arrived earliest gets their ticket first.

Queues have diverse applications and are used in task scheduling, managing requests in web servers, breadth-first search algorithms, and more. Python is powerful and versatile and provides developers tools and implementations for working with queues.

This article is designed to provide clear understanding of queues, why they are essential, and their applications. Then, we'll explore the operations associated with queues, the methods available for queue manipulation, and various implementations, including collections.deque, queue.Queue, and the built-in Python list.

What is a Queue?

A queue is a linear data structure that follows the First-In-First-Out (FIFO) principle. In a queue, the first element added is the first one to be removed. Imagine it as a line of people waiting for a service; the person who arrives first is the first to be served.

Queues are fundamental in computer science and have many applications, such as task scheduling, data processing, and synchronization and play a crucial role in maintaining order and ensuring fairness.

Queues have a wide range of applications across various scenarios:

  1. Task Scheduling: In operating systems, queues are used to manage processes and tasks.

  2. Data Processing: Queues are used to ensure that data is processed in the order received using data processing pipelines.

  3. Synchronization: By managing access to shared resources, queues find application in a multithreaded or multiprocessing environment to ensure orderly access to critical sections of code.

  4. Print Queue: A print queue ensures that documents are printed in the order received This ensures fairness in networked office environments.

  5. Background Task Processing: In web applications, background tasks such as sending emails, generating reports, or processing uploaded files can be managed using queues. Tasks are added to the queue and processed by background workers, ensuring that the application remains responsive to user requests.

Queues are, therefore essential to maintain order and ensure fairness in various applications. Queues help manage and streamline processes by following the FIFO principle, making them an indispensable tool in computer science and programming.

Operations Associated with Queue

To effectively manage data using queues, we need to understand the fundamental operations associated with them:

Operations Associated with Queue:

In the world of computer science and data structures, queues are defined by their distinctive set of operations that facilitate the orderly management of data. Understanding these fundamental operations is crucial for effectively working with queues in Python. Let's delve deeper into each operation:

1. Enqueue (Adding an Element):
Enqueueing is the process of placing an item at the rear of the queue. This operation ensures that the most recently arrived element becomes the last in line to be processed.

2. Dequeue (Removing an Element):
Dequeueing involves the removal of an element from the front of the queue. Dequeueing maintains the order in which elements entered the queue, adhering to the First-In-First-Out (FIFO) principle.

3. Peek (Viewing the Front Element):
Peek is an operation that allows you to view the front element of the queue without removing it. This operation is valuable when you need to inspect the next item to be processed without altering the queue's state.

4. Size (Determining the Number of Elements):
Measuring the size of a queue involves determining the number of elements currently residing within it. This operation provides insight into the queue's occupancy, helping you monitor its capacity and make informed decisions about when to enqueue or dequeue elements.

These operations are the building blocks for working with queues in Python.

Methods Available in Queue

There are different key methods available implementations to implement queues, each with its own set of methods and characteristics:

  • append(item): Adds an item to the rear of the queue (used in Python lists).

  • pop(index): Removes and returns an item from a specific index (used in Python lists).

  • popleft(): Removes and returns the leftmost item (used in collections.deque).

  • appendleft(item): Adds an item to the left end (used in collections.deque).

  • put(item): Enqueues an item at the rear of the queue (used in queue.Queue).

  • get(): Dequeues and returns an item from the front of the queue (used in queue.Queue).

  • empty(): Checks if the queue is empty.

  • qsize(): Returns the number of items in the queue.

  • full(): Checks if the queue is full (used in queue.Queue).

An understanding these methods is crucial for effectively working with queues in Python.


Now, let's explore different implementations of queues in Python and how to use them.

Implementation using collections.deque

Python's collections module provides a double-ended queue, known as deque, which can be used to implement a queue. A deque supports efficient append and pop operations from both ends, making it a suitable choice for implementing a queue.


This implementation is efficient for small to medium-sized queues.

•We import the deque class from the collections module and create a deque object named my_queue in this example. 

•We then use the append() method to enqueue elements and the popleft() method to dequeue elements. 

Implementation using queue.Queue

Using the queue.Queue class from the queue module provides a thread-safe and efficient queue implementation.



In this example, 

•We import the queue module and create a queue.Queue object named my_queue. 

•We use the put() method to enqueue elements and the get() method to dequeue elements.

 This implementation is thread-safe, making it suitable for multi-threaded applications.

The Built-in Python List

While Python provides dedicated data structures like queue.Queue and collections.deque for implementing queues, you can also use the built-in Python list to create a simple queue.


 In this example,

·         We use a Python list to implement a queue.

·         We add elements to the end of the list using the append() method and remove elements from the front using the pop(0) method.

The built-in Python list may not be as efficient as using queue.Queue or collections.deque for larger queues.

How to Add Elements to a Queue in Python?

Adding elements to a queue, also known as enqueueing, is a fundamental operation. You can use various methods depending on the queue implementation.

For collections.deque:

For queue.Queue:

For a Python list (not recommended for large queues):

You can choose the one that fits your specific requirements from these different methods.

How to Remove Elements From a Queue in Python?

Dequeueing or removing elements from a queue is as important as enqueueing. Let us understand how to dequeue elements from different queue implementations.

For collections.deque:

For queue.Queue:

For a Python list (not recommended for large queues):

Remember to choose the appropriate queue implementation based on your needs, especially for larger queues.

Sorting the Queue

In some cases, you might need to sort the elements in a queue based on certain criteria. While queues primarily follow the FIFO principle, you can create a sorted queue using a priority queue.


In this example, we create a queue.PriorityQueue and enqueue elements with associated priorities. When dequeuing, elements are retrieved in priority order.

The Queue Module

Python's queue module provides various classes for implementing queues apart from queue.Queue, such as queue.LifoQueue for implementing stack-like structures (Last-In-First-Out) and queue.PriorityQueue for priority-based queues.

Working With queue.Queue Class

queue.Queue class provides a thread-safe queue implementation.


In this example, we create a queue.Queue and use multiple threads to process items concurrently. The put() method enqueues items, and worker threads dequeue and process them. This demonstrates the thread-safe nature of queue.Queue.


Queues, guided by the First-In-First-Out (FIFO) principle, ensure that tasks are executed in the order they are received, creating a sense of order and fairness in various scenarios.

In the world of computer science and programming, effective data management through queues play an indispensable role in achieving this goal. Queues assist in managing data efficiently, ensuring that the first item added is the first to be processed.

Python, with its flexible and powerful features, offers a multiple options for queue implementation. By mastering the art of queues in Python, you can manage data efficiently and create robust and responsive applications. 


1. What is the primary purpose of a queue in Python?

The primary purpose of a queue in Python is to manage and process items in a specific order by following the First-In-First-Out (FIFO) principle and ensuring that the item added first is the first to be processed.

2. When should I use collections.deque to implement a queue?

When you need a simple and efficient queue implementation, especially for small to medium-sized queues, collections.deque offers fast operations for both enqueueing and dequeueing.

3. When is it appropriate to use queue.Queue in Python?

queue.Queue ensures that multiple threads can safely enqueue and dequeue items without conflicts. It is suitable for a thread-safe queue implementation, which is essential for multi-threaded applications. It

4. What is the difference between a stack and a queue?

A stack follows the Last-In-First-Out (LIFO) principle, processing the last-added element first. In contrast, a queue adheres to the First-In-First-Out (FIFO) rule, processing the first-added element first. Stacks are suitable for managing function calls, while queues are ideal for tasks requiring order preservation.

5. Can I use queues for priority-based ordering?

Yes, you can use queue.PriorityQueue from the queue module to implement a priority queue in Python. It allows you to enqueue priority items and dequeue them based on their priority levels.                                 

6. What is the maximum size limit of a queue in Python?

The maximum size limit of a queue in Python depends on the available system memory. However, you can specify a maximum size when initializing queue.Queue objects.

7. How do I handle exceptions in queue operations in Python?

To handle exceptions, you can use try-except blocks when performing queue operations. Exceptions for this include Queue.Empty and Queue.Full for queue.Queue or IndexError.

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