Stacks and Queues in Python: A Complete Beginner’s Guide

Introduction

In Python, data structures like Stacks and Queues play an important role in solving real-world problems. Whether you’re preparing for coding interviews, building compilers, or working with task scheduling systems, these two structures are widely used.

In this tutorial, we’ll cover:

• What is a Stack?

• What is a Queue?

• Their operations (push, pop, enqueue, dequeue)

• Implementation in Python with examples

1. What is a Stack?

A Stack is a linear data structure that follows the LIFO (Last In, First Out) principle.
Think of a stack of plates — the last plate you put on top is the first one you take out.

Common Stack Operations

• Push → Add an element to the stack

• Pop → Remove the top element

• Peek/Top → View the top element without removing it

• isEmpty → Check if stack is empty

Python Implementation of Stack (Using List)

stack = []

# Push
stack.append(10)
stack.append(20)
stack.append(30)

print("Stack:", stack)

# Pop
print("Popped:", stack.pop())
print("Stack after pop:", stack)

# Peek
print("Top element:", stack[-1])

✅ Output:

Stack: [10, 20, 30]
Popped: 30
Stack after pop: [10, 20]
Top element: 20

2. What is a Queue?

A Queue is a linear data structure that follows the FIFO (First In, First Out) principle.
Think of people standing in a line — the first person in line is the first to be served.

Common Queue Operations

• Enqueue → Add an element to the queue

• Dequeue → Remove the front element

• Peek/Front → Get the first element without removing it

• isEmpty → Check if queue is empty

Python Implementation of Queue (Using collections.deque)

from collections import deque

queue = deque()

# Enqueue
queue.append(10)
queue.append(20)
queue.append(30)

print("Queue:", queue)

# Dequeue
print("Dequeued:", queue.popleft())
print("Queue after dequeue:", queue)

# Peek
print("Front element:", queue[0])

✅ Output:

Queue: deque([10, 20, 30])
Dequeued: 10
Queue after dequeue: deque([20, 30])
Front element: 20

3. Queue Implementation Using Python’s queue Module

Python also provides a built-in queue module for more advanced operations.

from queue import Queue

q = Queue()

# Enqueue
q.put(1)
q.put(2)
q.put(3)

# Dequeue
print("Dequeued:", q.get())
print("Dequeued:", q.get())

Comparison: Stack vs Queue

Conclusion

Both Stacks and Queues are fundamental data structures in Python.

• Use Stacks when you need LIFO operations (undo operations, function calls).

• Use Queues when you need FIFO operations (task scheduling, message passing).

Mastering these concepts will help you write efficient code and ace coding interviews. 🚀

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