**If you want to understand with Graphical Representation click on it.**

You can find the explanatory video at the bottom of this page.

A Beginner's Guide to Stacks in Python

1. Introduction

What is DSA?

In the world of computers and programming, we need smart ways to organize information. A Data Structure is just a special way of storing and organizing data so we can use it effectively. Think of it like a bookshelf for your books or a folder for your files. DSA stands for Data Structures and Algorithms, which are the fundamental building blocks of programming.

Why is the Stack Important for Beginners?

The Stack is one of the simplest and most important data structures. For a beginner, understanding the Stack is a great first step because it's easy to grasp and used in many real-world applications. You see it in the "Undo" feature of a text editor, the "Back" button in your web browser, and even in video games. Learning about Stacks is a key skill that is often tested in coding interviews.

2. A Very Simple Explanation of Stack

Definition

A Stack is a data structure where items are added and removed according to a specific rule: the last item you put in is the first item you take out.

Understanding LIFO

This rule is called LIFO, which stands for Last-In, First-Out. Let's break it down:

L = Last: The very last item you add.
I = In: When you put an item into the stack.
F = First: The very first item you remove.
O = Out: When you take an item out of the stack.

So, LIFO simply means: The last item to go in is the first item to come out.

Daily Life Examples

You already use stacks every day without even realizing it!

A Stack of Plates: Imagine you are washing plates and stacking them. You place one plate, then another on top of it, and another on top of that. When you need a plate, you take the one from the very top—the last one you put on the stack.
A School Bag: When you pack your school bag, you might put your math book in, then your science book on top of it. To get to the math book, you first have to take out the science book, which was the last one you put in.
The Undo Feature: When you're typing and make a mistake, you hit "Undo." The very last thing you did is the first thing that gets undone. This is a perfect example of a stack in action.

3. Step-by-Step Concept Breakdown

A stack has a few basic actions, which we call "operations."

Push: This means adding a new item to the very top of the stack.
Pop: This means removing the item from the very top of the stack. Because of the LIFO rule, this will always be the last item that was added.
Peek: This means looking at the item on top of the stack without removing it. It’s like peeking at the top plate to see what it looks like before you take it.
isEmpty: This is a simple check to see if the stack is empty or not.

4. Visualization Section

To really understand a stack, let's create a mental picture of it. Imagine a container that is open only at the top.

Here is a stack with three items: 10, 20, and 30. The number 10 was added first, so it's at the bottom. The number 30 was added last, so it's at the top.

30	← Top (The last item added)
20
10

The Top of the Stack is a very important concept. It always refers to the last element that was added, and it's the only element we can directly access (to either pop or peek).

Now, let's see what happens when we perform operations:

1. If we Push(40) onto the stack: The new item, 40, is placed on top of the existing items. Now, 40 is the new Top.

40	← New Top
30
20
10

2. If we then Pop() the stack: The item at the Top (40) is removed, because it was the last one in. The stack goes back to how it was before.

5. Python Implementation (Beginner Level)

In Python, we can easily create a stack using a simple List.

Creating an Empty Stack: To start, we create an empty list. This list will be our stack. stack = [] Right now, our stack is empty.
Push Operation with append(): To add an item to the top of our stack, we use the list's append() method. This adds the item to the end of the list, which we treat as our "top."
Output: [10, 20, 30] In this list, 10 is at the bottom of the stack, 20 is in the middle, and 30 is at the top.
Pop Operation with pop(): To remove the top item from the stack, we use the list's pop() method. By default, pop() removes the last item from the list, which perfectly follows the LIFO rule.
Output: [10, 20] Why was 30 removed? Because it was the last item added (Last-In, First-Out).
Peek Operation with stack[-1]: To see the top item without removing it, we can look at the last element of the list. In Python, [-1] is a handy shortcut to get the last item.
Output: 20 This shows us that 20 is now the top element of the stack.
Checking if the Stack is Empty: We can check the length of our list. If the length is 0, the stack is empty.

A Complete Small Program

Here is a short program putting it all together:

# 1. Create an empty stack
stack = []

# 2. Push some items onto the stack
stack.append(5)
stack.append(10)
stack.append(15)
print("Stack:", stack)

# 3. Pop an item from the stack
stack.pop()
print("After pop:", stack)

# 4. Peek at the top item
print("Top element:", stack[-1])

Program Output:

Stack: [5, 10, 15]
After pop: [5, 10]
Top element: 10

6. Common Beginner Mistakes

When you're first learning about stacks, it's easy to make a few common mistakes.

Popping from an Empty Stack: If you try to pop() an item from a stack that has nothing in it, your program will crash with an error. Always check if a stack is empty before trying to pop from it.
Forgetting the LIFO Rule: It's easy to forget that you can only remove the top item. A stack is not like a list where you can remove an item from the middle. Always remember: Last-In, First-Out.
Confusing Stack with Queue: There is another data structure called a Queue that is the opposite of a stack. A Queue follows a "First-In, First-Out" (FIFO) rule, like a line of people waiting for a bus. Don't mix them up! (Stack = LIFO, Queue = FIFO).

7. Quiz Section

Test your understanding with these short questions.

In your own words, what is the main rule that a Stack follows?
What does the acronym LIFO stand for?
What is the key difference between the Push and Pop operations?
If you start with the stack [2, 4, 6, 8], what will the stack be after you pop() two times?
In Python, which list method is typically used to perform a Push operation?
How can you look at the top item of a stack in Python without removing it from the stack?
Using the stack of plates example, explain why it demonstrates the LIFO principle.
What is the risk of trying to Pop an item from a stack that is empty?
Name one real-world software feature that uses a Stack.
If you Push the numbers 5, then 10, then 15 onto an empty stack, which number is at the Top?

8. Answer Key

A Stack follows the "Last-In, First-Out" (LIFO) rule. This means the last item that you add to the stack is always the very first one you remove.
LIFO stands for Last-In, First-Out.
The Push operation adds a new item to the top of the stack. The Pop operation removes the existing item from the top of the stack.
After popping twice, the final stack would be [2, 4]. The numbers 8 and then 6 would be removed because they were the last ones in.
The append() method is used to Push (add) an item onto the stack, as it adds the element to the end of the list.
You can peek at the top item using index [-1] (e.g., stack[-1]). This shows you the last element of the list without deleting it.
When you stack plates, you place them on top of each other. To get a plate, you must take the one at the very top, which was the last one you placed. This is exactly how LIFO works.
If you try to Pop an item from an empty stack, your program will stop and show an error because there is nothing there to remove.
A common software feature that uses a Stack is the "Undo" function in a text editor or the "Back" button in a web browser.
The number 15 would be at the Top of the stack because it was the last one to be pushed.

9. Essay-Type Questions

For a deeper understanding, think about and try to answer these questions.

Explain the LIFO principle using the school bag example. Describe the process of adding three books (Math, Science, and History, in that order) and then explain which book comes out first and why.
Imagine you are designing a simple text editor with an "Undo" feature. How would you use a Stack to keep track of the user's actions (like typing a word) so they can be undone correctly?
Describe the visual representation of a stack. Start with an empty stack, then explain step-by-step what happens visually when you Push 'A', Push 'B', Pop, and then Peek.

10. Glossary Section

Term	Definition
Stack	A data structure that stores items and follows the LIFO (Last-In, First-Out) rule.
Data Structure	A formal way of organizing and storing data in a computer program.
LIFO	An acronym for "Last-In, First-Out." It is the core principle of a stack.
Push	The operation to add a new element to the top of the stack.
Pop	The operation to remove the top element from the stack.
Peek	The operation to look at the top element of the stack without removing it.
Top	The position of the last element added to the stack. It is the only accessible element.
Element	A single item or piece of data stored within a data structure like a stack.
List (Python)	A built-in Python data type that is ordered and changeable. It can be used to implement a stack.
Operation	A specific action that can be performed on a data structure, such as Push, Pop, or Peek.

FSPWD - Full Stack Python Web Development

Friday, January 23, 2026

DSA using Python: Stack