Stack in Python

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

Imagine a stack of plates: you can only add a new plate to the top, and you can only remove the plate that is currently on the top.

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Stack Operations

A stack has three primary operations, all of which occur at the Top of the stack:

1. Push: Adds an element to the top of the stack.
2. Pop: Removes and returns the element from the top of the stack.
3. Peek: Returns the element at the top.

Operation	Description	Analogy
Push	Add element to the Top	Placing a plate on top of the stack
Pop	Remove element from the Top	Taking the top plate off the stack
Peek	View the element at the Top	Looking at the top plate

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Stack Operations Using Python

ListIn the simplest Python implementation, we use a built-in list as the underlying storage. For a true LIFO stack, all primary operations are performed at the end of the list:

• Push is performed using list.append().
• Pop is performed using list.pop().
• Peek is performed using list indexing list[-1].

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The Stack Class

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

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

    def push(self, data):
        """Adds an element to the top of the stack (list.append())."""
        self._items.append(data)
        print(f"Pushed: {data}")

    def pop(self):
        """Removes and returns the top element (list.pop())."""
        if self.is_empty():
            raise IndexError("Error: Cannot pop from an empty stack.")
        
        # Pop removes the last item (the top of the stack)
        return self._items.pop()

    def peek(self):
        """Returns the top element without removing it."""
        if self.is_empty():
            raise IndexError("Error: Cannot peek at an empty stack.")
        
        # Access the last item (-1 index)
        return self._items[-1]

Example

my_stack = Stack()
my_stack.push("A") 
my_stack.push("B") 
my_stack.push("C") 

print("\nTop element (Peek):", my_stack.peek()) 

removed_item = my_stack.pop()
print("Popped element:", removed_item)

print("Current size:", len(my_stack._items))

my_stack.pop()
my_stack.pop()

# my_stack.pop() # Uncommenting this line will raise an IndexError

Output:

Pushed: A
Pushed: B
Pushed: C

Top element (Peek): C
Popped element: C
Current size: 2