python-stack

A simple, from-scratch implementation of the Stack data structure in Python, built on a fixed-size list. The size is chosen when the stack is created and never changes — when the stack is full, new items are rejected.

What is a Stack?

A stack is a linear data structure that follows the LIFO principle (Last In, First Out): the last item you add is the first one you remove. Think of a stack of plates — you can only add or remove from the top.

Features

• push — add an item to the top of the stack (prints a message if the stack is full)
• pop — remove and return the top item (prints a message if the stack is empty)
• peak — view the top item without removing it
• is_empty — check if the stack has no items
• show_size_top — see how many slots are filled vs. empty
• __len__ — use Python's built-in len() to get the number of items

Installation

No dependencies needed — just Python 3. Clone the repo or copy stack.py into your project.

git clone https://github.com/soshjant/python-stack.git

Usage

The size is optional and defaults to 10. Once chosen, it never changes.

from stack import stack

s = stack(3)   # a stack that can hold at most 3 items
# s = stack() would create a stack with the default size of 10

s.push(10)
s.push(20)
s.push(30)
s.push(40)     # prints: stack is full (the item is not added)

print(s.peak())            # 30
print(s.show_size_top())   # (3, 0)  -> 3 filled, 0 empty
print(len(s))              # 3

print(s.pop())             # 30
print(s.show_size_top())   # (2, 1)  -> 2 filled, 1 empty
print(s.is_empty())        # False

How it works

The stack uses a plain Python list pre-filled with None and an index called top that points to the last filled slot:

size = 5, after pushing 10, 20, 30

index:   0    1    2     3     4
list:  [10,  20,  30, None, None]
                  ↑
               top = 2

• push moves top up by one and writes the new item there.
• pop returns the item at top, then moves top down by one.
• When top == -1 the stack is empty; when top == size - 1 it is full.

Class Reference

Method	Description
stack(size=10)	Creates a new stack that can hold size items (default: 10)
push(x)	Adds x to the top; prints stack is full if there is no space
pop()	Removes and returns the top item; prints stack is empty if empty
peak()	Returns the top item without removing it; prints stack is empty if empty
is_empty()	Returns True if the stack has no items
show_size_top()	Returns a tuple: (filled_slots, empty_slots)
len(stack)	Returns the number of items currently in the stack

Notes

• This project was built as a learning exercise to understand how a stack works internally.
• Popped items are not physically cleared from the internal list; only the top pointer moves. This is normal and doesn't cause any bugs.
• A version of this stack that grows automatically when full (dynamic resizing) is also possible — see the python-dynamic-stack project.

License

Feel free to use, modify, and learn from this code.