Python Packages Overview — Function, Module & Package

From functions to packages: Understand how Python organizes reusable code — Function → Module → Package.

Use the playground and quiz to climb the reuse ladder.

What you will learn

• Differentiate function, module, and package
• Understand the reuse ladder
• See how packages organize multiple modules
• Know when to use each level in real projects

---

The reuse ladder

A package solves a bigger problem than a module, and a module solves a bigger problem than a function.

Summary table

Level	What is it?	Reuse scope
Function	A reusable block of code	Same file
Module	One .py file	Same folder
Package	Folder containing modules	Across projects

---

Level 1: Function

A function is useful when everything lives in one Python file.

def sumop(a, b):
    return a + b

print(sumop(10, 20))

Best for: small programs and reusable logic within a single script.

You cannot automatically use sumop from another .py file unless you copy it or move it into a module.

---

Level 2: Module

A module is simply a Python file.

arithops.py

def sumop(a, b):
    return a + b

def mulop(a, b):
    return a * b

main.py

import arithops

print(arithops.sumop(10, 20))
print(arithops.mulop(4, 5))

The important rule: both files are in the same folder. Move main.py to another directory and the import breaks.

---

Level 3: Package

A package is a folder that contains multiple modules.

Package structure

bankkit/
│
├── __init__.py
├── bankinfo.py
├── arithops.py
└── greetmod.py

Once Python knows where bankkit/ lives, any project can use:

import bankkit.bankinfo

print(bankkit.bankinfo.BNAME)

The package name becomes the top-level namespace — that is the jump from "same folder only" to cross-project reuse.

---

How they compare

Feature	Function	Module	Package
Physical unit	Function	.py file	Folder
Contains	Code	Variables, functions, classes	Multiple modules
Import needed	No	Yes	Yes
Reuse	One file	Same project folder	Multiple projects

---

Real-world example

Imagine you are building a banking application.

Module	Responsibility
bankinfo.py	Customer and bank details
transactions.py	Deposit and withdraw
interest.py	Interest calculations

The bankkit package groups related modules into one reusable library — the same pattern professional teams use for APIs, data pipelines, and ML toolkits.

---

Interview memory card

Question	Answer
What is a function?	Reusable code inside one file
What is a module?	A Python .py file
What is a package?	A folder containing multiple modules
Why use packages?	Organize and reuse code across projects

---

Key takeaways

• Function = reuse within one file
• Module = reuse across files in the same folder
• Package = organize multiple modules into a reusable library
• Packages are the foundation of professional Python project structure
• Next: Creating Python Packages — build bankkit/ step by step