Modules and packages

What you'll learn

How to import modules, use the standard library, split your own code into files and understand packages and __name__.

Why it matters

Python's strength is its huge amount of ready-made code. Modules let you use it, and also let you keep your own project organised in files that each do one job.

Explanation

A module is simply a .py file. You load it with import:

Variations:

• from math import sqrt, pi: import specific names.
• import datetime as dt: give a module a shorter name.

The standard library comes with Python, so there is nothing to install: math, random, json, datetime, os, pathlib, collections and many more.

Your own module works the same way. If you save helpers.py next to your main file, then import helpers works.

A package is a folder of modules (with an __init__.py file). Third-party packages are installed with pip.

if __name__ == "__main__": runs code only when the file is run directly, not when it is imported. Use it for demo code at the bottom of a module.

Example

Step-by-step walkthrough

1. math.sqrt(16) is 4.0 and round(math.pi, 4) is 3.1416.
2. json.dumps turns a dictionary into JSON text; json.loads turns it back. The first score is 90.
3. date(2025, 3, 14).strftime("%d %b %Y") formats the date: 14 Mar 2025.
4. To show a module of our own, the code writes helpers.py to the file system and then imports it. Because the file is brand new, we add the current folder to sys.path and refresh Python's import cache first. helpers.shout("hello") prints HELLO!.
5. In a script run directly, __name__ is "__main__", which is why the guard works.

Try it yourself

Use the random module to pick a random item from a list. (The result changes each run.)

ModulesPython

Editable codeimport math import json print(math.sqrt(16), round(math.pi, 4)) data = {"name": "Asha", "scores": [90, 85]} text = json.dumps(data) print(text) print(json.loads(text)["scores"][0])

Run ResetCopyCtrl+Enter to run

Common mistakes

• Naming your file like a standard module, such as random.py, which then hides the real one.
• Circular imports, where two files import each other.
• from module import *, which pollutes your names. Import what you need.
• Running code at import time instead of guarding it with __name__.

Quick quiz

Summary

• import brings in modules; the standard library is built in.
• Your own .py files are modules; folders with __init__.py are packages.
• Guard demo code with if __name__ == "__main__":.

Next: keeping each project's packages separate.

import math
print(math.sqrt(16))

import math
import json
from datetime import date

print(math.sqrt(16), round(math.pi, 4))

data = {"name": "Asha", "scores": [90, 85]}
text = json.dumps(data)
print(text)
print(json.loads(text)["scores"][0])

print(date(2025, 3, 14).strftime("%d %b %Y"))

# Write our own module to a file, then import it
with open("helpers.py", "w") as f:
    f.write("def shout(text):\n    return text.upper() + '!'\n")

import sys, importlib
sys.path.append(".")          # make sure the current folder is searched
importlib.invalidate_caches() # notice the file we just created
import helpers
print(helpers.shout("hello"))
print(__name__)