Python Modules & pip

A module is simply a .py file whose code you can reuse with import. Python ships with hundreds of modules (the standard library), and pip installs thousands more from PyPI. This lesson shows how to use both, and how to organise your own code into modules.

Import a module

import mathprint(math.sqrt(81)) from math import pi, floorprint(round(pi, 4), floor(7.9)) import statistics as statsprint(stats.mean([4, 8, 15, 16, 23, 42]))

9.0
3.1416 7
18

Explore a module

import randomnames = [n for n in dir(random) if not n.startswith("_")]print(len(names) > 20)print("choice" in names, "shuffle" in names)print(random.choice.__doc__.splitlines()[0])

True
True True
Choose a random element from a non-empty sequence.

Create your own module

Any .py file is a module. Save this as pricing.py:

TAX_RATE = 0.18 def with_tax(amount):    return round(amount * (1 + TAX_RATE), 2) def discount(amount, percent):    return round(amount * (1 - percent / 100), 2)

Then import it from another file in the same folder:

import pricingfrom pricing import discount print(pricing.with_tax(1000))print(discount(1000, 15))print(pricing.TAX_RATE)

1180.0
850.0
0.18

if __name__ == “__main__”

When a file is run directly, its __name__ is "__main__". When it is imported, __name__ is the module’s name. This lets a file be both a reusable module and a runnable script:

def greet(name):    return f"Hello, {name}" print("__name__ is", __name__) if __name__ == "__main__":    print(greet("from the command line"))

__name__ is __main__
Hello, from the command line

import greeterprint(greeter.greet("from app.py"))

__name__ is greeter
Hello, from app.py

When app.py imports greeter, the top-level print runs with __name__ set to greeter, but the code under the if does not.

Packages

A package is a folder of modules. Import modules from it with dots:

myshop/    __init__.py    pricing.py    cart.pymain.py          ->  from myshop.pricing import with_tax

The standard library

Install packages with pip

pip downloads packages from PyPI. Always run it through the Python you are using: python -m pip install requests.

$ python -m pip --versionpip 26.2.2 from C:\Python3\Lib\site-packages\pip (python 3.15)

Use a virtual environment

A virtual environment is a private folder of packages for one project, so different projects can use different versions. Create one and check that its Python is used:

$ python -m venv .venv$ .venv\Scripts\python -c "import sys; print(sys.prefix.endswith('.venv'))"True

Activate it so python and pip use it automatically:

• Windows: .venv\Scripts\activate
• macOS and Linux: source .venv/bin/activate

Then install and record your packages:

• pip install requests pytest
• pip freeze > requirements.txt saves the exact versions
• pip install -r requirements.txt recreates them on another machine

Check your understanding

Program Practice

Python modules FAQ