Python — Modules

A module is just a .py file

Split code into files and pull in what you need:

# math_utils.py
def add(a, b):
    return a + b

PI = 3.14159

# app.py
import math_utils

math_utils.add(2, 3)      # 5
print(math_utils.PI)

The four import styles

import math                       # use math.sqrt(9)
import math as m                  # alias: m.sqrt(9)
from math import sqrt             # call sqrt(9) directly
from math import sqrt, pi         # multiple names
from math import *                # everything (avoid — pollutes namespace!)

Prefer import module or explicit from … import name. Star imports make "where did this come from?" unanswerable.

Your own package = a folder of modules

myapp/
├── main.py
└── shop/
    ├── __init__.py        ← marks it a package (may be empty)
    ├── cart.py
    └── pricing.py

from shop.cart import add_item
from shop import pricing

Dots walk folders. __init__.py can also re-export for shorter imports.

__name__ == "__main__" — dual-purpose files

A file that's both importable AND runnable:

# converter.py
def c_to_f(c): return c * 9 / 5 + 32

if __name__ == "__main__":
    # runs ONLY when executed directly: python converter.py
    print(c_to_f(float(input("°C? "))))

When another file imports converter, the block is skipped — no surprise prompts on import.

Standard library — batteries included

Module	Superpower
random	randint, choice, shuffle
datetime	dates, times, deltas
json	parse/emit JSON
os / pathlib	files, paths
math	floor, ceil, sqrt, pi
collections	Counter, defaultdict, deque
itertools	iteration superpowers

import random
random.randint(1, 6)          # dice
random.choice(["a", "b"])

from collections import Counter
Counter("mississippi").most_common(2)   # [('s',4),('i',4)]

Third-party modules via pip

pip install requests pandas

import requests            # now available everywhere in your project

Best practice: virtual environments keep each project's packages isolated (python -m venv .venv). Full PIP lesson coming.

Gotchas

1. Don't name files after stdlib modules — random.py shadows the real one and breaks import random
2. Circular imports (a imports b imports a) crash — extract shared code to a third module
3. Imports run module code ONCE per session; re-importing reuses the cached module

Mini Practice

1. Create greet.py with a function; import it from app.py.
2. Try all four import styles on one module.
3. Build a two-module package folder with init.py.
4. Add the if name guard; prove both behaviors.
5. Counter-ify a paragraph with collections.Counter.

Next: dates →