15-modules

15 · Modules & the Standard Library

Part 3 — Functions & Modules · Estimated time: 25–35 min · Prerequisite: 13 · Functions

A module is just a Python file full of useful code you can reuse. Python ships with a huge standard library of them (maths, random numbers, dates, and more), and you can write your own. Importing modules means you rarely have to build things from scratch.

What you'll learn

• Importing modules with import
• The handy import ... as alias and from ... import name styles
• A taste of the standard library: math and random
• Writing and importing your own module
• The if __name__ == "__main__" pattern

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1. Importing from the standard library

import math

print(math.sqrt(16))   # 4.0
print(math.pi)         # 3.141592653589793
print(math.floor(3.7)) # 3
print(math.ceil(3.2))  # 4

Once imported, you reach inside the module with a dot: math.sqrt.

▶️ Run it: python examples/01_import_stdlib.py

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2. Three ways to import

import random            # the whole module
import math as m         # give it a shorter alias
from math import sqrt, pi  # pull specific names in directly

print(m.floor(2.9))   # 2
print(sqrt(25))       # 5.0   (no "math." needed — we imported it directly)
print(pi)             # 3.141592653589793

Use an alias for long names; use from ... import when you only need a couple of things and want to type them without the prefix.

▶️ Run it: python examples/02_import_styles.py

💡 Try it yourself: import random, then random.seed(0) and print random.randint(1, 6). (Seeding makes the "random" result repeat, which is handy for testing.)

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3. Writing your own module

A module is just a .py file. The file examples/greetings.py defines two functions:

# greetings.py
def hello():
    return "Hello!"

def hey(name):
    return f"Hey {name}!"

Another file imports it by filename (without the .py):

import greetings

print(greetings.hello())     # Hello!
print(greetings.hey("Sam"))  # Hey Sam!

▶️ Run it (from the examples/ folder): python examples/03_use_custom_module.py

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4. if __name__ == "__main__"

When you run a file directly, Python sets its __name__ to "__main__". When the file is imported, __name__ is the module's name instead. This lets a file include a demo that runs only when launched directly — not when imported:

def hello():
    return "Hello!"

if __name__ == "__main__":
    print(hello())   # runs only via `python greetings.py`, not when imported

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Common mistakes

Mistake	What happens	Fix
import Math	ModuleNotFoundError	Module names are lowercase: import math.
sqrt(9) without importing it	NameError	from math import sqrt first, or use math.sqrt.
Naming your file random.py	it shadows the real module	Don't name files after standard modules.
Expecting a module's demo to run on import	the __main__ block is skipped	That's the point — guard demos with if __name__ == "__main__".

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Recap / cheat-sheet

import math               # math.sqrt(16)
import math as m          # m.sqrt(16)
from math import sqrt     # sqrt(16)
from math import *        # import everything (use sparingly)

# your own module is just a .py file:
import greetings          # uses greetings.py in the same folder
greetings.hello()

if __name__ == "__main__":
    ...                   # runs only when this file is run directly

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_square_root.py — use math.sqrt.
2. exercises/02_circle_area.py — use math.pi.
3. exercises/03_from_import.py — import a single name with from.

Try each yourself before opening the solution.

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Next → 16 · Iterators & Generators (coming soon)