Week 3 — Modules & Packages

What you'll learn

• 1Import standard library modules and third-party packages
• 2Create your own module (.py file) and import it
• 3Organise a package with __init__.py
• 4Control public API with __all__
• 5Install and use third-party packages with pip

1. Importing Modules

import math
print(math.sqrt(16))     # 4.0
print(math.pi)           # 3.14159...

from math import sqrt, pi
print(sqrt(25))          # 5.0

import math as m         # alias
print(m.factorial(5))    # 120

2. Creating a Module

Any .py file is a module. Create utils.py and import from it.

# utils.py
def greet(name):
    return f"Hello, {name}!"

PI = 3.14159

# main.py
from utils import greet, PI
print(greet("Alice"))
print(PI)

3. Packages

A package is a directory with an __init__.py file. It groups related modules.

mypackage/
  __init__.py      # makes it a package
  geometry.py      # module with shapes
  statistics.py    # module with stats

from mypackage import geometry
from mypackage.statistics import mean

4. pip

pip install requests          # install
pip install requests==2.31.0  # specific version
pip uninstall requests        # remove
pip list                      # show installed
pip freeze > requirements.txt # export for sharing
pip install -r requirements.txt  # install from file

5. Common Mistakes

1. Circular imports: A imports B, B imports A. Restructure to break the cycle.
2. Naming your file the same as a standard library module (e.g., random.py) shadows it.
3. from module import * pollutes the namespace. Use explicit imports or __all__ in the module.

💻 Examples

Run these examples and check the output yourself.

"""Geometry helpers."""
import math

__all__ = ["circle_area", "rect_area", "distance"]

def circle_area(r):
    return math.pi * r ** 2

def rect_area(w, h):
    return w * h

def distance(p1, p2):
    return math.sqrt((p2[0]-p1[0])**2 + (p2[1]-p1[1])**2)

from geometry import circle_area, rect_area, distance

print(f"Circle r=5:  {circle_area(5):.2f}")
print(f"Rect 4×6:    {rect_area(4, 6)}")
print(f"Distance:    {distance((0,0),(3,4)):.2f}")

Circle r=5:  78.54
Rect 4×6:    24
Distance:    5.00

📝 Exercises

Try them yourself first, then open the solution to compare.

# math_utils/__init__.py
from .algebra import quadratic
from .stats import mean, median, mode

# math_utils/algebra.py
import cmath
def quadratic(a, b, c):
    d = cmath.sqrt(b**2 - 4*a*c)
    return (-b+d)/(2*a), (-b-d)/(2*a)

# math_utils/stats.py
from statistics import mean, median, mode