Modules and Packages

As your programs grow, splitting code into separate files keeps things organised. Python's module system lets you do exactly that, and the wider Python ecosystem of packages gives you access to thousands of pre-built libraries.

What is a Module?

A module is simply a Python file. Any .py file is a module you can import into another file.

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

def square(x):
    return x * x

PI = 3.14159

# main.py
import math_utils

print(math_utils.add(3, 4))      # 7
print(math_utils.square(5))      # 25
print(math_utils.PI)             # 3.14159

Import Styles

# Import whole module
import math
print(math.sqrt(16))   # 4.0

# Import specific names
from math import sqrt, pi
print(sqrt(25))        # 5.0
print(pi)              # 3.141592653589793

# Import with an alias
import math as m
print(m.floor(3.7))    # 3

# Import everything (not recommended — pollutes namespace)
from math import *

The Standard Library

Python ships with a rich standard library — no installation needed:

import os           # operating system interfaces
import sys          # system-specific parameters
import math         # mathematical functions
import random       # random number generation
import datetime     # date and time handling
import json         # JSON encoding and decoding
import re           # regular expressions
import collections  # specialised container types

Example: math and random

import math
import random

print(math.ceil(4.2))    # 5
print(math.floor(4.9))   # 4
print(math.pow(2, 8))    # 256.0

print(random.randint(1, 6))          # random die roll
print(random.choice(["a", "b", "c"])) # random element

What is a Package?

A package is a directory containing multiple modules and an init.py file. Packages let you organise related modules into a hierarchy:

myapp/
    __init__.py
    utils/
        __init__.py
        string_helpers.py
        math_helpers.py
    models/
        __init__.py
        user.py

from myapp.utils.string_helpers import capitalise
from myapp.models.user import User

Installing Third-Party Packages

Python's package manager pip installs packages from the Python Package Index (PyPI):

pip install requests
pip install numpy
pip install pandas

Then import and use them:

import requests

response = requests.get("https://api.example.com/data")
print(response.status_code)    # 200
print(response.json())         # parsed JSON

The if name == "main" Guard

def greet(name):
    return f"Hello, {name}!"

if __name__ == "__main__":
    # This runs only when the file is executed directly,
    # not when it is imported as a module
    print(greet("World"))