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Functions & Modules

Modules, Packages & the Standard Library

Organise code into modules, import in different ways, use if name == "main", install packages with pip, and tour the standard library: math, random, datetime, os, collections and more.

3 min read3 runnable examples4-question quiz

A module is simply a .py file. Any Python file can be imported by another, giving you its functions, classes and variables.

Pythongeometry.py
PI = 3.14159

def circle_area(r):
    return PI * r * r

def rectangle_area(l, w):
    return l * w

if __name__ == "__main__":
    print("This runs only when geometry.py is executed directly")
Python
import geometry
from geometry import rectangle_area
import geometry as geo

print(geometry.circle_area(2))
print(rectangle_area(3, 4))
print(geo.PI)
Output
12.56636
12
3.14159

(The demo message in geometry.py did not print, because the module was imported, not run.)

Ways to import

Form Use as
import math math.sqrt(2)
import numpy as np np.array(...) — an alias
from math import sqrt, pi sqrt(2), pi
from math import * everything — avoid: unclear where names come from

if name == "main"

Every module has a __name__. When a file is run directly, its __name__ is "__main__"; when it is imported, __name__ is the module's name. Putting test or demo code under this check lets a file work both as a script and as a library.

Packages and pip

A package is a folder of modules (with an __init__.py). Third-party packages are installed from PyPI with pip:

Terminal
pip install requests          # install a package
pip install numpy pandas      # several at once
pip list                      # what is installed
python3 -m venv .venv         # create an isolated virtual environment

Tour of the standard library

Python
import math, random, datetime, statistics

print(math.sqrt(144), math.factorial(10), math.gcd(84, 36), math.comb(5, 2))
print(round(math.pi, 4), math.ceil(4.1), math.floor(-4.1), math.log2(1024))

random.seed(7)                          # same "random" numbers on every run
print(random.randint(1, 6), random.choice(["heads", "tails"]))
deck = list(range(1, 11))
random.shuffle(deck)
print(deck, random.sample(range(100), 3))

print(statistics.mean([70, 85, 90]), statistics.median([3, 1, 4, 1, 5]))

today = datetime.date(2026, 11, 14)
exam = datetime.date(2026, 12, 10)
print((exam - today).days, "days to the exam")
print(today.strftime("%d %B %Y, %A"))
Output · yours may differ
12.0 3628800 12 10
3.1416 5 -5 10.0
3 heads
[8, 6, 4, 3, 9, 5, 10, 2, 1, 7] [64, 27, 4]
81.66666666666667 3
26 days to the exam
14 November 2026, Saturday
Python
from collections import Counter, deque, defaultdict, OrderedDict
import itertools, string

print(Counter("abracadabra").most_common(3))
print(list(itertools.permutations([1, 2, 3], 2)))
print(list(itertools.combinations("ABCD", 2)))
print(list(itertools.accumulate([1, 2, 3, 4])))      # running totals
print(string.ascii_uppercase[:5], string.digits)
Output
[('a', 5), ('b', 2), ('r', 2)]
[(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]
[('A', 'B'), ('A', 'C'), ('A', 'D'), ('B', 'C'), ('B', 'D'), ('C', 'D')]
[1, 3, 6, 10]
ABCDE 0123456789
Module Useful for
math maths functions and constants
random random numbers, shuffling, sampling
datetime, time dates, times, timing code
os, pathlib, shutil files, folders, paths
sys command-line arguments, interpreter settings
collections Counter, deque, defaultdict, namedtuple
itertools permutations, combinations, infinite iterators
json, csv reading and writing data formats
re regular expressions
statistics mean, median, mode, standard deviation

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    What is __name__ when a module is imported?

  2. 2

    After from math import sqrt, how do you call it?

  3. 3

    Which command installs a third-party package?

  4. 4

    Which module gives you Counter and deque?

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