The Standard Library

Big idea

Python comes with batteries included: a standard library of modules for maths, randomness, dates, counting, and much more. Before writing something yourself, check whether it's already there.

math

import math
math.sqrt(2)        # 1.4142135623730951
math.gcd(12, 18)    # 6 — greatest common divisor
math.isclose(0.1 + 0.2, 0.3)   # True — compare floats safely

random

import random

print(random.randint(1, 6))
print(random.choice(["rock", "paper", "scissors"]))
cards = ["A", "K", "Q", "J"]
random.shuffle(cards)
print(cards)

• randint(a, b) — a whole number from a to b, both included.
• choice(items) — one random item.
• shuffle(items) — mixes a list in place.
• random.seed(42) — makes the "random" sequence repeat exactly; handy for testing.

datetime

from datetime import date, timedelta

launch = date(2026, 10, 3)
print(launch)
print(launch.strftime("%d %B %Y"))
print(launch + timedelta(days=30))
print((date(2026, 12, 25) - launch).days, "days to go")

output
2026-10-03
03 October 2026
2026-11-02
83 days to go

date.today() and datetime.now() give the current date and time. Subtracting dates gives a timedelta, a length of time.

collections

Two ready-made containers you'll use constantly:

from collections import Counter, defaultdict

votes = ["tea", "coffee", "tea", "juice", "tea", "coffee"]
tally = Counter(votes)
print(tally.most_common(2))
print(tally["juice"], tally["water"])

by_letter = defaultdict(list)
for word in ["apple", "avocado", "banana"]:
    by_letter[word[0]].append(word)
print(dict(by_letter))

[('tea', 3), ('coffee', 2)]
1 0
{'a': ['apple', 'avocado'], 'b': ['banana']}

• Counter — counts things for you; a missing key counts as 0 instead of raising KeyError.
• defaultdict(list) — a dict that creates an empty list the first time you use a new key.

itertools

Tools for looping in clever ways:

from itertools import combinations, permutations, product

print(list(combinations("ABC", 2)))
print(len(list(permutations("ABC"))))
print(list(product([0, 1], repeat=2)))

output
[('A', 'B'), ('A', 'C'), ('B', 'C')]
6
[(0, 0), (0, 1), (1, 0), (1, 1)]

• combinations(items, k) — every group of k, order ignored.
• permutations(items) — every ordering.
• product(a, b) — every pairing, like nested loops.

from collections import Counter

counts = Counter("mississippi")
print(counts.most_common(1)[0][1])

output
i

most_common(1) is a list holding one (letter, count) pair. [0] takes the pair; [1] took the count. The letter is at [0][0] — but careful: i and s both appear 4 times, and ties keep the order they were first seen, so it's i.

A few more worth knowing

• os and pathlib — files and folders.
• json — read and write JSON data (you'll use it in the project).
• statistics — mean, median, standard deviation.
• time — time.sleep(1) pauses for a second.

Watch it

Think of it like

Moving into a furnished flat. The kitchen already has pans, the drawers have cutlery. You could forge your own spoon — but why would you?

Key terms

• Standard library — The modules that come with every Python install.
• Counter — A dict subclass that counts items.
• timedelta — A length of time, from subtracting dates or for adding to one.