The Standard Library

Python ships with hundreds of modules for everyday jobs: dates, randomness, maths, statistics, files, data formats, even talking to the internet. This collection is the standard library, and it is why Python is often described as "batteries included". Most scripts need nothing installed at all, as long as you know how to find what is already there.

Think of moving into a furnished flat. Before buying a kettle, you open the cupboards. The standard library is the cupboards, and a quick look through them saves both money and the risk of bringing home something you did not need.

What Ships with Python

You have already used several standard modules: decimal for money, csv for spreadsheet files, json for data, pathlib for paths. A few more come up again and again. random makes random choices. statistics does averages, medians and spreads. math has square roots, rounding up and down, and constants like pi. datetime handles dates and times, and has the next topic to itself.

A Few in Action

The program below uses three of them. statistics.median gives the middle value of a week of temperatures, which, unlike the average, is not dragged up by one unusually hot day. math.ceil rounds up, which is what you want when you need 20 guests at tables of 6: three tables are not enough, so the answer is four.

import math
import statistics

temps = [18, 22, 25, 19, 30, 27, 21]
print("Median temperature:", statistics.median(temps))
print("Mean temperature:", round(statistics.mean(temps), 1))

guests = 20
seats_per_table = 6
print("Tables needed:", math.ceil(guests / seats_per_table))

Median temperature: 22
Mean temperature: 23.1
Tables needed: 4

random.choice picks one item from a list, and random.shuffle puts a list in a random order. Because the result is random, your output will differ from the one below, which is exactly the point.

import random

names = ["Anna", "Omar", "Lena", "Sam"]
print("This week's winner:", random.choice(names))

This week's winner: Lena

One note of care: random is fine for games, draws and tests, and wrong for anything secret, such as passwords or security codes. For those, the standard library has a separate module, secrets. This book does not need it, but you should know it exists.

Finding the Rest

The official documentation at docs.python.org lists every standard module in a page called The Python Standard Library. Each module has its own page, and every one starts the same way: a short summary of what the module is for, followed by its functions, each with a description and usually an example. Reading one of these pages is a skill worth practising. It outlasts any course, including this one.

Before You Install Anything

When you need a new capability, search the standard library first. If it is there, you get code that is maintained alongside Python itself, documented in one place, and installed on every computer that has Python. The next two topics are about installing other people's packages, and that is the right move only when the standard library genuinely does not cover the job.

You need to	Module	Example
count money exactly	decimal	Decimal("4.20")
read a spreadsheet export	csv	csv.DictReader(f)
save and load structured data	json	json.dump(data, f)
work with paths	pathlib	Path("data") / "notes.txt"
find a median or mean	statistics	statistics.median(values)
pick at random	random	random.choice(names)
work with dates	datetime	date.today()