The Standard Library

Introduction

Python is famous for being "batteries included". Installed with the language is a large standard library: ready-made modules for maths, dates, random numbers, files, the web and much more. Before you write something yourself, check whether it is already there.

Importing

A module is a file of Python code. Use import to bring one in.

You can import specific names, or give a module a shorter name:

math and statistics

random

random makes pseudo-random numbers. Setting a seed makes the sequence repeatable, which is useful for tests.

Run it twice and you get the same answers, because of the seed. Remove the random.seed(7) line and each run will differ. random is not safe for passwords. For those, use the secrets module.

datetime

strftime formats a date as text, and strptime parses text into a date. date.today() and datetime.now() give the current time, but their output changes each time you run them.

collections: smarter containers

Counter counts things for you:

defaultdict creates missing entries automatically, which avoids get checks:

deque is a list-like queue that is fast at both ends:

itertools

Finding more

The official documentation lists every module. Other favourites: json and csv for data, pathlib for files, re for pattern matching, argparse for command-line programs, unittest for tests, and urllib for the web. Beyond the standard library, the Python Package Index (PyPI) has hundreds of thousands of third-party packages you can install with pip.

Try it yourself

Use Counter to find the three most common letters in "the quick brown fox jumps over the lazy dog", ignoring spaces. Then use datetime to print how many days there are from 2025-01-01 to 2025-12-31.

Summary

• import module, from module import name and import module as short bring code in.
• math, statistics, random and datetime cover numbers, chance and dates. A seed makes random repeatable.
• Counter, defaultdict and deque are handy containers, and itertools builds combinations and chains.
• Check the standard library, and then PyPI, before writing something yourself.