Python comes with a huge standard library — modules that ship with every Python install. Knowing what’s there saves you from reinventing things and from reaching for a third-party package when you don’t need one.

This lesson is a quick tour. Section 14 covers the most useful modules in depth.

Maths and numbers

  • math — everyday maths: sqrt, log, cos, pi, floor, ceil.
  • statistics — basic stats: mean, median, stdev, variance.
  • decimal — exact decimal arithmetic for money.
  • fractions — exact fractions like 1/3.

Randomness

  • random — random numbers, choices, shuffles.
  • secrets — cryptographically safe random values (for tokens, passwords).

Dates and times

  • datetime — dates, times, durations.
  • time — lower-level clock and sleep functions.
  • calendar — calendar utilities.
  • zoneinfo — timezone-aware times.

Collections and iteration

  • collections — Counter, defaultdict, deque, OrderedDict, namedtuple.
  • itertools — chain, groupby, combinations, product, islice.
  • functools — partial, reduce, lru_cache, cache.

Files and the filesystem

  • pathlib — modern path handling (covered in Section 10).
  • os — OS-level operations: env vars, processes, low-level file ops.
  • shutil — copying, moving, deleting trees of files.
  • tempfile — temporary files and directories.
  • glob — finding files by pattern (also via pathlib.Path.glob).

Data formats

  • json — JSON (covered in Section 10).
  • csv — CSV (covered in Section 10).
  • tomllib — TOML reading (Python 3.11+).
  • xml.etree.ElementTree — XML parsing.

System and process

  • sys — interpreter info, command-line arguments, exit.
  • argparse — building command-line interfaces.
  • subprocess — running other programs.
  • logging — proper logging (covered in Section 15).

Networking

  • urllib — basic HTTP requests.
  • http — lower-level HTTP tools.
  • socket — raw socket programming.

For real HTTP work, almost everyone uses the third-party requests library or the modern httpx. urllib is fine for quick tasks.

Concurrency

  • threading — threads.
  • multiprocessing — processes.
  • asyncio — async/await for I/O concurrency.
  • concurrent.futures — high-level “run these in parallel” interface.

We won’t cover concurrency in this course — it’s a substantial topic for a follow-up.

Text and strings

  • re — regular expressions.
  • string — string constants (ascii_letters, digits, punctuation), templates.
  • textwrap — wrap and dedent text.

Debugging and testing

  • unittest — built-in testing framework.
  • pdb — Python debugger (covered in Section 15).
  • pprint — pretty-print complex data structures.
  • traceback — work with tracebacks programmatically.

For testing, most modern projects use the third-party pytest instead of unittest — but unittest is always available.

Miscellany

  • enum — enumeration types.
  • typing — type-hinting helpers (we’ll see more in Section 13).
  • dataclasses — boilerplate-free classes (covered in Section 12).
  • hashlib — hashing (SHA, MD5).
  • uuid — UUIDs.
  • gzip, zipfile, tarfile — compressed files.

How to discover more

When you find yourself thinking “I bet Python has a built-in for this”, check first:

  • Search the Python docs.
  • Type help(module_name) in the REPL.
  • Skim the standard library index — even a quick scroll teaches you what’s available.

A surprising amount of common work — parsing dates, counting words, downloading files, walking directories — has a clean built-in answer.

What’s next

Final lesson of the section — running modules as scripts, the right way.

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