Python's slogan is "batteries included." Before installing anything, check whether the standard library already does the job. Here are five modules worth knowing today.

math

import math

print(math.sqrt(16))
print(math.floor(3.7), math.ceil(3.2))
print(round(math.pi, 4))
print(math.pow(2, 10))

math.floor rounds down, math.ceil rounds up, and round() is a built-in that rounds to nearest. Also useful: math.inf, math.factorial(n), math.gcd(a, b).

random

The module the Module 2 guessing game wanted:

import random

random.seed(42)

print(random.randint(1, 10))
print(random.choice(["rock", "paper", "scissors"]))

deck = [1, 2, 3, 4, 5]
random.shuffle(deck)
print(deck)
print(random.sample(range(1, 50), 3))

random.seed(42) makes the sequence reproducible — the same seed always gives the same "random" numbers, which is exactly what you want in a test. Leave the seed out for genuinely varying results.

randint(a, b) includes both ends, unlike range().

random is fine for games and sampling, but not for security. Passwords and tokens need the secrets module instead.

datetime

import datetime

d = datetime.date(2026, 1, 15)
print(d)
print(d.year, d.month, d.day)

later = d + datetime.timedelta(days=30)
print(later)
print((later - d).days)

date is a day, datetime a day plus time, and timedelta a duration you can add or subtract. Subtracting two dates gives a timedelta. Module 9 covers this properly.

Formatting uses strftime:

import datetime

d = datetime.date(2026, 1, 15)
print(d.strftime("%d/%m/%Y"))
print(d.strftime("%B %d, %Y"))

os

Interacting with the operating system, mostly paths and files:

import os

os.makedirs("demo/sub", exist_ok=True)
with open("demo/sub/file.txt", "w") as f:
    f.write("hi")

print(os.path.exists("demo/sub/file.txt"))
print(os.path.basename("demo/sub/file.txt"))
print(os.path.splitext("report.csv"))
print(sorted(os.listdir("demo")))

os.path.join("a", "b") builds paths with the right separator for the platform — always prefer it over "a" + "/" + "b".

os.environ reads environment variables, the standard way to pass configuration and secrets to a program:

import os

print(os.environ.get("MY_SETTING", "not set"))

Use .get() with a default so a missing variable doesn't crash you.

Modern code often uses pathlib instead, which is more object-oriented:

from pathlib import Path

p = Path("demo") / "sub" / "file.txt"
print(p.name)
print(p.suffix)
print(p.parent.name)

The / operator joins paths. Both are fine; pathlib reads better for path-heavy work.

sys

Information about the interpreter itself:

import sys

print(sys.version_info.major)
print(isinstance(sys.path, list))
print(sys.platform.startswith(("linux", "darwin", "win", "emscripten")))

The two you'll actually use are sys.argv — command-line arguments, which you'll need for CLI tools — and sys.exit(1) to stop with an error code.

Others worth a look

| Module | For | | --- | --- | | collections | Counter, defaultdict, deque | | itertools | combinations, permutations, grouping | | re | regular expressions (Module 9) | | unittest | testing (Module 9) | | statistics | mean, median, stdev | | textwrap | wrapping and indenting text |

collections.Counter deserves a mention — it replaces the counting loop you wrote in Module 3:

from collections import Counter

counts = Counter("banana")
print(counts["a"])
print(counts.most_common(2))

Try it yourself

Checkpoint

• Check the standard library before installing a package.
• math for numbers, random for chance (seed it for reproducibility).
• datetime for dates and timedelta for durations.
• os for paths, files, and environment variables; pathlib is the modern alternative.
• sys for interpreter and command-line details; collections for better containers.