Files, Errors and Modules · Working at Scale · lesson 6 of 8

A standard library tour

about 15 minutes · free · runs in your browser

Step 1 of 2

Batteries included

Python ships with a large library. Four modules earn their place in your head:

math — sqrt, floor, ceil, pi, inf.

random — randint(a, b) inclusive at both ends, choice(seq), shuffle(list), sample(seq, k). Call random.seed(n) first and you get the same "random" numbers every time, which is how you test code that uses it.

datetime — date, datetime and timedelta:

from datetime import date, timedelta
launch = date(2026, 1, 15)
launch + timedelta(days=30)      # date(2026, 2, 14)
(other - launch).days            # difference in days

collections — Counter counts things in one line, and defaultdict removes the "is this key there yet?" dance:

from collections import Counter
Counter("mississippi").most_common(2)   # [('i', 4), ('s', 4)]

Your turn: use Counter for the two most common words, and timedelta to find the date 45 days after 1 March 2026.

You start from this, and edit it in the browser:

from collections import Counter
from datetime import date, timedelta

words = ["red", "blue", "red", "green", "blue", "red"]

# Set top_two and deadline.

Step 2 of 2

Deterministic randomness

Code that uses random is untestable until you seed it. random.seed(n) makes the sequence repeatable, so the same seed always produces the same draws.

Your turn: write roll_dice(seed, count) that seeds the generator and returns count dice rolls, each from 1 to 6.

You start from this, and edit it in the browser:

import random


def roll_dice(seed, count):
    # Seed, then return count rolls of a six-sided die.
    pass