Applied PythonLesson 18 of 1913 min

The Standard Library

A tour of the batteries included: math, random, datetime, json, and collections.

By the end you will be able to

Reach for the standard library before writing it yourself
Work with dates, JSON, and random values
Use Counter and defaultdict to simplify tallying code

Python ships with a large standard library — no installation required. Knowing roughly what is in it is one of the highest-leverage things a beginner can learn, because the alternative is reinventing it badly.

Python

Output

9.0 2 3
3.1416 120
18
15.5
— expected output; press Run to execute it yourself

random produces different values on every run, so there is no fixed output to compare against — press Run a few times and watch it change. Calling random.seed(n) first makes a run reproducible, which is what you want in a test.

Python

Output

Press Run to execute this code.

Dates and times

Python

Output

2026-08-09
Sunday, 09 August 2026
2026 8 9
2026-09-23
45 days apart
2026-12-25
— expected output; press Run to execute it yourself

JSON

JSON is how programs exchange structured data. json.dumps turns Python objects into a JSON string; json.loads turns one back.

Python

Output

{
  "name": "Ada",
  "skills": [
    "Python",
    "maths"
  ],
  "active": true,
  "score": null
}
Python <class 'dict'>
— expected output; press Run to execute it yourself

collections

Counter and defaultdict replace whole blocks of hand-written tallying code.

Python

Output

Counter({'the': 3, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1, 'end': 1})
[('the', 3), ('cat', 1)]
3 0
{'t': ['the', 'the', 'the'], 'c': ['cat'], 's': ['sat'], 'o': ['on'], 'm': ['mat'], 'e': ['end']}
— expected output; press Run to execute it yourself

Exercise

Using collections.Counter, write most_common_word(text) that returns the single most frequent word in a sentence, lowercased.

Python

Output

Press Run to execute this code.

Check your understanding

0/2 answered

1.What does json.loads(text) do?
2.What does Counter(['a','b','a'])['z'] return?