Unit 11: Patterns & Data

match, JSON and finding text.

Unit 11 of 31 in Python for kids. Its 5 lessons are Matching Cases, Saving Data as Text, Finding Patterns, Checking and Changing and Pattern Master — below is everything each one explains, and a question or two from it to try.

Every sample on this page was run through real Python before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

🎛️ Matching Cases

A tidier if/elif chain

When you are checking one value against lots of possibilities, match reads better. Each case is one possibility.

Python

colour = "red"

match colour:
    case "red":
        print("Stop")
    case "green":
        print("Go")

It prints

Stop

The catch-all case

case _ means "anything else". It goes last, and it is how you stop your program silently doing nothing.

Python

animal = "fox"

match animal:
    case "cat":
        print("Meow")
    case _:
        print("I do not know that one")

It prints

I do not know that one

Several values in one case

A | between values means "any of these".

Python

day = "Sunday"

match day:
    case "Saturday" | "Sunday":
        print("Weekend!")
    case _:
        print("School day")

It prints

Weekend!

Try it yourself

What does this print?

Python

n = 3

match n:
    case 1:
        print("one")
    case 2:
        print("two")
    case _:
        print("lots")

What does case _: mean?

An empty case
Anything that none of the cases above matched
A mistake
Match nothing

Answer them in the app

💾 Saving Data as Text

What is JSON?

A dictionary lives in your program and vanishes when it stops. JSON is that same data written out as plain text, so it can be saved in a file or sent across the internet.

Python

import json

player = {"name": "Ada", "score": 10}
print(json.dumps(player))

It prints

{"name": "Ada", "score": 10}

And back again

dumps turns data into text. loads turns the text back into real Python data you can use.

Python

import json

text = '{"name": "Ada", "score": 10}'
player = json.loads(text)
print(player["name"])
print(player["score"] + 5)

It prints

Ada
15

Making it readable

indent spreads the text over several lines so a human can read it.

Python

import json

print(json.dumps({"a": 1, "b": 2}, indent=2))

It prints

{
  "a": 1,
  "b": 2
}

Try it yourself

Which way round is dumps?

Text into data
Data into text
It deletes the data
It prints the data

What does this print?

Python

import json

data = json.loads('{"pets": ["cat", "dog"]}')
print(len(data["pets"]))

Answer them in the app

🔍 Finding Patterns

Searching for a shape, not a word

in finds an exact word. A regular expression finds a *shape* — like "some digits" — wherever it appears. \d means one digit and + means one or more.

Python

import re

print(re.findall(r"\d+", "I have 3 cats and 12 fish"))

It prints

['3', '12']

Why the r?

The r before the quotes means "leave the backslashes alone". Without it Python tries to read \d itself and gets confused. Always put the r there.

Python

import re

print(re.findall(r"\w+", "hello there"))

It prints

['hello', 'there']

The full stop matches anything

A . in a pattern means "any single character". So c.t matches cat, cot and cut.

Python

import re

print(re.findall(r"c.t", "cat cot dog cut"))

It prints

['cat', 'cot', 'cut']

Try it yourself

What does \d mean in a pattern?

The letter d
Any one digit
A dot
A dollar

What does this print?

Python

import re

print(re.findall(r"\d+", "a1b22c333"))

Answer them in the app

✂️ Checking and Changing

Does the whole thing fit?

^ means "the start" and $ means "the end", so ^c.t$ matches only words that are exactly three letters starting c and ending t. fullmatch answers with something truthy, or None when it does not fit.

Python

import re

print(bool(re.fullmatch(r"^c.t$", "cat")))
print(bool(re.fullmatch(r"^c.t$", "chart")))

It prints

True
False

Swapping things out

re.sub finds every match and replaces it. Great for hiding things.

Python

import re

print(re.sub(r"\d", "*", "my code is 1234"))

It prints

my code is ****

Try it yourself

What does this print?

Python

import re

print(re.sub(r"cat", "dog", "cat and cat"))

What does ^ mean at the start of a pattern?

Not this
Match must begin at the start of the text
Go up a line
A power, like 2^3

Answer them in the app

🏆 Pattern Master

Try it yourself

What does this print?

Python

grade = "B"

match grade:
    case "A" | "B":
        print("Great")
    case _:
        print("Keep going")

Answer it in the app