Data in JSON and YAML: Free Lesson

An order a person can read

Priya orders two teas and a cake on a shop's website. The website sends her order to the kitchen's system.

Both programs must understand it. A developer fixing a fault at midnight must read it too.

So the order travels as plain text in an agreed shape.

By the end of this lesson

Note

I can read and write data in JSON and YAML.

Key words

Word	What it means	Example
JSON	a text format for data, built from objects and lists
Object	a group of keys, each with a value	{"qty": 2}
Key	the name of one value in an object	"qty"
List	values in order, inside square brackets	["tea", "cake"]
Nesting	putting an object or list inside another
YAML	a format for the same data that uses indentation

The cells students fill in are left blank in this preview.

JSON is objects and lists

Curly brackets { } hold an object: keys and values, such as "customer": "Priya".

Square brackets [ ] hold a list. Text goes in double quotes. Numbers have no quotes.

Yes and no are true and false. In Python, json.loads turns JSON text into dictionaries and lists.

Read an order

Code: python

import json

# An order, as the shop's website sends it.
text = '{"id": 1042, "customer": "Priya", "paid": true, "items": [{"name": "Tea", "qty": 2}, {"name": "Cake", "qty": 1}]}'

order = json.loads(text)
print(order["customer"])
print(order["paid"])
print(order["items"][0]["name"])

# Your turn: print the qty of the cake.

Follow nesting one step at a time

The order holds a list called items. Each item is an object of its own. That is nesting.

To reach a value, follow the path one step at a time. order["items"] is the list. [0] is the first item, because counting starts at 0. ["name"] is its name.

Worked example: paths into the order

Path in Python	What it gives
order["id"]	1042
order["items"]	The list of two items
order["items"][0]	{"name": "Tea", "qty": 2}
order["items"][0]["qty"]	2
order["items"][1]["name"]	Cake

The cells students fill in are left blank in this preview.

A common mistake

Warning

Many people think JSON accepts single quotes, because Python does. It does not: {'qty': 2} makes json.loads fail. Use double quotes, and put no comma after the last item.

Write a support ticket as JSON

Code: python

import json

ticket = {
    "id": 77,
    "subject": "Payment failed",
    "tags": ["payments", "urgent"],
    "customer": {"name": "Tom", "city": "Leeds"},
}

# indent=2 makes the JSON easy for a person to read.
print(json.dumps(ticket, indent=2))

# Your turn: add "open": True to the ticket and run it again.
# Notice that Python's True is written true in JSON.

YAML says the same with indentation

YAML holds the same objects and lists with fewer brackets. A key and its value sit on one line: name: Tom.

A list item starts with - . Nesting is shown by indenting two spaces.

You meet YAML in settings files for servers and build tools. The spaces matter, so never use a tab.

The ticket in YAML

Code: bash

# The same ticket in YAML
id: 77
subject: Payment failed
tags:
  - payments
  - urgent
customer:
  name: Tom
  city: Leeds