JSON – saving data like the pros

What you'll learn

Understand what JSON is and where it's used
Save and load dictionaries and lists with json
Keep working with saved data

What is JSON?

JSON (pronounced “Jason”) is a text format for data. It looks almost exactly like Python dictionaries and lists – and it’s used everywhere: apps store settings with it, websites send data back and forth with it, and Codelix even stores your progress in the browser as JSON.

import json

player = {"name": "Mia", "level": 7, "items": ["sword", "shield"], "active": True}
text = json.dumps(player)
print(text)
print(type(text))

json.dumps turns Python data into text. Note the differences: JSON writes true instead of True and always uses double quotes.

And back again

import json

text = '{"name": "Ben", "points": [3, 5, 8]}'
data = json.loads(text)
print(data["name"], sum(data["points"]))

json.loads turns the text back into real Python data.

With files: dump and load

Without the s at the end, the functions work directly with files:

import json

settings = {"volume": 80, "language": "en"}
with open("settings.json", "w") as file:
    json.dump(settings, file)

with open("settings.json") as file:
    loaded = json.load(file)
print(loaded)

Remember: dumps/loads = with strings (text), dump/load = with files.

Quick check

What does this program print?

import json
print(json.dumps({"ok": True, "n": None}))

Your task

Your turn

Write two functions. save(data, filename) writes a dictionary as JSON into a file. load(filename) reads the file back in and returns the data. If the file doesn't exist yet, load should return an empty dictionary (tip: catch FileNotFoundError).

import json


def save(data, filename):
    pass


def load(filename):
    pass


highscores = {"Ana": 120, "Ben": 95}
save(highscores, "highscores.json")
print(load("highscores.json"))

Something to think about
In save use json.dump(data, file), in load return json.load(file) – each inside a with open(...) block.
Which command helps?
Write the file with json.dump, not with str(data) – then it contains real JSON.
Put the opening inside try and catch except FileNotFoundError – then return {}.

Gap code – fill in the ___

                        import json


def save(data, filename):
    with ___ as file:
        json.dump(___)


def load(filename):
    try:
        with ___ as file:
            return ___
    except FileNotFoundError:
        return ___


highscores = {"Ana": 120, "Ben": 95}
save(highscores, "highscores.json")
print(load("highscores.json"))

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

import json


def save(data, filename):
    with open(filename, "w") as file:
        json.dump(data, file)


def load(filename):
    try:
        with open(filename) as file:
            return json.load(file)
    except FileNotFoundError:
        return {}


highscores = {"Ana": 120, "Ben": 95}
save(highscores, "highscores.json")
print(load("highscores.json"))