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JSON: serializar e analisar

JSON is the most widespread data exchange format. The json module in the standard library handles it natively.

From Python to JSON: dumps

json.dumps(object) serializes a Python object into a JSON string.

Python
import json

dati = {"nome": "Ada", "anni": 36, "skills": ["python", "matematica"]}
json.dumps(dati)
# '{"nome": "Ada", "anni": 36, "skills": ["python", "matematica"]}'

json.dumps(dati, indent=2, sort_keys=True)
# multi-line formatted string, keys sorted

From JSON to Python: loads

json.loads(string) parses a JSON string and returns Python objects.

Python
testo = '{"nome": "Ada", "anni": 36}'
d = json.loads(testo)
d["nome"]    # 'Ada'
type(d)      # <class 'dict'>

Type mapping

JSONPython
objectdict
arraylist
stringstr
numberint / float
trueTrue
falseFalse
nullNone

From file: dump / load

Python
with open("config.json", "w") as f:
    json.dump(dati, f, indent=2)

with open("config.json") as f:
    dati = json.load(f)

Without the trailing "s" they work on files (file-like), with the "s" on strings.

Non-serializable types: default

datetime, set, your custom classes are not serializable by default:

Python
import datetime, json
json.dumps({"ora": datetime.datetime.now()})
# TypeError: Object of type datetime is not JSON serializable

You can pass a default function that tells how to transform them:

Python
def converti(obj):
    if isinstance(obj, datetime.datetime):
        return obj.isoformat()
    if isinstance(obj, set):
        return list(obj)
    raise TypeError(f"non serializzabile: {type(obj)}")

json.dumps({"ora": datetime.datetime.now()}, default=converti)

JSON and file operations

The standard library also provides json.dump() and json.load() (without the 's') to write and read JSON data directly to and from files, pairing naturally with context managers:

Python
with open("data.json", "w") as f:
    json.dump(data, f)

Try it

Exercício#python.m8.l1.e1
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Given `data = {'nome': 'Ada', 'anni': 36}`, serialize it to JSON with indent=2 and assign to the variable `json_str`. Evaluate `json_str`.

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json.dumps(data, indent=2)

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Review exercise

Exercício#python.m8.l1.e2
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Given the JSON string s = '[{"nome": "a", "v": 1}, {"nome": "b", "v": 2}]', parse it with json.loads into data and compute the sum of the 'v' values in total. Evaluate total.

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json.loads returns a list of dicts.

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Additional challenge

Exercício#python.m8.l1.e3
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Given the JSON string `json_data = '{"name": "Alice", "age": 30}'`, decode it into a Python dictionary using `json.loads`. Extract the value associated with the `'name'` key and store it in `user_name`. Finally, evaluate `user_name`.

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Use json.loads(json_data) to decode, then access via data['name'].

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