json — JSON Encoder and Decoder

The json module provides functions for encoding Python objects to JSON strings and decoding JSON strings to Python objects.

import json

Encoding (Python → JSON)

import json

data = {
    "name": "Alice",
    "age": 30,
    "languages": ["Python", "JavaScript"],
    "active": True,
    "address": None
}

# Convert to JSON string
json_str = json.dumps(data)
print(json_str)
# {"name": "Alice", "age": 30, "languages": ["Python", "JavaScript"], "active": true, "address": null}

# Pretty print
json_pretty = json.dumps(data, indent=2)
print(json_pretty)

# Write to file
with open('data.json', 'w') as f:
    json.dump(data, f, indent=2)

# Sort keys
json.dumps(data, sort_keys=True)

# Non-ASCII characters
json.dumps({"name": "Алиса"}, ensure_ascii=False)
# '{"name": "Алиса"}' instead of '{"name": "\\u0410\\u043b\\u0438\\u0441\\u0430"}'

Decoding (JSON → Python)

import json

json_str = '{"name": "Alice", "age": 30, "active": true}'

# Parse JSON string
data = json.loads(json_str)
print(data['name'])   # Alice
print(type(data))     # <class 'dict'>

# Read from file
with open('data.json', 'r') as f:
    data = json.load(f)

Type Mapping

Custom Serialization

import json
from datetime import datetime

class DateTimeEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return obj.isoformat()
        return super().default(obj)

data = {"created": datetime.now()}
json.dumps(data, cls=DateTimeEncoder)
# '{"created": "2024-01-15T10:30:00"}'

# Using default parameter
json.dumps(data, default=str)
# '{"created": "2024-01-15 10:30:00"}'

Working with Complex Objects

import json

class User:
    def __init__(self, name, email):
        self.name = name
        self.email = email

# Serialize using __dict__
user = User("Alice", "alice@example.com")
json.dumps(user.__dict__)

# Custom decoder
def user_decoder(d):
    if 'name' in d and 'email' in d:
        return User(d['name'], d['email'])
    return d

user = json.loads('{"name":"Alice","email":"alice@example.com"}',
                  object_hook=user_decoder)

Common Pitfalls

• Single quotes are invalid JSON — use "double quotes" only
• Trailing commas are invalid — {"a": 1,} will fail
• Python sets, tuples can't be serialized — convert to list first
• json.load vs json.loads — load reads from file, loads from string
• NaN and Infinity — not valid JSON, Python allows by default

Official Documentation

json — JSON encoder and decoder

API Reference

Important Functions

Important Classes