Python: 'TypeError: Object of type datetime is not JSON serializable' (also Decimal, UUID, set, bytes) when calling json.dumps or returning JSON. How do I serialize these types?

Cause: Python's json module only knows how to encode dict, list, tuple, str, int, float, bool and None. Anything else, such as datetime/date, Decimal (common from database drivers), UUID, set, bytes, or your own classes and dataclasses, raises this TypeError unless you tell json how to convert it. Fix: 1. Pass a default function that converts unknown types: json.dumps(data, default=to_jsonable), where to_jsonable returns obj.isoformat() for datetime and date, str(obj) for UUID and Decimal (or float(obj) if you accept precision loss), list(obj) for sets, dataclasses.asdict(obj) for dataclasses, and otherwise raises TypeError so real mistakes still surface. 2. Or subclass json.JSONEncoder and override its default method, then use json.dumps(data, cls=MyEncoder); useful when a framework lets you plug in an encoder class. 3. Convert at the edge instead: build the payload with plain types yourself (created_at=dt.isoformat()), or use a model library that serializes for you (Pydantic's model_dump(mode='json') or model_dump_json(), Django REST Framework serializers, FastAPI's jsonable_encoder). 4. Use ISO 8601 strings for times and include the time zone (timezone-aware datetimes produce an offset in isoformat()), so the consumer can parse them back with datetime.fromisoformat(). 5. Avoid default=str as a blanket fix in production: it silently turns any object, including unexpected ones, into strings. Verify: json.loads(json.dumps(data, default=to_jsonable)) round-trips, and the consumer parses the dates back correctly. Sources: https://docs.python.org/3/library/json.html https://docs.python.org/3/library/datetime.html