Custom JSON Encoding

When you try to serialize types JSON doesn't support (dates, sets, custom objects), you need a custom encoder.

import json
from datetime import datetime

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

Use it with the cls parameter:

data = {"event": "meeting", "when": datetime.now()}
json.dumps(data, cls=CustomEncoder)
# '{"event": "meeting", "when": "2023-08-06T14:30:00"}'

For simpler cases, use the default parameter:

def convert(obj):
    if isinstance(obj, datetime):
        return obj.isoformat()
    raise TypeError(f"Can't serialize {type(obj)}")

json.dumps(data, default=convert)

The encoder handles each unserializable object and returns something JSON can understand.

I cover custom encoding in my JSON with Python course.