from datetime import datetime
import orjson
from pydantic import BaseModel

def orjson_dumps(v, *, default):
    # orjson.dumps returns bytes, to match standard json.dumps we need to decode
    return orjson.dumps(v, default=default).decode()

class User(BaseModel):
    id: int
    name = 'John Doe'
    signup_ts: datetime = None

    class Config:
        json_loads = orjson.loads
        json_dumps = orjson_dumps


user = User.parse_raw('{"id": 123, "signup_ts": 1234567890, "name": "John Doe"}')
print(user.json())
#> {"id":123,"signup_ts":"2009-02-13T23:31:30+00:00","name":"John Doe"}