from pydantic import BaseModel
from datetime import datetime, timezone
from fastapi.encoders import jsonable_encoder


class ModelWithCustomEncoder(BaseModel):
    class Config:
        json_encoders = {
            datetime: lambda dt: dt.replace(
                microsecond=0, tzinfo=timezone.utc
            ).isoformat()
        }


class ModelWithCustomEncoderSubclass(ModelWithCustomEncoder):
    dt_field: datetime

    class Config:
        pass


model = ModelWithCustomEncoderSubclass(dt_field=datetime(2019, 1, 1, 8))

print(model.json())
# {"dt_field": "2019-01-01T08:00:00+00:00"}

print(jsonable_encoder(model))
# {'dt_field': '2019-01-01T08:00:00'}