I am using pydantic v2 and have a custom schema for a FutureAwareDatetime.

class FutureAwareDatetime:
    """A datetime that requires timezone info and must be in the future."""

    @classmethod
    def __get_pydantic_core_schema__(
        cls,
        source: type[Any],
        handler: GetCoreSchemaHandler,
    ) -> CoreSchema:
        if cls is source:
            # used directly as a type
            return datetime_schema(
                now_op="future",
                tz_constraint="aware",
            )
        else:
            schema = handler(source)
            _check_annotated_type(schema["type"], "datetime", cls.__name__)
            schema["now_op"] = "future"
            schema["tz_constraint"] = "aware"
            return schema

    def __repr__(self) -> str:
        return "FutureAwareDatetime"

While this is working fine, I want to be able to use get dt.isoformat(timespec="seconds") when I serialize the data with e.g. model_dump()

I do not want to use the field_serializer decorator but use it directly inside the class.

Thank you.