[Python] Pydantic Model with exactly one input out of two optionals

I want a pydantic Model that can take exactly one of two optional arguments, where the missing argument will be calculated from the other.

Consider this:

from pydantic import BaseModel, model_validator
from typing import Optional

class Object:
id: int

def get_object_by_id(id: int) -> Object:
...

class ObjectContent(BaseModel):
_object_id: Optional[int] = None
_object: Optional[Object] = None

@model_validator(mode="before")
@classmethod
def object_reference(cls, values):
if values.get("_object_id") is None and values.get("_object") is None:
raise ValueError("Define either _object_id or _object")

@property
def object_id(self) -> int:
if self._object_id is None:
self._object_id = self.object.id
return self._object_id # type check complains that this might still be None

@object_id.setter
def object_id(self, id: int):
self._object_id = id

@property
def object(self) -> Object:
if self._object is None:
self._object = get_object_by_id(self.object_id)
return self._object # type check complains that this might still be None

@object.setter
def object(self, object: Object):
self._object = object


ObjectContent(_object_id=5) # I would prefer to initialize with the unprotected argument, i.e. ObjectContent(object_id=5) or ObjectContent(object=my_object)

1. I would expecte pydantic to have a more concise solution?
2. Since get_object_from_id would be a costly io-operation it should stay lazy loaded

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