I am testing the contents of some messages, and I have a bit of a conundrum. I have a spec for the messages, which includes a number of required and optional fields (the spec is AsyncAPI 2.5.0, if that helps). The spec gives a number of fields that are explicitly required, but are allowed to have a value of null.

I'm using marshmallow-dataclass to build the schema:

from typing import ClassVar, Type

from marshmallow import Schema
from marshmallow_dataclass import dataclass


@dataclass
class RequiredButNullable:
    Schema: ClassVar[Type[Schema]]
    required_field: str  # Required, cannot be null
    required_but_nullable: int  # this field must be present, but can be null
    some_other_field: float | None  # optional field, can be missing

If I load a valid message with required_but_nullable set to null, it throws a validation error, but if I change the typing to int | None, it allows an invalid message that is missing required_but_nullable.

Is there a way to define a custom type, e.g. Nullable[int], or do I need to add some form of validation function?