MustSetProperty -- descriptor that raises exception if its value is None.

demo.py

from typing import Any, Generic, Optional, TypeVar, Union, overload

T = TypeVar("T")


class UnsetError(Exception):
    pass


class MustSetProperty(Generic[T]):
    def __set_name__(self, owner: Any, name: str):
        """
        Customized names -- Descriptor HowTo Guide
        https://docs.python.org/3/howto/descriptor.html#customized-names
        """
        self.public_name = name
        self.private_name = f"__must_set_{name}"

    @overload
    def __get__(self, obj: None, objtype: Any) -> "MustSetProperty[T]":
        pass

    @overload
    def __get__(self, obj: Any, objtype: Any) -> T:
        pass

    def __get__(self, obj: Any, objtype: Any) -> Union["MustSetProperty[T]", T]:
        if obj is None:
            return self

        if (value := getattr(obj, self.private_name, None)) is None:
            raise UnsetError(f"{self.public_name} is missing")

        return value

    def __set__(self, obj: Any, value: Optional[T]):
        setattr(obj, self.private_name, value)


class Bar:
    name = MustSetProperty[str]()


assert isinstance(Bar.name, MustSetProperty)

bar = Bar()
try:
    bar.name  # raise UnsetError
except UnsetError:
    print("accesing error")

bar.name = "bar"
print(bar.name)  # accesing OK

bar.name = None  # set None
try:
    bar.name  # raise UnsetError
except UnsetError:
    print("accesing error")