I have an abstract class Task that describes the volume_id attribute. The int value is the ID of the volume that the specific task is for. In the case that the task is not for a specific volume (a “general” task), then it’s value is just None. See example code below:

from abc import ABC, abstractmethod
from typing import Union


class Task(ABC):
    volume_id: Union[int, None]

    @abstractmethod
    def run(self) -> None:
        return


class GeneralTask(Task):
    volume_id = None

    def run(self) -> None:
        # Do something without using volume_id
        return


class SpecificTask(Task):
    volume_id = None
    
    def __init__(self, volume_id: int) -> None:
        self.volume_id = volume_id
        return
    
    def run(self) -> None:
        # Do something that uses volume_id
        int(self.volume_id)
        return

The problem is that my type checker/static analyser complains in the SpecificTask.run function that the value of self.volume_id could potentially be None. This is only true in the scenario where an instance of the class is created, the value of the attribute is manually changed to None, and then the run() function is run. This scenario doesn’t come up in my code base and should be prohibited anyway.

How can I make clear and enforce that the type of volume_id will not change after running __init__ (it can be an int or None, we set it to an int and it won’t change back)? I tried changing the typehint to Final[int] in the __init__ function, but then the static analyser complains that I’m overriding the type hint from the abstract class…