gistfile1.txt

from abc import ABCMeta, abstractmethod
from typing import Set


class Observer(metaclass=ABCMeta):
    @abstractmethod
    def notify(self, volume: int) -> None:
        pass


class Store:
    def __init__(self) -> None:
        self.volume_observers: Set[Observer] = set()
        self.volume = 0

    def register_volume_observer(self, observer: Observer) -> None:
        self.volume_observers.add(observer)

    def _notify_volume(self) -> None:
        for observer in self.volume_observers:
            observer.notify(self.volume)

    def change_volume(self, new_value: int) -> None:
        self.volume = new_value
        self._notify_volume()


class VolumeLogger(Observer):
    def notify(self, volume: int) -> None:
        print(volume)


def main() -> None:
    store = Store()
    volume_logger = VolumeLogger()
    store.register_volume_observer(volume_logger)

    store.change_volume(1)


if __name__ == "__main__":
    main()