from abc import ABC
from typing import Protocol, Type


class Base(ABC):
    pass


class Foo(Base):
    pass


class Bar(Base):
    pass


class IApp(Protocol):
    foo_or_bar: Type[Base]


class Application(IApp, ABC):
    def __init__(self, flag):
        if flag:
            self.foo_or_bar = Foo()
        else:
            self.foo_or_bar = Bar()