dependency.py

class Dependency:
    def get_cheese_from_singleton(self):
        from foobar_controller import FoobarController
        liked = FoobarController.foo_the_bar()
        return 'I like {}'.format(liked)

    def get_cheese_from_di(self, foobar_controller):
        liked = foobar_controller.foo_the_bar()
        return 'I like {}'.format(liked)

foobar_controller.py

class FoobarController:
    foobar = None

    @classmethod
    def foo_the_bar(cls):
        return cls.foobar

main.py

if __name__ == '__main__':
    import sys
    from foobar_controller import FoobarController
    from dependency import Dependency

    # Get values for service class from somewhere and apply them
    FoobarController.foobar = sys.argv[1]

    dep = Dependency()

    # Let the dependency figure out how to get the service class.
    # For testing you could mock it.
    print(dep.get_cheese_from_singleton())

    # DI
    print(dep.get_cheese_from_di(FoobarController))