class HelloDescriptor:
    def __get__(self, obj, objtype=None):
        return self.hello


class Foo(HelloDescriptor):
    def hello(self):
        return 42


class Bar(HelloDescriptor):
    hello = Foo()


class Baz(HelloDescriptor):
    hello = Bar()


class Spam(HelloDescriptor):
    hello = Baz()


spam_instance = Spam()
print(spam_instance.hello())