async def METHOD_NAME():
    class EmptyRadioSetApp(App[None]):
        def compose(self) -> ComposeResult:
            yield RadioSet()
        def on_mount(self) -> None:
            # This isn't encouraged; but neither is it currently prohibited;
            # so let's test.
            for n in range(5):
                self.query_one(RadioSet).mount(RadioButton(id=f"rb{n}"))
    async with EmptyRadioSetApp().run_test() as pilot:
        assert pilot.app.query_one(RadioSet)._selected is None
        await pilot.press("up")
        assert pilot.app.query_one(RadioSet)._selected == 0