def METHOD_NAME(self, *, router, app, assignor):
    topic = Mock()
    prepare_key = topic.prepare_key
    prepare_key.return_value = [Mock(name="v1"), Mock(name="v2")]
    assert (
        router.external_topic_key_store(topic, "k")
        is assignor.external_key_store.return_value
    )
    assignor.external_key_store.assert_called_once_with(
        topic.get_topic_name(),
        topic.prepare_key.return_value[0],
    )