How to use the pykka.ActorRegistry.get_by_urn function in pykka

def on_start(self):
            self.events.on_start_was_called.set()
            if ActorRegistry.get_by_urn(self.actor_urn) is not None:
                self.events.actor_registered_before_on_start_was_called.set()

def test_get_by_urn_returns_none_if_not_found():
    result = ActorRegistry.get_by_urn('urn:foo:bar')

    assert result is None

def test_actors_may_be_looked_up_by_urn(actor_ref):
    result = ActorRegistry.get_by_urn(actor_ref.actor_urn)

    assert result == actor_ref

def reply(self, message, text, opts=None):
        """
        Reply to the sender of the provided message with a message \
        containing the provided text.

        :param message: the message to reply to
        :param text: the text to reply with
        :param opts: A dictionary of additional values to add to metadata
        :return: None
        """
        metadata = Metadata(source=self.actor_urn,
                            dest=message['metadata']['source']).__dict__
        metadata['opts'] = opts
        message = Message(text=text, metadata=metadata,
                          should_log=message['should_log']).__dict__
        dest_actor = ActorRegistry.get_by_urn(message['metadata']['dest'])
        if dest_actor is not None:
            dest_actor.tell(message)
        else:
            raise("Tried to send message to nonexistent actor")