Proper way to test actor private field value changed with pytest

[SomeAkk]

Hello!
I got actor with method which change private field of that actor, it looks like that:

class BufferActorClass(pykka.ThreadingActor):

    def __init__(self):
        super().__init__()
        self._buffer = []

    def buffer_flush(self):
        for message in self._buffer:
            self.actor_ref.tell(message)
        self._buffer = []

Now i writing unit test for that function: test check if buffer_flush clear _buffer or not. Look at code below:

def mocked_consumer_actor():
    pass

def mocked_buffer_actor_with(buffer):
    proxy = BufferActor.start(parent_actor=mocked_consumer_actor).proxy()
    proxy._actor._buffer = buffer
    return proxy

class TestBufferActorClassMethods:

    @pytest.mark.parametrize(
        argnames="buffer",
        argvalues=(
            ["message1", "message2", "message3"],
            ["message1"],
            []
        ),
    )
    def test_buffer_flush_must_clear_buffer(self, buffer):
        actor = mocked_buffer_actor_with(buffer)
        actor._actor._buffer = buffer
        actor.buffer_flush().get()
        assert actor._actor._buffer is []

After actor.buffer_flush().get() i am waiting that _buffer will be [], but this not going such, instead i got ["message1", "message2", "message3"] or another pytest parameters. What is wrong in my test? How to propper check field states of actors?

Updated
Also i am found that this problem coased by self.actor_ref.tell(message) - if delete this line all is ok and test is passed.

[SomeAkk]

Found problem: it was in my code, any help unneeded)
Problem was that after self.actor_ref.tell(message) in on_receive() i added messages in ._buffer again.
So in test i saw buffer with messages ["message1", "message2", "message3"] again)

[jodal]

Thanks for updating you question with the solution!