Thanks @jeffkpayne!

@lukesneeringer NP! Funny, though, I was just working on a unit test to test that the calls actually happen asynchronously... Should I continue with that and open a new PR?

It would be along the lines of:

def test_on_response():
    call_times = []

    def side_effect(*args, **kwargs):
        # list.append() is thread-safe.
        call_times.append(datetime.now())
        time.sleep(2)

    callback = mock.Mock(spec=())
    callback.side_effect = side_effect

    # Then, after the calls happen, test that the times are both more than a second ago, or something like that...

I would accept that as a system test (but not a unit test). :-) And yes, additional PR is of course welcome.

P. S. pubsub 0.28.4 is going through CI to release now. (This was a serious enough bug that I wanted it out immediately.)

@lukesneeringer The Pub / Sub unit tests seem to be broken (or heisen-broken) on Python 2.7:

• https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/3738
• https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/3741