Race Condition

runner.py

class Runner(threading.Thread):

    def __init__(self, in_queue, out_queue):
        super(Runner, self).__init__()
        self.in_queue = in_queue
        self.out_queue = out_queue
        self._keep_running = True

    def run(self):
        while self._keep_running or not self.in_queue.empty():
            task, args, kwargs = self.in_queue.get()
            if task:
                results = task(*args, **kwargs)
                self.out_queue.put(results)
                self.in_queue.task_done()

    def stop(self):
        self._keep_running = False

test_runner.py

    def test_run_polls_in_queue_for_tasks(self):
        def fake_empty():
            return self.in_queue.get.called

        mock_callable = mock.Mock(return_value='fake result')
        task = (mock_callable, ('a1', 'a2'), {'k1': 'k1', 'k2': 'k2'})
        self.in_queue.get.return_value = task
        self.in_queue.empty = fake_empty

        self.runner.start()
        self.runner.stop()

        # the next line sometimes fails
        self.assertTrue(self.in_queue.get.called)
        self.assertTrue(mock_callable.called)
        mock_callable.assert_called_with('a1', 'a2', k1='k1', k2='k2')
        self.out_queue.put.assert_called_with('fake result')
        self.assertTrue(self.in_queue.task_done.called)