asyncio.sleep required after cancelling tasks?

Question

To test a man-in-the-middle tcp proxy I have coded an echo tcp server and a tcp client. After each one of the tests I want the proxy and the server to go down, to make sure each test starts on a clean environment, so I have coded:

class TestConnections(TestCase):

    loop = None

    @classmethod
    def setUpClass(cls) -> None:
        TestConnections.loop = asyncio.new_event_loop()
        asyncio.set_event_loop(TestConnections.loop)

    def setUp(self) -> None:
        EnergyAgent.GP = GrowattParser.GrowattParser()
        self.server = self.loop.create_task(tcp_server(1235))
        self.gp = self.loop.create_task(EnergyAgentProxy(1234, 1235))

    def tearDown(self) -> None:
        logger.debug('Cancelling the tasks')
        self.server.cancel()
        self.gp.cancel()
        self.loop.run_until_complete(asyncio.sleep(0.5))

    @classmethod
    def tearDownClass(cls) -> None:
        logger.debug('Closing the event loop')
        TestConnections.loop.close()

    def test_client2server(self):
        # start the client process, and wait until done
        result = self.loop.run_until_complete(asyncio.wait_for(tcp_client(1235, message), timeout=2))
        self.assertEqual(message, result)

Now, the problem is: unless I add in the method tearDown the last line

self.loop.run_until_complete(asyncio.sleep(0.5))

I get an error about tasks on the proxy not having finished:

ERROR:asyncio:Task was destroyed but it is pending!

Is there any way to just for all the tasks to finish? I have tried running

self.loop.run_until_complete(asyncio.wait([self.server, self.gp]))

and asyncio.gather... unsuccessfully.

Answer 1

i assume that right instance tearDown method, the class tearDownClass is invoked?

that error usually comes when you close the event loop before the tasks were finished. when you cancel a task, it immediately returns, but the task itself is not yet canceled but rather waits for the loop to throw it a CancelledError. so you should await for the task to receive it's termination error. if you close the event loop before the task got the chance to be thrown, it wont be considered canceled and youll get that error.

by making the self.loop.run_until_complete(asyncio.sleep(0.5)) call you did you actually did that in a non deliberate way.

the docs elaborate about that also checkout this answer