def test_errback(self):
        """
        The L{Deferred} returned by L{task.deferLater} is errbacked if the
        supplied function raises an exception.
        """
        def callable():
            raise TestException()

        clock = task.Clock()
        d = task.deferLater(clock, 1, callable)
        clock.advance(1)
        return self.assertFailure(d, TestException)

        The L{Deferred} returned by L{task.deferLater} is errbacked if the
        supplied function raises an exception.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_task.py