def test_noCallback(self):
        """
        The L{Deferred} returned by L{task.deferLater} fires with C{None}
        when no callback function is passed.
        """
        clock = task.Clock()
        d = task.deferLater(clock, 2.0)
        self.assertNoResult(d)

        clock.advance(2.0)
        self.assertIs(None, self.successResultOf(d))

        The L{Deferred} returned by L{task.deferLater} fires with C{None}
        when no callback function is passed.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_task.py