def test_cancelDelayedCall(self):
        """
        If the only delayed call is canceled, L{None} is the timeout passed
        to C{doIteration}.
        """
        reactor = TimeoutReportReactor()
        call = reactor.callLater(50, lambda: None)
        call.cancel()
        timeout = self._checkIterationTimeout(reactor)
        self.assertIsNone(timeout)

        If the only delayed call is canceled, L{None} is the timeout passed
        to C{doIteration}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_posixbase.py