def test_stopCooperatorReentrancy(self):
        """
        If a callback of a L{Deferred} from L{CooperativeTask.whenDone} calls
        C{Cooperator.stop} on its L{CooperativeTask._cooperator}, the
        L{Cooperator} will stop, but the L{CooperativeTask} whose callback is
        calling C{stop} should already be considered 'stopped' by the time the
        callback is running, and therefore removed from the
        L{CoooperativeTask}.
        """
        callbackPhases = []
        def stopit(result):
            callbackPhases.append(result)
            self.cooperator.stop()
            # "done" here is a sanity check to make sure that we get all the
            # way through the callback; i.e. stop() shouldn't be raising an
            # exception due to the stopped-ness of our main task.
            callbackPhases.append("done")
        self.task.whenDone().addCallback(stopit)
        self.stopNext()
        self.scheduler.pump()
        self.assertEqual(callbackPhases, [self.task._iterator, "done"])

        If a callback of a L{Deferred} from L{CooperativeTask.whenDone} calls
        C{Cooperator.stop} on its L{CooperativeTask._cooperator}, the
        L{Cooperator} will stop, but the L{CooperativeTask} whose callback is
        calling C{stop} should already be considered 'stopped' by the time the
        callback is running, and therefore removed from the
        L{CoooperativeTask}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_cooperator.py