def test_cancelDelayedCall(self):
        """
        Test that when a DelayedCall is cancelled it does not run.
        """
        called = []
        def function():
            called.append(None)
        call = reactor.callLater(0, function)
        call.cancel()

        # Schedule a call in two "iterations" to check to make sure that the
        # above call never ran.
        d = Deferred()
        def check():
            try:
                self.assertEqual(called, [])
            except:
                d.errback()
            else:
                d.callback(None)
        reactor.callLater(0, reactor.callLater, 0, check)
        return d

        Test that when a DelayedCall is cancelled it does not run.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_internet.py