def test_callLater(self):
        """
        Test that a DelayedCall really calls the function it is
        supposed to call.
        """
        d = Deferred()
        reactor.callLater(0, d.callback, None)
        d.addCallback(self.assertEqual, None)
        return d

        Test that a DelayedCall really calls the function it is
        supposed to call.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_internet.py