def test_fail(self):
        """
        A test which fails in the callback of the returned L{defer.Deferred}.
        """
        d = defer.Deferred()
        d.addCallback(self._later)
        reactor.callLater(0, d.callback, None)
        return d

        A test which fails in the callback of the returned L{defer.Deferred}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/erroneous.py