def test_doubleEncodingError(self):
        """
        If it is not possible to encode a response to the request (for example,
        because L{xmlrpclib.dumps} raises an exception when encoding a
        L{Fault}) the exception which prevents the response from being
        generated is logged and the request object is finished anyway.
        """
        logObserver = EventLoggingObserver()
        filtered = FilteringLogObserver(
            logObserver,
            [LogLevelFilterPredicate(defaultLogLevel=LogLevel.critical)]
        )
        globalLogPublisher.addObserver(filtered)
        self.addCleanup(lambda: globalLogPublisher.removeObserver(filtered))
        d = self.proxy().callRemote("echo", "")

        # *Now* break xmlrpclib.dumps.  Hopefully the client already used it.
        def fakeDumps(*args, **kwargs):
            raise RuntimeError("Cannot encode anything at all!")
        self.patch(xmlrpclib, 'dumps', fakeDumps)

        # It doesn't matter how it fails, so long as it does.  Also, it happens
        # to fail with an implementation detail exception right now, not
        # something suitable as part of a public interface.
        d = self.assertFailure(d, Exception)

        def cbFailed(ignored):
            # The fakeDumps exception should have been logged.
            self.assertEquals(1, len(logObserver))
            self.assertIsInstance(
                logObserver[0]["log_failure"].value,
                RuntimeError
            )
            self.assertEqual(len(self.flushLoggedErrors(RuntimeError)), 1)
        d.addCallback(cbFailed)
        return d

        If it is not possible to encode a response to the request (for example,
        because L{xmlrpclib.dumps} raises an exception when encoding a
        L{Fault}) the exception which prevents the response from being
        generated is logged and the request object is finished anyway.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_xmlrpc.py