def test_clientPresentsCertificate(self):
        """
        When the server verifies and the client presents a valid certificate
        for that verification by passing it to
        L{sslverify.optionsForClientTLS}, communication proceeds.
        """
        cProto, sProto, cWrapped, sWrapped, pump = self.serviceIdentitySetup(
            u"valid.example.com",
            u"valid.example.com",
            validCertificate=True,
            serverVerifies=True,
            clientPresentsCertificate=True,
        )

        self.assertEqual(cWrapped.data,
                         b'greetings!')

        cErr = cWrapped.lostReason
        sErr = sWrapped.lostReason
        self.assertIsNone(cErr)
        self.assertIsNone(sErr)

        When the server verifies and the client presents a valid certificate
        for that verification by passing it to
        L{sslverify.optionsForClientTLS}, communication proceeds.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_sslverify.py