def METHOD_NAME(self):
    for parser in self.parsers:
        with self.subTest(parser=parser.__name__):
            MyPolicy = default.clone(message_factory=self.MyMessage)
            msg = parser("To: foo\n\ntest", Message, policy=MyPolicy)
            self.assertNotIsInstance(msg, self.MyMessage)
            self.assertIsInstance(msg, Message)