def test_authSuccess(self):
        """
        The checker created by the C{--auth} command-line option returns a
        L{Deferred} that returns the avatar id when presented with credentials
        that are known to that checker.
        """
        self.options.parseOptions(['--auth', 'file:' + self.filename])
        checker = self.options['credCheckers'][-1]
        correct = credentials.UsernamePassword(*self.usernamePassword)
        return checker.requestAvatarId(correct).addCallback(
            lambda username: self.assertEqual(username, correct.username)
        )

        The checker created by the C{--auth} command-line option returns a
        L{Deferred} that returns the avatar id when presented with credentials
        that are known to that checker.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_ftp_options.py