def test_unspecifiedRecipients(self):
        """
        If no recipients are given in the argument list and there is no
        recipient header in the message text, L{parseOptions} raises
        L{SystemExit} with a string describing the problem.
        """
        self.patch(sys, 'stdin', NativeStringIO(
            'Subject: foo\n'
            '\n'
            'Hello, goodbye.\n'))
        exc = self.assertRaises(SystemExit, parseOptions, [])
        self.assertEqual(exc.args, ('No recipients specified.',))

        If no recipients are given in the argument list and there is no
        recipient header in the message text, L{parseOptions} raises
        L{SystemExit} with a string describing the problem.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_mailmail.py