def test_arg_astringEmptyLine(self):
        """
        An empty string argument raises L{imap4.IllegalClientResponse}.
        """
        for empty in [b"", b"\r\n", b" "]:
            self.assertRaises(imap4.IllegalClientResponse,
                              self.server.arg_astring, empty)

        An empty string argument raises L{imap4.IllegalClientResponse}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/mail/test/test_imap.py