t be exactly the same as the url inputted to
        :func:`.parse_url`, but it should be equivalent by the RFC (e.g., urls
        with a blank port will have : removed).

        Example: ::

            >>> U = parse_url('http://google.com/mail/')
            >>> U.url
            'http://google.com/mail/'
            >>> Url('http', 'username:password', 'host.com', 80,
            ... '/path', 'query', 'fragment').url
            'http://username:password@host.com:80/path?query#fragment'
        Ú