<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def url(self):
 """ Convert self into a url This function should more or less round-trip with :func:`.parse_url`. The returned url may not 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: :: 'http://google.com/mail/' 'http://username:password@host.com:80/path?query#fragment' """

        scheme, auth, host, port, path, query, fragment = self
        url = ''

        # We use "is not None" we want things to happen with empty strings (or 0 port)
        if scheme is not None:
            url += scheme + '://'
        if auth is not None:
            url += auth + '@'
        if host is not None:
            url += host
        if port is not None:
            url += ':' + str(port)
        if path is not None:
            url += path
        if query is not None:
            url += '?' + query
        if fragment is not None:
            url += '#' + fragment

        return url