<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def site_url(url):
 """ Determine the server URL. """

    base_url = 'http://%s' % socket.gethostname()

    if server.port is not 80:
        base_url += ':%d' % server.port

    return urlparse.urljoin(base_url, url)