code: def get_subdomain(url):
          
          allowed_domain = UrlExtractor.get_allowed_domain(url)
          return allowed_domain[:len(allowed_domain) - len(
              UrlExtractor.get_allowed_domain(url, False))]
docs: Determines the domain's subdomains.
  
          :param str url: the url to extract any subdomains from
          :return str: subdomains of url