code: def _should_queue(self, link, referrer, rel):
          
          scheme, netloc, path, _, _, _ = urlparse(link)
          if path.endswith(self.source_extensions + self.binary_extensions +
                           self.excluded_extensions):
              result = False
          elif self.skip_externals and not link.startswith(self.base_url):
              result = False
          elif not referrer.startswith(self.base_url):
              result = False
          elif rel not in ('homepage', 'download'):
              result = False
          elif scheme not in ('http', 'https', 'ftp'):
              result = False
          elif self._is_platform_dependent(link):
              result = False
          else:
              host = netloc.split(':', 1)[0]
              if host.lower() == 'localhost':
                  result = False
              else:
                  result = True
          logger.debug('should_queue: %s (%s) from %s -> %s', link, rel,
                       referrer, result)
          return result
docs: Determine whether a link URL from a referring page and with a
          particular "rel" attribute should be queued for scraping.