code: def log_url (self, url_data):
          
          key = (url_data.parent_url, url_data.cache_url)
          key = repr(key)
          if key in self.blacklist:
              if url_data.valid:
                  del self.blacklist[key]
              else:
                  self.blacklist[key] += 1
          else:
              if not url_data.valid:
                  self.blacklist[key] = 1
docs: Put invalid url in blacklist, delete valid url from blacklist.