code: def cache_add(self, resource_url, cache_id):
          
          # Add entry to cache index
          self.cache[resource_url] = cache_id
          # Write cache index content to database file
          with open(self.db_file, 'w') as f:
              for resource in self.cache:
                  f.write(resource + '\t' + self.cache[resource] + '\n')
docs: Add entry permanently to local cache.
  
          Parameters
          ----------
          resource_url : string
              Resource Url
          cache_id : string
              Unique cache identifier for resource