code: def init_app(self, app):
          
          self.init_config(app)
          app.extensions['invenio-indexer'] = self
  
          hooks = app.config.get('INDEXER_BEFORE_INDEX_HOOKS', [])
          for hook in hooks:
              if isinstance(hook, six.string_types):
                  hook = import_string(hook)
              before_record_index.connect_via(app)(hook)
docs: Flask application initialization.
  
          :param app: The Flask application.