docs: def init_app(self, app, conf_key=None):
          """
          :type app: flask.Flask
          :parm str conf_key: Key of flask config.
          """
          conf_key = conf_key or self.conf_key or 'PYMEMCACHE'
          self.conf_key = conf_key
          conf = app.config[conf_key]
          if not isinstance(conf, dict):
              raise TypeError("Flask-PyMemcache conf should be dict")
  
          close_on_teardown = conf.pop('close_on_teardown', False)
  
          if isinstance(conf['server'], list):
              conf['servers'] = conf.pop('server')
              client = pymemcache.client.hash.HashClient(**conf)
          elif isinstance(conf['server'], tuple):
              client = pymemcache.client.Client(**conf)
          else:
              raise TypeError("Flask-PyMemcache conf['server'] should be tuple or list of tuples")
  
          app.extensions.setdefault('pymemcache', {})
          app.extensions['pymemcache'][self] = client
  
          if close_on_teardown:
              @app.teardown_appcontext
              def close_connection(exc=None):
                  client.close()