code: def xsrf_secret_key():
      
      secret = memcache.get(XSRF_MEMCACHE_ID, namespace=OAUTH2CLIENT_NAMESPACE)
      if not secret:
          # Load the one and only instance of SiteXsrfSecretKey.
          model = SiteXsrfSecretKey.get_or_insert(key_name='site')
          if not model.secret:
              model.secret = _generate_new_xsrf_secret_key()
              model.put()
          secret = model.secret
          memcache.add(XSRF_MEMCACHE_ID, secret,
                       namespace=OAUTH2CLIENT_NAMESPACE)
  
      return str(secret)
docs: Return the secret key for use for XSRF protection.
  
      If the Site entity does not have a secret key, this method will also create
      one and persist it.
  
      Returns:
          The secret key.