code: def access_token(self):
          
          self._check_appid_appsecret()
  
          if callable(self.__access_token_getfunc):
              self.__access_token, self.__access_token_expires_at = self.__access_token_getfunc()
  
          if self.__access_token:
              now = time.time()
              if self.__access_token_expires_at - now > 60:
                  return self.__access_token
  
          self.grant_access_token()  # 从腾讯服务器获取 access token 并更新
          return self.__access_token
docs: 获取当前 access token 值, 本方法会自行维护 access token 有效性