code: def _req_offline_space(self):
          
          url = 'http://115.com/'
          params = {
              'ct': 'offline',
              'ac': 'space',
              '_': get_timestamp(13)
          }
          _sign = os.environ.get('U115_BROWSER_SIGN')
          if _sign is not None:
              _time = os.environ.get('U115_BROWSER_TIME')
              if _time is None:
                  msg = 'U115_BROWSER_TIME is required given U115_BROWSER_SIGN.'
                  raise APIError(msg)
              params['sign'] = _sign
              params['time'] = _time
              params['uid'] = self.user_id
          req = Request(url=url, params=params)
          r = self.http.send(req)
          if r.state:
              self._signatures['offline_space'] = r.content['sign']
              self._lixian_timestamp = r.content['time']
          else:
              msg = 'Failed to retrieve signatures.'
              raise RequestFailure(msg)
docs: Required before accessing lixian tasks