code: def _private(self, command, **params):
          
          if not self._apikey or not self._secret:
              raise PoloniexCredentialsException('missing apikey/secret')
  
          with self.nonce_lock:
              params.update({'command': command, 'nonce': next(self.nonce_iter)})
              response = self.session.post(
                  self._private_url, data=params,
                  auth=Poloniex._PoloniexAuth(self._apikey, self._secret))
              return response
docs: Invoke the 'command' public API with optional params.