<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def flush(self, multithread=True, **kwargs):
        '''
        Flushes the internal write buffer.
        '''
        if self._write_buf.tell() > 0:
            data = self._write_buf.getvalue()
            self._write_buf = BytesIO()

            if multithread:
                self._async_upload_part_request(data, index=self._cur_part, **kwargs)
            else:
                self.upload_part(data, self._cur_part, **kwargs)

            self._cur_part += 1

        if len(self._http_threadpool_futures) > 0:
            dxpy.utils.wait_for_all_futures(self._http_threadpool_futures)
            try:
                for future in self._http_threadpool_futures:
                    if future.exception() != None:
                        raise future.exception()
            finally:
                self._http_threadpool_futures = set()