code: def _attempt_resumable_download(self, key, fp, headers, cb, num_cb,
                                      torrent, version_id):
          
          cur_file_size = get_cur_file_size(fp, position_to_eof=True)
  
          if (cur_file_size and 
              self.etag_value_for_current_download and
              self.etag_value_for_current_download == key.etag.strip('"\'')):
              # Try to resume existing transfer.
              if cur_file_size > key.size:
                raise ResumableDownloadException(
                    '%s is larger (%d) than %s (%d).\nDeleting tracker file, so '
                    'if you re-try this download it will start from scratch' %
                    (fp.name, cur_file_size, str(storage_uri_for_key(key)),
                     key.size), ResumableTransferDisposition.ABORT)
              elif cur_file_size == key.size:
                  if key.bucket.connection.debug >= 1:
                      print 'Download complete.'
                  return
              if key.bucket.connection.debug >= 1:
                  print 'Resuming download.'
              headers = headers.copy()
              headers['Range'] = 'bytes=%d-%d' % (cur_file_size, key.size - 1)
              cb = ByteTranslatingCallbackHandler(cb, cur_file_size).call
              self.download_start_point = cur_file_size
          else:
              if key.bucket.connection.debug >= 1:
                  print 'Starting new resumable download.'
              self._save_tracker_info(key)
              self.download_start_point = 0
              # Truncate the file, in case a new resumable download is being
              # started atop an existing file.
              fp.truncate(0)
  
          # Disable AWSAuthConnection-level retry behavior, since that would
          # cause downloads to restart from scratch.
          key.get_file(fp, headers, cb, num_cb, torrent, version_id,
                       override_num_retries=0)
          fp.flush()
docs: Attempts a resumable download.
  
          Raises ResumableDownloadException if any problems occur.