code: def prefetch(self, file_size=None):
          
          if file_size is None:
              file_size = self.stat().st_size
  
          # queue up async reads for the rest of the file
          chunks = []
          n = self._realpos
          while n < file_size:
              chunk = min(self.MAX_REQUEST_SIZE, file_size - n)
              chunks.append((n, chunk))
              n += chunk
          if len(chunks) > 0:
              self._start_prefetch(chunks)
docs: Pre-fetch the remaining contents of this file in anticipation of future
          `.read` calls.  If reading the entire file, pre-fetching can
          dramatically improve the download speed by avoiding roundtrip latency.
          The file's contents are incrementally buffered in a background thread.
  
          The prefetched data is stored in a buffer until read via the `.read`
          method.  Once data has been read, it's removed from the buffer.  The
          data may be read in a random order (using `.seek`); chunks of the
          buffer that haven't been read will continue to be buffered.
  
          :param int file_size:
              When this is ``None`` (the default), this method calls `stat` to
              determine the remote file size. In some situations, doing so can
              cause exceptions or hangs (see `#562
              <https://github.com/paramiko/paramiko/pull/562>`_); as a
              workaround, one may call `stat` explicitly and pass its value in
              via this parameter.
  
          .. versionadded:: 1.5.1
          .. versionchanged:: 1.16.0
              The ``file_size`` parameter was added (with no default value).
          .. versionchanged:: 1.16.1
              The ``file_size`` parameter was made optional for backwards
              compatibility.