docs: def _WritePartial(self, data):
      """Writes at most one chunk of data."""
  
      chunk = self.offset // self.chunksize
      chunk_offset = self.offset % self.chunksize
      data = utils.SmartStr(data)
  
      available_to_write = min(len(data), self.chunksize - chunk_offset)
  
      fd = self._GetChunkForWriting(chunk)
      fd.seek(chunk_offset)
  
      fd.write(data[:available_to_write])
      self.offset += available_to_write
  
      return data[available_to_write:]