code: def __flush_data(self, data):
          
          # Ensure the index, even if there's nothing to write, so
          # the filemd5 command always succeeds.
          self.__ensure_indexes()
          self._file['md5'].update(data)
  
          if not data:
              return
          assert(len(data) <= self.chunk_size)
  
          chunk = {"files_id": self._file["_id"],
                   "n": self._chunk_number,
                   "data": Binary(data)}
  
          try:
              self._chunks.insert_one(chunk)
          except DuplicateKeyError:
              self._raise_file_exists(self._file['_id'])
          self._chunk_number += 1
          self._position += len(data)
docs: Flush `data` to a chunk.