code: def src_vocab(self):
          
          if self._src_vocab is None:
              src_vocab_file_name, src_vocab_hash = \
                  self._data_file[self._pair_key]['vocab' + '_' + self._src_lang]
              [src_vocab_path] = self._fetch_data_path([(src_vocab_file_name, src_vocab_hash)])
              with io.open(src_vocab_path, 'r', encoding='utf-8') as in_file:
                  self._src_vocab = Vocab.from_json(in_file.read())
          return self._src_vocab
docs: Source Vocabulary of the Dataset.
  
          Returns
          -------
          src_vocab : Vocab
              Source vocabulary.