Source Vocabulary of the Dataset.

        Returns
        -------
        src_vocab : Vocab
            Source vocabulary.

def src_vocab(self):
        """Source Vocabulary of the Dataset.

        Returns
        -------
        src_vocab : Vocab
            Source vocabulary.
        """
        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