def to_json(self, path=None):
        """
        Summarizes some information of vocab as JSON string. If path is gaven,
        the JSON string will be saved into files. The JSON string and the saved
        file all can be used to reconstruct the :class:`Vocab` by calling
        :meth:`from_json` method.

        Args:
            path (str, optional): The path to save JSON string. If None, the
                JSON will not be saved. Default: None.

        Returns:
            str: The JSON string including information of vocab.

        Example:
            .. code-block:: python

                from paddlenlp.data import Vocab
                # The vocab file. The sample file can be downloaded firstly.
                # wget https://bj.bcebos.com/paddlenlp/data/senta_word_dict.txt
                vocab_file_path = './senta_word_dict.txt'
                # Initialize the Vocab
                vocab = Vocab.load_vocabulary(
                    vocab_file_path,
                    unk_token='[UNK]',
                    pad_token='[PAD]')
                json_str = vocab.to_json(path='./vocab.json')
        """
        vocab_dict = {}
        vocab_dict['idx_to_token'] = dict(self.idx_to_token)
        vocab_dict['token_to_idx'] = dict(self.token_to_idx)
        vocab_dict['unk_token'] = self.unk_token
        vocab_dict['identifiers_to_tokens'] = self._identifiers_to_tokens
        json_str = json.dumps(vocab_dict)
        if path:
            with io.open(path, 'w', encoding='utf-8') as f:
                f.write(json_str)
        return json_str

        Summarizes some information of vocab as JSON string. If path is gaven,
        the JSON string will be saved into files. The JSON string and the saved
        file all can be used to reconstruct the :class:`Vocab` by calling
        :meth:`from_json` method.

        Args:
            path (str, optional): The path to save JSON string. If None, the
                JSON will not be saved. Default: None.

        Returns:
            str: The JSON string including information of vocab.

        Example:
            .. code-block:: python

                from paddlenlp.data import Vocab
                # The vocab file. The sample file can be downloaded firstly.
                # wget https://bj.bcebos.com/paddlenlp/data/senta_word_dict.txt
                vocab_file_path = './senta_word_dict.txt'
                # Initialize the Vocab
                vocab = Vocab.load_vocabulary(
                    vocab_file_path,
                    unk_token='[UNK]',
                    pad_token='[PAD]')
                json_str = vocab.to_json(path='./vocab.json')
        
ptstructure/vqa/pytorchnlp/data/vocab.py