def from_json(cls, json_str):
        """Deserialize Vocab object from json string.

        Parameters
        ----------
        json_str : str
            Serialized json string of a Vocab object.


        Returns
        -------
        Vocab
        """
        vocab_dict = json.loads(json_str)

        unknown_token = vocab_dict.get('unknown_token')
        vocab = cls(unknown_token=unknown_token)
        vocab._idx_to_token = vocab_dict.get('idx_to_token')
        vocab._token_to_idx = vocab_dict.get('token_to_idx')
        if unknown_token:
            vocab._token_to_idx = DefaultLookupDict(vocab._token_to_idx[unknown_token],
                                                    vocab._token_to_idx)
        vocab._reserved_tokens = vocab_dict.get('reserved_tokens')
        vocab._padding_token = vocab_dict.get('padding_token')
        vocab._bos_token = vocab_dict.get('bos_token')
        vocab._eos_token = vocab_dict.get('eos_token')
        return vocab
Deserialize Vocab object from json string.

        Parameters
        ----------
        json_str : str
            Serialized json string of a Vocab object.


        Returns
        -------
        Vocab