code: def from_json(cls, json_str):
          
          vocab_dict = json.loads(json_str)
  
          unknown_token = vocab_dict.get('unknown_token')
          bert_vocab = cls(unknown_token=unknown_token)
          bert_vocab._idx_to_token = vocab_dict.get('idx_to_token')
          bert_vocab._token_to_idx = vocab_dict.get('token_to_idx')
          if unknown_token:
              bert_vocab._token_to_idx = DefaultLookupDict(bert_vocab._token_to_idx[unknown_token],
                                                           bert_vocab._token_to_idx)
          bert_vocab._reserved_tokens = vocab_dict.get('reserved_tokens')
          bert_vocab._padding_token = vocab_dict.get('padding_token')
          bert_vocab._bos_token = vocab_dict.get('bos_token')
          bert_vocab._eos_token = vocab_dict.get('eos_token')
          bert_vocab._mask_token = vocab_dict.get('mask_token')
          bert_vocab._sep_token = vocab_dict.get('sep_token')
          bert_vocab._cls_token = vocab_dict.get('cls_token')
  
          return bert_vocab
docs: Deserialize BERTVocab object from json string.
  
          Parameters
          ----------
          json_str : str
              Serialized json string of a BERTVocab object.
  
          Returns
          -------
          BERTVocab