<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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