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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def from_json(cls, json_str):
 """Deserialize BERTVocab object from json string. Parameters json_str : str Serialized json string of a BERTVocab object. Returns ------- BERTVocab """

        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