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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 _unpack(self, data):
 """ Unpacks the given byte string and parses the result from JSON. Returns None on failure and saves data into "self.buffer". """

        msg_magic, msg_length, msg_type = self._unpack_header(data)
        msg_size = self._struct_header_size + msg_length
        # XXX: Message shouldn't be any longer than the data
        payload = data[self._struct_header_size:msg_size]
        return payload.decode('utf-8', 'replace')