<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_hash(self, keys=[]):
 """Return a unique hash associated with the listed keys."""

        if not len(keys):
            keys = list(self.keys())

        string_rep = ''
        oself = self._ordered(deepcopy(self))
        for key in keys:
            string_rep += json.dumps(oself.get(key, ''), sort_keys=True)

        return hashlib.sha512(string_rep.encode()).hexdigest()[:16]