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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 _field_names_(self):
 """ get list of field names defined in Swagger spec :return: a list of field names :rtype: a list of str """

        ret = []
        for n in six.iterkeys(self.__swagger_fields__):
            new_n = self.__swagger_rename__.get(n, None)
            ret.append(new_n) if new_n else ret.append(n)
        
        return ret