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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 list(self, domain_id, sub_domain=None):
        '''Get a list of records, for a specific domain

        :param str domain_id: Domain ID
        :param str sub_domain: Optional. Subdomain of domain
        :return: list of records
        '''
        optional_args = {}
        if sub_domain != None:
            optional_args['sub_domain'] = sub_domain
        r = self._api.do_post('Record.List', domain_id=domain_id,
                              **optional_args)
        record_list = []
        for record in r['records']:
            record['domain_id'] = domain_id
            record_list.append(Record(record))
        return record_list