def METHOD_NAME(self, s3_transfer_allow, s3_transfer_buckets):
    if s3_transfer_allow and s3_transfer_buckets:
        external_policy = _get_external_bucket_policy(s3_transfer_buckets)
        if not isinstance(external_policy, dict):
            _build_external_bucket_json(s3_transfer_buckets)
            external_policy = _get_external_bucket_policy(s3_transfer_buckets)
        if external_policy:
            self._external_policy = external_policy