Factory: create a policy from a protobuf message.

        Args:
            policy_pb (google.iam.policy_pb2.Policy): message returned by
            ``get_iam_policy`` gRPC API.

        Returns:
            :class:`Policy`: the parsed policy

def from_pb(cls, policy_pb):
        """Factory: create a policy from a protobuf message.

        Args:
            policy_pb (google.iam.policy_pb2.Policy): message returned by
            ``get_iam_policy`` gRPC API.

        Returns:
            :class:`Policy`: the parsed policy
        """
        policy = cls(policy_pb.etag, policy_pb.version)

        for binding in policy_pb.bindings:
            policy[binding.role] = sorted(binding.members)

        return policy