def set_iam_policy(self, policy, client=None):
        """Update the IAM policy for the bucket.

        See
        https://cloud.google.com/storage/docs/json_api/v1/buckets/setIamPolicy

        If :attr:`user_project` is set, bills the API request to that project.

        :type policy: :class:`google.api_core.iam.Policy`
        :param policy: policy instance used to update bucket's IAM policy.

        :type client: :class:`~google.cloud.storage.client.Client` or
                      ``NoneType``
        :param client: Optional. The client to use.  If not passed, falls back
                       to the ``client`` stored on the current bucket.

        :rtype: :class:`google.api_core.iam.Policy`
        :returns: the policy instance, based on the resource returned from
                  the ``setIamPolicy`` API request.
        """
        client = self._require_client(client)
        query_params = {}

        if self.user_project is not None:
            query_params["userProject"] = self.user_project

        resource = policy.to_api_repr()
        resource["resourceId"] = self.path
        info = client._connection.api_request(
            method="PUT",
            path="%s/iam" % (self.path,),
            query_params=query_params,
            data=resource,
            _target_object=None,
        )
        return Policy.from_api_repr(info)