def dataset(self, dataset_id, project=None):
        """Construct a reference to a dataset.

        :type dataset_id: str
        :param dataset_id: ID of the dataset.

        :type project: str
        :param project: (Optional) project ID for the dataset (defaults to
                        the project of the client).

        :rtype: :class:`google.cloud.bigquery.dataset.DatasetReference`
        :returns: a new ``DatasetReference`` instance
        """
        if project is None:
            project = self.project

        return DatasetReference(project, dataset_id)