def METHOD_NAME(self):
    """Ordered indices for batching."""
    if self._ordered_indices is None:
        # Call the underlying dataset's ordered_indices() here, so that we
        # get the same random ordering as we would have from using the
        # underlying dataset directly.
        self._ordered_indices = OrderedDict(
            [
                (key, dataset.METHOD_NAME())
                for key, dataset in self.datasets.items()
            ]
        )
    return np.arange(len(self))