def _convert_scalars_to_slices(indexers):
    """Convert a dict of xarray dimension-index pairs to solely use slices.

    Assumes that the index values have been validated already in
    _validate_indexers.

    Parameters
    ----------
    indexers : dict
        Dictionary mapping dimension names to integers or slices.

    Returns
    -------
    dict
    """
    result = {}
    for k, v in indexers.items():
        if isinstance(v, slice):
            result[k] = v
        else:
            if v == -1:
                result[k] = slice(v, None)
            else:
                result[k] = slice(v, v + 1)
    return result