def _readone_slice(dset, positional_indices):
    """Read a slice from an xarray.Dataset.

    Parameters
    ----------

    dset                : file_object
                         xarray.Dataset object
    positional_indices  : dict
                          dict containing positional indices for each dimension
                          e.g. {'lat': 0, 'lon': 0}

    Returns
    ---------
    chunk               : xarray.Dataset
                         a subset of the Xarray Dataset

    """

    # Change the positional indices into slice objects
    # e.g {'lat': 0, 'lon': 0} ---> {'lat': slice(0, 1, None),  'lon': slice(0, 1, None)}
    positional_slices = {dim: slice(positional_indices[dim], positional_indices[dim]+1) 
                                                         for dim in positional_indices}

    # Read a slice for the given positional_slices
    chunk = dset[positional_slices]
    return chunk