def parse_slice_idx_to_str(slice_idx):
    """
    Parse the slice index to a three digit string for saving and reading the
    2D .npy files generated by io.preprocess.Preprocessor.
    Naming convention: {type of slice}_{case}_{slice_idx}
        * adding 0s to slice_idx until it reaches 3 digits,
        * so sorting files is easier when stacking
    """
    return f"{slice_idx:03}"