def get_input_output_filenames_from_args(args):
    """Return strings inserted to input and output filenames."""
    input_filename = args.input_filename
    output_filename = args.output_filename
    if args.input_output_filename is not None:
        input_filename = args.input_output_filename
        output_filename = args.input_output_filename
    return input_filename, output_filename