def _start_string(num_total, tag, num_processes):
    """Returns a string to be displayed before processing begins. It contains
    the number of total items and the number of processes. It is prefixed by the `tag`.

    Where information are not available or a division by zero would occur, a `-` or `0ms` is returned for
    that field.
    """
    fmt_string = "{tag}: Start processing {num_total} items with {num_processes} processes..."
    return fmt_string.format(
        tag=tag,
        num_total=num_total,
        num_processes=num_processes,
    )