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)
