def prettify_delta(delta):
    """
    Returns a human-readable string representing the given timedelta. The
    biggest unit is the hour, because a working day is too ambiguous.
    """
    d = {}

    d['minutes'], d['seconds'] = divmod(int(delta.total_seconds()), 60)
    d['hours'], d['minutes'] = divmod(d['minutes'], 60)

    li = []

    for unit in ('hours', 'minutes', 'seconds'):
        if d[unit]:
            s = str(d[unit])+' '+unit
            if d[unit] == 1:
                s = s[:-1]
            li.append(s)

    s = ', '.join(li)
    if not s:
        s = '-'

    return s