root@dangerzone:~# chcat -l -- +SystemLow,+SystemHigh john
/usr/bin/chcat: Requires at least one category
root@dangerzone:~#

def chcat_add(orig, newcat, objects,login_ind):
    if len(newcat) == 1:
        aise ValueError(_("Requires at least one category"))

cats = cmds[0].split(",")
mod_ind = 0
errors = 0
objects = cmds[1:]
try:
    if check_replace(cats):
        errors = chcat_replace(translate(cats), objects, login_ind)
    else:
        for c in cats:
            l = []
            l.append(c[1:])
            if len(c) > 0 and c[0] == "+":
                errors += chcat_add(c[1:],translate(l), objects, login_ind)
                continue
            [...]