Here's a slightly more interesting example: record the fact that
"-a" is seen, but blow up if it comes after "-b" in the
command-line.
def check_order(option, opt_str, value, parser):
if parser.values.b:
raise OptionValueError("can't use -a after -b")
parser.values.a = 1
[...]
parser.add_option("-a", action="callback", callback=check_order)
parser.add_option("-b", action="store_true", dest="b")