6.21.4.5 Callback example 2: check option order

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")

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