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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")