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14.3.4.8 Callback example 5: fixed arguments

Things get slightly more interesting when you define callback options that take a fixed number of arguments. Specifying that a callback option takes arguments is similar to defining a `store` or `append` option: if you define `type`, then the option takes one argument that must be convertible to that type; if you further define `nargs`, then the option takes `nargs` arguments.

Here's an example that just emulates the standard `store` action:

    def store_value(option, opt_str, value, parser):
        setattr(parser.values, option.dest, value)
    [...]
    parser.add_option("--foo",
                      action="callback", callback=store_value,
                      type="int", nargs=3, dest="foo")

Note that `optparse` takes care of consuming 3 arguments and converting them to integers for you; all you have to do is store them. (Or whatever; obviously you don't need a callback for this example.)