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14.3.4.1 Defining a callback option

As always, the easiest way to define a callback option is by using the `parser.add_option()` method. Apart from `action`, the only option attribute you must specify is `callback`, the function to call:

    parser.add_option("-c", action="callback", callback=my_callback)

`callback` is a function (or other callable object), so you must have already defined `my_callback()` when you create this callback option. In this simple case, `optparse` doesn't even know if -c takes any arguments, which usually means that the option takes no arguments--the mere presence of -c on the command-line is all it needs to know. In some circumstances, though, you might want your callback to consume an arbitrary number of command-line arguments. This is where writing callbacks gets tricky; it's covered later in this section.

`optparse` always passes four particular arguments to your callback, and it will only pass additional arguments if you specify them via `callback_args` and `callback_kwargs`. Thus, the minimal callback function signature is:

    def my_callback(option, opt, value, parser):

The four arguments to a callback are described below.

There are several other option attributes that you can supply when you define a callback option:

`type`
  has its usual meaning: as with the `store` or `append` actions, it instructs `optparse` to consume one argument and convert it to `type`. Rather than storing the converted value(s) anywhere, though, `optparse` passes it to your callback function.
`nargs`
  also has its usual meaning: if it is supplied and > 1, `optparse` will consume `nargs` arguments, each of which must be convertible to `type`. It then passes a tuple of converted values to your callback.
`callback_args`
  a tuple of extra positional arguments to pass to the callback
`callback_kwargs`
  a dictionary of extra keyword arguments to pass to the callback