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6.21.4 Option Callbacks

When `optparse`'s built-in actions and types aren't quite enough for your needs, you have two choices: extend `optparse` or define a callback option. Extending `optparse` is more general, but overkill for a lot of simple cases. Quite often a simple callback is all you need.

There are two steps to defining a callback option:

- define the option itself using the `callback` action

- write the callback; this is a function (or method) that takes at least four arguments, as described below