As always, you can define a callback option either by directly instantiating the Option class, or by using the add_option() method of your OptionParser object. The only option attribute you must specify is callback, the function to call:
Note that you supply a function object here--so you must have already defined a function my_callback() when you define the callback option. In this simple case, optparse knows nothing about the arguments the -c option expects to take. Usually, this means that the option doesn't take any 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 document.
There are several other option attributes that you can supply when you define an option attribute:
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