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:
parser.add_option("-c", callback=my_callback)
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:
has its usual meaning: as with the ``store'' or ``append'' actions, it
instructs optparse to consume one argument that must be
convertible to type. Rather than storing the value(s) anywhere,
though, optparse converts it to type and passes it to
your callback function.
also has its usual meaning: if it is supplied and "nargs > 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.
