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<title>6.21.4.1 Defining a callback option
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<H3><A NAME=
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6.21.4.1 Defining a callback option
As always, the easiest way to define a callback option is by using the
<code>parser.add_option()
</code> method. Apart from
<tt class=
"member">action
</tt>, the only option
attribute you must specify is
<code>callback
</code>, the function to call:
<div class=
"verbatim"><pre>
parser.add_option(
"-c",
action=
"callback", callback=my_callback)
<code>callback
</code> is a function (or other callable object), so you must have
already defined
<code>my_callback()
</code> when you create this callback option.
In this simple case,
<tt class=
"module">optparse
</tt> doesn't even know if
<b class=
"programopt">-c
</b> takes any
arguments, which usually means that the option takes no arguments--the
mere presence of
<b class=
"programopt">-c
</b> 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.
<tt class=
"module">optparse
</tt> always passes four particular arguments to your callback, and it
will only pass additional arguments if you specify them via
<code>callback_args
</code> and
<code>callback_kwargs
</code>. Thus, the minimal callback
<div class=
"verbatim"><pre>
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:
<DT><STRONG><tt class=
"member">type
</tt></STRONG></DT>
<DD>has its usual meaning: as with the
<code>store
</code> or
<code>append
</code> actions,
it instructs
<tt class=
"module">optparse
</tt> to consume one argument and convert it to
<tt class=
"member">type
</tt>. Rather than storing the converted value(s) anywhere,
though,
<tt class=
"module">optparse
</tt> passes it to your callback function.
<DT><STRONG><code>nargs
</code></STRONG></DT>
<DD>also has its usual meaning: if it is supplied and
> 1,
<tt class=
"module">optparse
</tt> will
consume
<code>nargs
</code> arguments, each of which must be convertible to
<tt class=
"member">type
</tt>. It then passes a tuple of converted values to your
<DT><STRONG><code>callback_args
</code></STRONG></DT>
<DD>a tuple of extra positional arguments to pass to the callback
<DT><STRONG><code>callback_kwargs
</code></STRONG></DT>
<DD>a dictionary of extra keyword arguments to pass to the callback
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<span class=
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