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<H3><A NAME="SECTION0082141000000000000000"></A><A NAME="optparse-defining-callback-option"></A>
<BR>
6.21.4.1 Defining a callback option
</H3>

<P>
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)
</pre></div>

<P>
<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.

<P>
<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
function signature is:
<div class="verbatim"><pre>
def my_callback(option, opt, value, parser):
</pre></div>

<P>
The four arguments to a callback are described below.

<P>
There are several other option attributes that you can supply when you
define a callback option:
<DL>
<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.
</DD>
<DT><STRONG><code>nargs</code></STRONG></DT>
<DD>also has its usual meaning: if it is supplied and &gt; 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
callback.
</DD>
<DT><STRONG><code>callback_args</code></STRONG></DT>
<DD>a tuple of extra positional arguments to pass to the callback
</DD>
<DT><STRONG><code>callback_kwargs</code></STRONG></DT>
<DD>a dictionary of extra keyword arguments to pass to the callback
</DD>
</DL>

<P>

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