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<H3><A NAME="SECTION0082147000000000000000"></A><A NAME="optparse-callback-example-4"></A>
<BR>
6.21.4.7 Callback example 4: check arbitrary condition
</H3>

<P>
Of course, you could put any condition in there--you're not limited
to checking the values of already-defined options.  For example, if
you have options that should not be called when the moon is full, all
you have to do is this:
<div class="verbatim"><pre>
def check_moon(option, opt_str, value, parser):
    if is_moon_full():
        raise OptionValueError("%s option invalid when moon is full"
                               % opt_str)
    setattr(parser.values, option.dest, 1)
[...]
parser.add_option("--foo",
                  action="callback", callback=check_moon, dest="foo")
</pre></div>

<P>
(The definition of <code>is_moon_full()</code> is left as an exercise for the
reader.)

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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