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<H2><A NAME="SECTION0082140000000000000000"></A><A NAME="optparse-option-callbacks"></A>
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6.21.4 Option Callbacks
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<P>
When <tt class="module">optparse</tt>'s built-in actions and types aren't quite enough for your
needs, you have two choices: extend <tt class="module">optparse</tt> or define a callback option.
Extending <tt class="module">optparse</tt> is more general, but overkill for a lot of simple
cases.  Quite often a simple callback is all you need.

<P>
There are two steps to defining a callback option:

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<LI>
define the option itself using the <code>callback</code> action

<P>
</LI>
<LI>
write the callback; this is a function (or method) that
takes at least four arguments, as described below

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<LI><A href="optparse-defining-callback-option.html">6.21.4.1 Defining a callback option</a>
<LI><A href="optparse-how-callbacks-called.html">6.21.4.2 How callbacks are called</a>
<LI><A href="optparse-raising-errors-in-callback.html">6.21.4.3 Raising errors in a callback</a>
<LI><A href="optparse-callback-example-1.html">6.21.4.4 Callback example 1: trivial callback</a>
<LI><A href="optparse-callback-example-2.html">6.21.4.5 Callback example 2: check option order</a>
<LI><A href="optparse-callback-example-3.html">6.21.4.6 Callback example 3: check option order (generalized)</a>
<LI><A href="optparse-callback-example-4.html">6.21.4.7 Callback example 4: check arbitrary condition</a>
<LI><A href="optparse-callback-example-5.html">6.21.4.8 Callback example 5: fixed arguments</a>
<LI><A href="optparse-callback-example-6.html">6.21.4.9 Callback example 6: variable arguments</a>
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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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