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<H3><A NAME="SECTION0082052000000000000000"><!--x--></A><A NAME="optparse-adding-actions"><!--z--></A>
<BR>
6.20.5.2 Adding new actions
</H3>
<P>
Adding new actions is a bit trickier, because you have to understand
that <tt class="module">optparse</tt> has a couple of classifications for actions:
<P>
<dl class="definitions">
<dt><b><a id='l2h-2040'>``store'' actions</a></b></dt>
<dd>
actions that result in <tt class="module">optparse</tt> storing a value to an attribute
of the OptionValues instance; these options require a <var>dest</var>
attribute to be supplied to the Option constructor
<dt><b><a id='l2h-2041'>``typed'' actions</a></b></dt>
<dd>
actions that take a value from the command line and expect it to be
of a certain type; or rather, a string that can be converted to a
certain type. These options require a <var>type</var> attribute to the
Option constructor.
</dl>
<P>
Some default ``store'' actions are <var>store</var>, <var>store_const</var>,
<var>append</var>, and <var>count</var>. The default ``typed'' actions are
<var>store</var>, <var>append</var>, and <var>callback</var>.
<P>
When you add an action, you need to decide if it's a ``store'' action,
a ``typed'', neither, or both. Three class attributes of
<tt class="class">Option</tt> (or your <tt class="class">Option</tt> subclass) control this:
<P>
<dl><dt><b><tt id='l2h-2037' class="member">ACTIONS</tt></b></dt>
<dd>
All actions must be listed as strings in ACTIONS.
</dl>
<dl><dt><b><tt id='l2h-2038' class="member">STORE_ACTIONS</tt></b></dt>
<dd>
``store'' actions are additionally listed here.
</dl>
<dl><dt><b><tt id='l2h-2039' class="member">TYPED_ACTIONS</tt></b></dt>
<dd>
``typed'' actions are additionally listed here.
</dl>
<P>
In order to actually implement your new action, you must override
<tt class="class">Option</tt>'s <tt class="method">take_action()</tt> method and add a case that
recognizes your action.
<P>
For example, let's add an ``extend'' action. This is similar to the
standard ``append'' action, but instead of taking a single value from
the command-line and appending it to an existing list, ``extend'' will
take multiple values in a single comma-delimited string, and extend an
existing list with them. That is, if <b class="programopt">--names</b> is an
``extend'' option of type string, the command line:
<P>
<div class="verbatim"><pre>
--names=foo,bar --names blah --names ding,dong
</pre></div>
<P>
would result in a list:
<P>
<div class="verbatim"><pre>
["foo", "bar", "blah", "ding", "dong"]
</pre></div>
<P>
Again we define a subclass of <tt class="class">Option</tt>:
<P>
<div class="verbatim"><pre>
class MyOption (Option):
ACTIONS = Option.ACTIONS + ("extend",)
STORE_ACTIONS = Option.STORE_ACTIONS + ("extend",)
TYPED_ACTIONS = Option.TYPED_ACTIONS + ("extend",)
def take_action (self, action, dest, opt, value, values, parser):
if action == "extend":
lvalue = value.split(",")
values.ensure_value(dest, []).extend(lvalue)
else:
Option.take_action(
self, action, dest, opt, value, values, parser)
</pre></div>
<P>
Features of note:
<P>
<UL>
<LI>``extend'' both expects a value on the command-line and stores that
value somewhere, so it goes in both <tt class="member">STORE_ACTIONS</tt> and
<tt class="member">TYPED_ACTIONS</tt>.
<P>
</LI>
<LI><tt class="method">MyOption.take_action()</tt> implements just this one new
action, and passes control back to <tt class="method">Option.take_action()</tt> for
the standard <tt class="module">optparse</tt> actions.
<P>
</LI>
<LI><var>values</var> is an instance of the <tt class="class">Values</tt> class, which
provides the very useful <tt class="method">ensure_value()</tt>
method. <tt class="method">ensure_value()</tt> is essentially <tt class="function">getattr()</tt>
with a safety valve; it is called as:
<P>
<div class="verbatim"><pre>
values.ensure_value(attr, value)
</pre></div>
</LI>
</UL>
<P>
If the <tt class="member">attr</tt> attribute of <var>values</var> doesn't exist or is
<code>None</code>, then <tt class="method">ensure_value()</tt> first sets it to <var>value</var>, and
then returns <var>value</var>. This is very handy for actions like
``extend'', ``append'', and ``count'', all of which accumulate data in
a variable and expect that variable to be of a certain type (a list
for the first two, an integer for the latter). Using
<tt class="method">ensure_value()</tt> means that scripts using your action don't
have to worry about setting a default value for the option
destinations in question; they can just leave the default as <code>None</code> and
<tt class="method">ensure_value()</tt> will take care of getting it right when it's
needed.
<P>
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<span class="release-info">Release 2.3.4, documentation updated on May 20, 2004.</span>
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