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<H3><A NAME="SECTION0082133000000000000000"></A><A NAME="optparse-defining-options"></A>
<BR>
6.21.3.3 Defining options
</H3>

<P>
Each Option instance represents a set of synonymous command-line option
strings, e.g. <b class="programopt">-f</b> and <b class="programopt">--file</b>.  You can
specify any number of short or long option strings, but you must specify
at least one overall option string.

<P>
The canonical way to create an Option instance is with the
<tt class="method">add_option()</tt> method of <tt class="class">OptionParser</tt>:
<div class="verbatim"><pre>
parser.add_option(opt_str[, ...], attr=value, ...)
</pre></div>

<P>
To define an option with only a short option string:
<div class="verbatim"><pre>
parser.add_option("-f", attr=value, ...)
</pre></div>

<P>
And to define an option with only a long option string:
<div class="verbatim"><pre>
parser.add_option("--foo", attr=value, ...)
</pre></div>

<P>
The keyword arguments define attributes of the new Option object.  The
most important option attribute is <tt class="member">action</tt>, and it largely determines
which other attributes are relevant or required.  If you pass irrelevant
option attributes, or fail to pass required ones, <tt class="module">optparse</tt> raises an
OptionError exception explaining your mistake.

<P>
An options's <em>action</em> determines what <tt class="module">optparse</tt> does when it encounters this
option on the command-line.  The standard option actions hard-coded into
<tt class="module">optparse</tt> are:
<DL>
<DT><STRONG><code>store</code></STRONG></DT>
<DD>store this option's argument (default)
</DD>
<DT><STRONG><code>store_const</code></STRONG></DT>
<DD>store a constant value
</DD>
<DT><STRONG><code>store_true</code></STRONG></DT>
<DD>store a true value
</DD>
<DT><STRONG><code>store_false</code></STRONG></DT>
<DD>store a false value
</DD>
<DT><STRONG><code>append</code></STRONG></DT>
<DD>append this option's argument to a list
</DD>
<DT><STRONG><code>count</code></STRONG></DT>
<DD>increment a counter by one
</DD>
<DT><STRONG><code>callback</code></STRONG></DT>
<DD>call a specified function
</DD>
<DT><STRONG><tt class="member">help</tt></STRONG></DT>
<DD>print a usage message including all options and the
documentation for them
</DD>
</DL>

<P>
(If you don't supply an action, the default is <code>store</code>.  For this
action, you may also supply <tt class="member">type</tt> and <tt class="member">dest</tt> option attributes; see
below.)

<P>
As you can see, most actions involve storing or updating a value
somewhere.  <tt class="module">optparse</tt> always creates a special object for this,
conventionally called <code>options</code> (it happens to be an instance of
<code>optparse.Values</code>).  Option arguments (and various other values) are
stored as attributes of this object, according to the <tt class="member">dest</tt>
(destination) option attribute.

<P>
For example, when you call
<div class="verbatim"><pre>
parser.parse_args()
</pre></div>

<P>
one of the first things <tt class="module">optparse</tt> does is create the <code>options</code> object:
<div class="verbatim"><pre>
options = Values()
</pre></div>

<P>
If one of the options in this parser is defined with
<div class="verbatim"><pre>
parser.add_option("-f", "--file", action="store", type="string", dest="filename")
</pre></div>

<P>
and the command-line being parsed includes any of the following:
<div class="verbatim"><pre>
-ffoo
-f foo
--file=foo
--file foo
</pre></div>

<P>
then <tt class="module">optparse</tt>, on seeing this option, will do the equivalent of
<div class="verbatim"><pre>
options.filename = "foo"
</pre></div>

<P>
The <tt class="member">type</tt> and <tt class="member">dest</tt> option attributes are almost as important as
<tt class="member">action</tt>, but <tt class="member">action</tt> is the only one that makes sense for <em>all</em>
options.

<P>

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