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55<H3><A NAME="SECTION0082133000000000000000"></A><A NAME="optparse-defining-options"></A>
56<BR>
576.21.3.3 Defining options
58</H3>
59
60<P>
61Each Option instance represents a set of synonymous command-line option
62strings, e.g. <b class="programopt">-f</b> and <b class="programopt">--file</b>. You can
63specify any number of short or long option strings, but you must specify
64at least one overall option string.
65
66<P>
67The canonical way to create an Option instance is with the
68<tt class="method">add_option()</tt> method of <tt class="class">OptionParser</tt>:
69<div class="verbatim"><pre>
70parser.add_option(opt_str[, ...], attr=value, ...)
71</pre></div>
72
73<P>
74To define an option with only a short option string:
75<div class="verbatim"><pre>
76parser.add_option("-f", attr=value, ...)
77</pre></div>
78
79<P>
80And to define an option with only a long option string:
81<div class="verbatim"><pre>
82parser.add_option("--foo", attr=value, ...)
83</pre></div>
84
85<P>
86The keyword arguments define attributes of the new Option object. The
87most important option attribute is <tt class="member">action</tt>, and it largely determines
88which other attributes are relevant or required. If you pass irrelevant
89option attributes, or fail to pass required ones, <tt class="module">optparse</tt> raises an
90OptionError exception explaining your mistake.
91
92<P>
93An options's <em>action</em> determines what <tt class="module">optparse</tt> does when it encounters this
94option on the command-line. The standard option actions hard-coded into
95<tt class="module">optparse</tt> are:
96<DL>
97<DT><STRONG><code>store</code></STRONG></DT>
98<DD>store this option's argument (default)
99</DD>
100<DT><STRONG><code>store_const</code></STRONG></DT>
101<DD>store a constant value
102</DD>
103<DT><STRONG><code>store_true</code></STRONG></DT>
104<DD>store a true value
105</DD>
106<DT><STRONG><code>store_false</code></STRONG></DT>
107<DD>store a false value
108</DD>
109<DT><STRONG><code>append</code></STRONG></DT>
110<DD>append this option's argument to a list
111</DD>
112<DT><STRONG><code>count</code></STRONG></DT>
113<DD>increment a counter by one
114</DD>
115<DT><STRONG><code>callback</code></STRONG></DT>
116<DD>call a specified function
117</DD>
118<DT><STRONG><tt class="member">help</tt></STRONG></DT>
119<DD>print a usage message including all options and the
120documentation for them
121</DD>
122</DL>
123
124<P>
125(If you don't supply an action, the default is <code>store</code>. For this
126action, you may also supply <tt class="member">type</tt> and <tt class="member">dest</tt> option attributes; see
127below.)
128
129<P>
130As you can see, most actions involve storing or updating a value
131somewhere. <tt class="module">optparse</tt> always creates a special object for this,
132conventionally called <code>options</code> (it happens to be an instance of
133<code>optparse.Values</code>). Option arguments (and various other values) are
134stored as attributes of this object, according to the <tt class="member">dest</tt>
135(destination) option attribute.
136
137<P>
138For example, when you call
139<div class="verbatim"><pre>
140parser.parse_args()
141</pre></div>
142
143<P>
144one of the first things <tt class="module">optparse</tt> does is create the <code>options</code> object:
145<div class="verbatim"><pre>
146options = Values()
147</pre></div>
148
149<P>
150If one of the options in this parser is defined with
151<div class="verbatim"><pre>
152parser.add_option("-f", "--file", action="store", type="string", dest="filename")
153</pre></div>
154
155<P>
156and the command-line being parsed includes any of the following:
157<div class="verbatim"><pre>
158-ffoo
159-f foo
160--file=foo
161--file foo
162</pre></div>
163
164<P>
165then <tt class="module">optparse</tt>, on seeing this option, will do the equivalent of
166<div class="verbatim"><pre>
167options.filename = "foo"
168</pre></div>
169
170<P>
171The <tt class="member">type</tt> and <tt class="member">dest</tt> option attributes are almost as important as
172<tt class="member">action</tt>, but <tt class="member">action</tt> is the only one that makes sense for <em>all</em>
173options.
174
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