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| 55 | <H3><A NAME="SECTION0082133000000000000000"></A><A NAME="optparse-defining-options"></A> |
| 56 | <BR> |
| 57 | 6.21.3.3 Defining options |
| 58 | </H3> |
| 59 | |
| 60 | <P> |
| 61 | Each Option instance represents a set of synonymous command-line option |
| 62 | strings, e.g. <b class="programopt">-f</b> and <b class="programopt">--file</b>. You can |
| 63 | specify any number of short or long option strings, but you must specify |
| 64 | at least one overall option string. |
| 65 | |
| 66 | <P> |
| 67 | The 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> |
| 70 | parser.add_option(opt_str[, ...], attr=value, ...) |
| 71 | </pre></div> |
| 72 | |
| 73 | <P> |
| 74 | To define an option with only a short option string: |
| 75 | <div class="verbatim"><pre> |
| 76 | parser.add_option("-f", attr=value, ...) |
| 77 | </pre></div> |
| 78 | |
| 79 | <P> |
| 80 | And to define an option with only a long option string: |
| 81 | <div class="verbatim"><pre> |
| 82 | parser.add_option("--foo", attr=value, ...) |
| 83 | </pre></div> |
| 84 | |
| 85 | <P> |
| 86 | The keyword arguments define attributes of the new Option object. The |
| 87 | most important option attribute is <tt class="member">action</tt>, and it largely determines |
| 88 | which other attributes are relevant or required. If you pass irrelevant |
| 89 | option attributes, or fail to pass required ones, <tt class="module">optparse</tt> raises an |
| 90 | OptionError exception explaining your mistake. |
| 91 | |
| 92 | <P> |
| 93 | An options's <em>action</em> determines what <tt class="module">optparse</tt> does when it encounters this |
| 94 | option 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 |
| 120 | documentation 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 |
| 126 | action, you may also supply <tt class="member">type</tt> and <tt class="member">dest</tt> option attributes; see |
| 127 | below.) |
| 128 | |
| 129 | <P> |
| 130 | As you can see, most actions involve storing or updating a value |
| 131 | somewhere. <tt class="module">optparse</tt> always creates a special object for this, |
| 132 | conventionally called <code>options</code> (it happens to be an instance of |
| 133 | <code>optparse.Values</code>). Option arguments (and various other values) are |
| 134 | stored as attributes of this object, according to the <tt class="member">dest</tt> |
| 135 | (destination) option attribute. |
| 136 | |
| 137 | <P> |
| 138 | For example, when you call |
| 139 | <div class="verbatim"><pre> |
| 140 | parser.parse_args() |
| 141 | </pre></div> |
| 142 | |
| 143 | <P> |
| 144 | one of the first things <tt class="module">optparse</tt> does is create the <code>options</code> object: |
| 145 | <div class="verbatim"><pre> |
| 146 | options = Values() |
| 147 | </pre></div> |
| 148 | |
| 149 | <P> |
| 150 | If one of the options in this parser is defined with |
| 151 | <div class="verbatim"><pre> |
| 152 | parser.add_option("-f", "--file", action="store", type="string", dest="filename") |
| 153 | </pre></div> |
| 154 | |
| 155 | <P> |
| 156 | and 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> |
| 165 | then <tt class="module">optparse</tt>, on seeing this option, will do the equivalent of |
| 166 | <div class="verbatim"><pre> |
| 167 | options.filename = "foo" |
| 168 | </pre></div> |
| 169 | |
| 170 | <P> |
| 171 | The <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> |
| 173 | options. |
| 174 | |
| 175 | <P> |
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