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55 | <H3><A NAME="SECTION0082152000000000000000"></A><A NAME="optparse-adding-new-actions"></A> | |
56 | <BR> | |
57 | 6.21.5.2 Adding new actions | |
58 | </H3> | |
59 | ||
60 | <P> | |
61 | Adding new actions is a bit trickier, because you have to understand | |
62 | that <tt class="module">optparse</tt> has a couple of classifications for actions: | |
63 | <DL> | |
64 | <DT><STRONG>``store'' actions</STRONG></DT> | |
65 | <DD>actions that result in <tt class="module">optparse</tt> storing a value to an attribute of the | |
66 | current OptionValues instance; these options require a <tt class="member">dest</tt> | |
67 | attribute to be supplied to the Option constructor | |
68 | </DD> | |
69 | <DT><STRONG>``typed'' actions</STRONG></DT> | |
70 | <DD>actions that take a value from the command line and expect it to be | |
71 | of a certain type; or rather, a string that can be converted to a | |
72 | certain type. These options require a <tt class="member">type</tt> attribute to the | |
73 | Option constructor. | |
74 | </DD> | |
75 | </DL> | |
76 | ||
77 | <P> | |
78 | These are overlapping sets: some default ``store'' actions are <code>store</code>, | |
79 | <code>store_const</code>, <code>append</code>, and <code>count</code>, while the default ``typed'' | |
80 | actions are <code>store</code>, <code>append</code>, and <code>callback</code>. | |
81 | ||
82 | <P> | |
83 | When you add an action, you need to decide if it's a ``store'' action, a | |
84 | ``typed'' action, neither, or both. Three class attributes of | |
85 | Option (or your Option subclass) control this: | |
86 | <DL> | |
87 | <DT><STRONG><tt class="member">ACTIONS</tt></STRONG></DT> | |
88 | <DD>all actions must be listed in ACTIONS | |
89 | </DD> | |
90 | <DT><STRONG><tt class="member">STORE_ACTIONS</tt></STRONG></DT> | |
91 | <DD>``store'' actions are additionally listed here | |
92 | </DD> | |
93 | <DT><STRONG><tt class="member">TYPED_ACTIONS</tt></STRONG></DT> | |
94 | <DD>``typed'' actions are additionally listed here | |
95 | </DD> | |
96 | </DL> | |
97 | ||
98 | <P> | |
99 | In order to actually implement your new action, you must override | |
100 | Option's <tt class="method">take_action()</tt> method and add a case that recognizes your | |
101 | action. | |
102 | ||
103 | <P> | |
104 | For example, let's add an <code>extend</code> action. This is similar to the | |
105 | standard <code>append</code> action, but instead of taking a single value from | |
106 | the command-line and appending it to an existing list, <code>extend</code> will | |
107 | take multiple values in a single comma-delimited string, and extend an | |
108 | existing list with them. That is, if <code>"-names"</code> is an <code>extend</code> | |
109 | option of type <code>string</code>, the command line | |
110 | <div class="verbatim"><pre> | |
111 | --names=foo,bar --names blah --names ding,dong | |
112 | </pre></div> | |
113 | ||
114 | <P> | |
115 | would result in a list | |
116 | <div class="verbatim"><pre> | |
117 | ["foo", "bar", "blah", "ding", "dong"] | |
118 | </pre></div> | |
119 | ||
120 | <P> | |
121 | Again we define a subclass of Option: | |
122 | <div class="verbatim"><pre> | |
123 | class MyOption (Option): | |
124 | ||
125 | ACTIONS = Option.ACTIONS + ("extend",) | |
126 | STORE_ACTIONS = Option.STORE_ACTIONS + ("extend",) | |
127 | TYPED_ACTIONS = Option.TYPED_ACTIONS + ("extend",) | |
128 | ||
129 | def take_action(self, action, dest, opt, value, values, parser): | |
130 | if action == "extend": | |
131 | lvalue = value.split(",") | |
132 | values.ensure_value(dest, []).extend(lvalue) | |
133 | else: | |
134 | Option.take_action( | |
135 | self, action, dest, opt, value, values, parser) | |
136 | </pre></div> | |
137 | ||
138 | <P> | |
139 | Features of note: | |
140 | ||
141 | <UL> | |
142 | <LI> | |
143 | <code>extend</code> both expects a value on the command-line and stores that | |
144 | value somewhere, so it goes in both <tt class="member">STORE_ACTIONS</tt> and | |
145 | <tt class="member">TYPED_ACTIONS</tt> | |
146 | ||
147 | <P> | |
148 | </LI> | |
149 | <LI> | |
150 | <tt class="method">MyOption.take_action()</tt> implements just this one new action, and | |
151 | passes control back to <tt class="method">Option.take_action()</tt> for the standard | |
152 | <tt class="module">optparse</tt> actions | |
153 | ||
154 | <P> | |
155 | </LI> | |
156 | <LI> | |
157 | <code>values</code> is an instance of the optparse_parser.Values class, | |
158 | which provides the very useful <tt class="method">ensure_value()</tt> method. | |
159 | <tt class="method">ensure_value()</tt> is essentially <tt class="function">getattr()</tt> with a safety valve; | |
160 | it is called as | |
161 | <div class="verbatim"><pre> | |
162 | values.ensure_value(attr, value) | |
163 | </pre></div> | |
164 | ||
165 | <P> | |
166 | If the <code>attr</code> attribute of <code>values</code> doesn't exist or is None, then | |
167 | ensure_value() first sets it to <code>value</code>, and then returns 'value. | |
168 | This is very handy for actions like <code>extend</code>, <code>append</code>, and | |
169 | <code>count</code>, all of which accumulate data in a variable and expect that | |
170 | variable to be of a certain type (a list for the first two, an integer | |
171 | for the latter). Using <tt class="method">ensure_value()</tt> means that scripts using | |
172 | your action don't have to worry about setting a default value for the | |
173 | option destinations in question; they can just leave the default as | |
174 | None and <tt class="method">ensure_value()</tt> will take care of getting it right when | |
175 | it's needed. | |
176 | ||
177 | <P> | |
178 | </LI> | |
179 | </UL> | |
180 | ||
181 | <P> | |
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