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| 55 | <H3><A NAME="SECTION0082141000000000000000"></A><A NAME="optparse-defining-callback-option"></A> | |
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| 57 | 6.21.4.1 Defining a callback option | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | As always, the easiest way to define a callback option is by using the | |
| 62 | <code>parser.add_option()</code> method. Apart from <tt class="member">action</tt>, the only option | |
| 63 | attribute you must specify is <code>callback</code>, the function to call: | |
| 64 | <div class="verbatim"><pre> | |
| 65 | parser.add_option("-c", action="callback", callback=my_callback) | |
| 66 | </pre></div> | |
| 67 | ||
| 68 | <P> | |
| 69 | <code>callback</code> is a function (or other callable object), so you must have | |
| 70 | already defined <code>my_callback()</code> when you create this callback option. | |
| 71 | In this simple case, <tt class="module">optparse</tt> doesn't even know if <b class="programopt">-c</b> takes any | |
| 72 | arguments, which usually means that the option takes no arguments--the | |
| 73 | mere presence of <b class="programopt">-c</b> on the command-line is all it needs to know. In | |
| 74 | some circumstances, though, you might want your callback to consume an | |
| 75 | arbitrary number of command-line arguments. This is where writing | |
| 76 | callbacks gets tricky; it's covered later in this section. | |
| 77 | ||
| 78 | <P> | |
| 79 | <tt class="module">optparse</tt> always passes four particular arguments to your callback, and it | |
| 80 | will only pass additional arguments if you specify them via | |
| 81 | <code>callback_args</code> and <code>callback_kwargs</code>. Thus, the minimal callback | |
| 82 | function signature is: | |
| 83 | <div class="verbatim"><pre> | |
| 84 | def my_callback(option, opt, value, parser): | |
| 85 | </pre></div> | |
| 86 | ||
| 87 | <P> | |
| 88 | The four arguments to a callback are described below. | |
| 89 | ||
| 90 | <P> | |
| 91 | There are several other option attributes that you can supply when you | |
| 92 | define a callback option: | |
| 93 | <DL> | |
| 94 | <DT><STRONG><tt class="member">type</tt></STRONG></DT> | |
| 95 | <DD>has its usual meaning: as with the <code>store</code> or <code>append</code> actions, | |
| 96 | it instructs <tt class="module">optparse</tt> to consume one argument and convert it to | |
| 97 | <tt class="member">type</tt>. Rather than storing the converted value(s) anywhere, | |
| 98 | though, <tt class="module">optparse</tt> passes it to your callback function. | |
| 99 | </DD> | |
| 100 | <DT><STRONG><code>nargs</code></STRONG></DT> | |
| 101 | <DD>also has its usual meaning: if it is supplied and > 1, <tt class="module">optparse</tt> will | |
| 102 | consume <code>nargs</code> arguments, each of which must be convertible to | |
| 103 | <tt class="member">type</tt>. It then passes a tuple of converted values to your | |
| 104 | callback. | |
| 105 | </DD> | |
| 106 | <DT><STRONG><code>callback_args</code></STRONG></DT> | |
| 107 | <DD>a tuple of extra positional arguments to pass to the callback | |
| 108 | </DD> | |
| 109 | <DT><STRONG><code>callback_kwargs</code></STRONG></DT> | |
| 110 | <DD>a dictionary of extra keyword arguments to pass to the callback | |
| 111 | </DD> | |
| 112 | </DL> | |
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