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| 55 | <H3><A NAME="SECTION0082125000000000000000"></A><A NAME="optparse-default-values"></A> | |
| 56 | <BR> | |
| 57 | 6.21.2.5 Default values | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | All of the above examples involve setting some variable (the | |
| 62 | ``destination'') when certain command-line options are seen. What happens | |
| 63 | if those options are never seen? Since we didn't supply any defaults, | |
| 64 | they are all set to <code>None</code>. This is usually fine, but sometimes you | |
| 65 | want more control. <tt class="module">optparse</tt> lets you supply a default value for each | |
| 66 | destination, which is assigned before the command line is parsed. | |
| 67 | ||
| 68 | <P> | |
| 69 | First, consider the verbose/quiet example. If we want <tt class="module">optparse</tt> to set | |
| 70 | <code>verbose</code> to <code>True</code> unless <code>"-q"</code> is seen, then we can do this: | |
| 71 | <div class="verbatim"><pre> | |
| 72 | parser.add_option("-v", action="store_true", dest="verbose", default=True) | |
| 73 | parser.add_option("-q", action="store_false", dest="verbose") | |
| 74 | </pre></div> | |
| 75 | ||
| 76 | <P> | |
| 77 | Since default values apply to the <em>destination</em> rather than to any | |
| 78 | particular option, and these two options happen to have the same | |
| 79 | destination, this is exactly equivalent: | |
| 80 | <div class="verbatim"><pre> | |
| 81 | parser.add_option("-v", action="store_true", dest="verbose") | |
| 82 | parser.add_option("-q", action="store_false", dest="verbose", default=True) | |
| 83 | </pre></div> | |
| 84 | ||
| 85 | <P> | |
| 86 | Consider this: | |
| 87 | <div class="verbatim"><pre> | |
| 88 | parser.add_option("-v", action="store_true", dest="verbose", default=False) | |
| 89 | parser.add_option("-q", action="store_false", dest="verbose", default=True) | |
| 90 | </pre></div> | |
| 91 | ||
| 92 | <P> | |
| 93 | Again, the default value for <code>verbose</code> will be <code>True</code>: the last | |
| 94 | default value supplied for any particular destination is the one that | |
| 95 | counts. | |
| 96 | ||
| 97 | <P> | |
| 98 | A clearer way to specify default values is the <tt class="method">set_defaults()</tt> | |
| 99 | method of OptionParser, which you can call at any time before calling | |
| 100 | <tt class="method">parse_args()</tt>: | |
| 101 | <div class="verbatim"><pre> | |
| 102 | parser.set_defaults(verbose=True) | |
| 103 | parser.add_option(...) | |
| 104 | (options, args) = parser.parse_args() | |
| 105 | </pre></div> | |
| 106 | ||
| 107 | <P> | |
| 108 | As before, the last value specified for a given option destination is | |
| 109 | the one that counts. For clarity, try to use one method or the other of | |
| 110 | setting default values, not both. | |
| 111 | ||
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