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| 55 | <H3><A NAME="SECTION0082128000000000000000"></A><A NAME="optparse-how-optparse-handles-errors"></A> | |
| 56 | <BR> | |
| 57 | 6.21.2.8 How <tt class="module">optparse</tt> handles errors | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | There are two broad classes of errors that <tt class="module">optparse</tt> has to worry about: | |
| 62 | programmer errors and user errors. Programmer errors are usually | |
| 63 | erroneous calls to <code>parser.add_option()</code>, e.g. invalid option strings, | |
| 64 | unknown option attributes, missing option attributes, etc. These are | |
| 65 | dealt with in the usual way: raise an exception (either | |
| 66 | <code>optparse.OptionError</code> or <code>TypeError</code>) and let the program crash. | |
| 67 | ||
| 68 | <P> | |
| 69 | Handling user errors is much more important, since they are guaranteed | |
| 70 | to happen no matter how stable your code is. <tt class="module">optparse</tt> can automatically | |
| 71 | detect some user errors, such as bad option arguments (passing <code>"-n | |
| 72 | 4x"</code> where <b class="programopt">-n</b> takes an integer argument), missing arguments | |
| 73 | (<code>"-n"</code> at the end of the command line, where <b class="programopt">-n</b> takes an argument | |
| 74 | of any type). Also, you can call <code>parser.error()</code> to signal an | |
| 75 | application-defined error condition: | |
| 76 | <div class="verbatim"><pre> | |
| 77 | (options, args) = parser.parse_args() | |
| 78 | [...] | |
| 79 | if options.a and options.b: | |
| 80 | parser.error("options -a and -b are mutually exclusive") | |
| 81 | </pre></div> | |
| 82 | ||
| 83 | <P> | |
| 84 | In either case, <tt class="module">optparse</tt> handles the error the same way: it prints the | |
| 85 | program's usage message and an error message to standard error and | |
| 86 | exits with error status 2. | |
| 87 | ||
| 88 | <P> | |
| 89 | Consider the first example above, where the user passes <code>"4x"</code> to an | |
| 90 | option that takes an integer: | |
| 91 | <div class="verbatim"><pre> | |
| 92 | $ /usr/bin/foo -n 4x | |
| 93 | usage: foo [options] | |
| 94 | ||
| 95 | foo: error: option -n: invalid integer value: '4x' | |
| 96 | </pre></div> | |
| 97 | ||
| 98 | <P> | |
| 99 | Or, where the user fails to pass a value at all: | |
| 100 | <div class="verbatim"><pre> | |
| 101 | $ /usr/bin/foo -n | |
| 102 | usage: foo [options] | |
| 103 | ||
| 104 | foo: error: -n option requires an argument | |
| 105 | </pre></div> | |
| 106 | ||
| 107 | <P> | |
| 108 | <tt class="module">optparse</tt>-generated error messages take care always to mention the option | |
| 109 | involved in the error; be sure to do the same when calling | |
| 110 | <code>parser.error()</code> from your application code. | |
| 111 | ||
| 112 | <P> | |
| 113 | If <tt class="module">optparse</tt>'s default error-handling behaviour does not suite your needs, | |
| 114 | you'll need to subclass OptionParser and override <code>exit()</code> and/or | |
| 115 | <tt class="method">error()</tt>. | |
| 116 | ||
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