projects / OpenSPARC-T2-SAM / blame
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| 54 | ||
| 55 | <H3><A NAME="SECTION0082126000000000000000"></A><A NAME="optparse-generating-help"></A> | |
| 56 | <BR> | |
| 57 | 6.21.2.6 Generating help | |
| 58 | </H3> | |
| 59 | ||
| 60 | <P> | |
| 61 | <tt class="module">optparse</tt>'s ability to generate help and usage text automatically is useful | |
| 62 | for creating user-friendly command-line interfaces. All you have to do | |
| 63 | is supply a <tt class="member">help</tt> value for each option, and optionally a short usage | |
| 64 | message for your whole program. Here's an OptionParser populated with | |
| 65 | user-friendly (documented) options: | |
| 66 | <div class="verbatim"><pre> | |
| 67 | usage = "usage: %prog [options] arg1 arg2" | |
| 68 | parser = OptionParser(usage=usage) | |
| 69 | parser.add_option("-v", "--verbose", | |
| 70 | action="store_true", dest="verbose", default=True, | |
| 71 | help="make lots of noise [default]") | |
| 72 | parser.add_option("-q", "--quiet", | |
| 73 | action="store_false", dest="verbose", | |
| 74 | help="be vewwy quiet (I'm hunting wabbits)") | |
| 75 | parser.add_option("-f", "--filename", | |
| 76 | metavar="FILE", help="write output to FILE"), | |
| 77 | parser.add_option("-m", "--mode", | |
| 78 | default="intermediate", | |
| 79 | help="interaction mode: novice, intermediate, " | |
| 80 | "or expert [default: %default]") | |
| 81 | </pre></div> | |
| 82 | ||
| 83 | <P> | |
| 84 | If <tt class="module">optparse</tt> encounters either <code>"-h"</code> or <code>"-help"</code> on the command-line, | |
| 85 | or if you just call <tt class="method">parser.print_help()</tt>, it prints the following to | |
| 86 | standard output: | |
| 87 | <div class="verbatim"><pre> | |
| 88 | usage: <yourscript> [options] arg1 arg2 | |
| 89 | ||
| 90 | options: | |
| 91 | -h, --help show this help message and exit | |
| 92 | -v, --verbose make lots of noise [default] | |
| 93 | -q, --quiet be vewwy quiet (I'm hunting wabbits) | |
| 94 | -f FILE, --filename=FILE | |
| 95 | write output to FILE | |
| 96 | -m MODE, --mode=MODE interaction mode: novice, intermediate, or | |
| 97 | expert [default: intermediate] | |
| 98 | </pre></div> | |
| 99 | ||
| 100 | <P> | |
| 101 | (If the help output is triggered by a help option, <tt class="module">optparse</tt> exits after | |
| 102 | printing the help text.) | |
| 103 | ||
| 104 | <P> | |
| 105 | There's a lot going on here to help <tt class="module">optparse</tt> generate the best possible | |
| 106 | help message: | |
| 107 | ||
| 108 | <UL> | |
| 109 | <LI> | |
| 110 | the script defines its own usage message: | |
| 111 | <div class="verbatim"><pre> | |
| 112 | usage = "usage: %prog [options] arg1 arg2" | |
| 113 | </pre></div> | |
| 114 | ||
| 115 | <P> | |
| 116 | <tt class="module">optparse</tt> expands <code>"%prog"</code> in the usage string to the name of the current | |
| 117 | program, i.e. <code>os.path.basename(sys.argv[0])</code>. The expanded string | |
| 118 | is then printed before the detailed option help. | |
| 119 | ||
| 120 | <P> | |
| 121 | If you don't supply a usage string, <tt class="module">optparse</tt> uses a bland but sensible | |
| 122 | default: ``<code>usage: %prog [options]"</code>, which is fine if your script | |
| 123 | doesn't take any positional arguments. | |
| 124 | ||
| 125 | <P> | |
| 126 | </LI> | |
| 127 | <LI> | |
| 128 | every option defines a help string, and doesn't worry about line- | |
| 129 | wrapping--<tt class="module">optparse</tt> takes care of wrapping lines and making the | |
| 130 | help output look good. | |
| 131 | ||
| 132 | <P> | |
| 133 | </LI> | |
| 134 | <LI> | |
| 135 | options that take a value indicate this fact in their | |
| 136 | automatically-generated help message, e.g. for the ``mode'' option: | |
| 137 | <div class="verbatim"><pre> | |
| 138 | -m MODE, --mode=MODE | |
| 139 | </pre></div> | |
| 140 | ||
| 141 | <P> | |
| 142 | Here, ``MODE'' is called the meta-variable: it stands for the argument | |
| 143 | that the user is expected to supply to <b class="programopt">-m</b>/<b class="programopt">--mode</b>. By default, | |
| 144 | <tt class="module">optparse</tt> converts the destination variable name to uppercase and uses | |
| 145 | that for the meta-variable. Sometimes, that's not what you want--for example, the <b class="programopt">--filename</b> option explicitly sets | |
| 146 | <code>metavar="FILE"</code>, resulting in this automatically-generated option | |
| 147 | description: | |
| 148 | <div class="verbatim"><pre> | |
| 149 | -f FILE, --filename=FILE | |
| 150 | </pre></div> | |
| 151 | ||
| 152 | <P> | |
| 153 | This is important for more than just saving space, though: the | |
| 154 | manually written help text uses the meta-variable ``FILE'' to clue the | |
| 155 | user in that there's a connection between the semi-formal syntax ``-f | |
| 156 | FILE'' and the informal semantic description ``write output to FILE''. | |
| 157 | This is a simple but effective way to make your help text a lot | |
| 158 | clearer and more useful for end users. | |
| 159 | ||
| 160 | <P> | |
| 161 | </LI> | |
| 162 | <LI> | |
| 163 | options that have a default value can include <code>%default</code> in | |
| 164 | the help string--<tt class="module">optparse</tt> will replace it with <tt class="function">str()</tt> of the | |
| 165 | option's default value. If an option has no default value (or the | |
| 166 | default value is <code>None</code>), <code>%default</code> expands to <code>none</code>. | |
| 167 | ||
| 168 | <P> | |
| 169 | </LI> | |
| 170 | </UL> | |
| 171 | ||
| 172 | <P> | |
| 173 | ||
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| 208 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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