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<H3><A NAME="SECTION0082126000000000000000"></A><A NAME="optparse-generating-help"></A>
<BR>
6.21.2.6 Generating help
</H3>
<P>
<tt class="module">optparse</tt>'s ability to generate help and usage text automatically is useful
for creating user-friendly command-line interfaces. All you have to do
is supply a <tt class="member">help</tt> value for each option, and optionally a short usage
message for your whole program. Here's an OptionParser populated with
user-friendly (documented) options:
<div class="verbatim"><pre>
usage = "usage: %prog [options] arg1 arg2"
parser = OptionParser(usage=usage)
parser.add_option("-v", "--verbose",
action="store_true", dest="verbose", default=True,
help="make lots of noise [default]")
parser.add_option("-q", "--quiet",
action="store_false", dest="verbose",
help="be vewwy quiet (I'm hunting wabbits)")
parser.add_option("-f", "--filename",
metavar="FILE", help="write output to FILE"),
parser.add_option("-m", "--mode",
default="intermediate",
help="interaction mode: novice, intermediate, "
"or expert [default: %default]")
</pre></div>
<P>
If <tt class="module">optparse</tt> encounters either <code>"-h"</code> or <code>"-help"</code> on the command-line,
or if you just call <tt class="method">parser.print_help()</tt>, it prints the following to
standard output:
<div class="verbatim"><pre>
usage: <yourscript> [options] arg1 arg2
options:
-h, --help show this help message and exit
-v, --verbose make lots of noise [default]
-q, --quiet be vewwy quiet (I'm hunting wabbits)
-f FILE, --filename=FILE
write output to FILE
-m MODE, --mode=MODE interaction mode: novice, intermediate, or
expert [default: intermediate]
</pre></div>
<P>
(If the help output is triggered by a help option, <tt class="module">optparse</tt> exits after
printing the help text.)
<P>
There's a lot going on here to help <tt class="module">optparse</tt> generate the best possible
help message:
<UL>
<LI>
the script defines its own usage message:
<div class="verbatim"><pre>
usage = "usage: %prog [options] arg1 arg2"
</pre></div>
<P>
<tt class="module">optparse</tt> expands <code>"%prog"</code> in the usage string to the name of the current
program, i.e. <code>os.path.basename(sys.argv[0])</code>. The expanded string
is then printed before the detailed option help.
<P>
If you don't supply a usage string, <tt class="module">optparse</tt> uses a bland but sensible
default: ``<code>usage: %prog [options]"</code>, which is fine if your script
doesn't take any positional arguments.
<P>
</LI>
<LI>
every option defines a help string, and doesn't worry about line-
wrapping--<tt class="module">optparse</tt> takes care of wrapping lines and making the
help output look good.
<P>
</LI>
<LI>
options that take a value indicate this fact in their
automatically-generated help message, e.g. for the ``mode'' option:
<div class="verbatim"><pre>
-m MODE, --mode=MODE
</pre></div>
<P>
Here, ``MODE'' is called the meta-variable: it stands for the argument
that the user is expected to supply to <b class="programopt">-m</b>/<b class="programopt">--mode</b>. By default,
<tt class="module">optparse</tt> converts the destination variable name to uppercase and uses
that for the meta-variable. Sometimes, that's not what you want--for example, the <b class="programopt">--filename</b> option explicitly sets
<code>metavar="FILE"</code>, resulting in this automatically-generated option
description:
<div class="verbatim"><pre>
-f FILE, --filename=FILE
</pre></div>
<P>
This is important for more than just saving space, though: the
manually written help text uses the meta-variable ``FILE'' to clue the
user in that there's a connection between the semi-formal syntax ``-f
FILE'' and the informal semantic description ``write output to FILE''.
This is a simple but effective way to make your help text a lot
clearer and more useful for end users.
<P>
</LI>
<LI>
options that have a default value can include <code>%default</code> in
the help string--<tt class="module">optparse</tt> will replace it with <tt class="function">str()</tt> of the
option's default value. If an option has no default value (or the
default value is <code>None</code>), <code>%default</code> expands to <code>none</code>.
<P>
</LI>
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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