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<title>6.21.2.6 Generating help
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<H3><A NAME=
"SECTION0082126000000000000000"></A><A NAME=
"optparse-generating-help"></A>
<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",
help=
"interaction mode: novice, intermediate, "
"or expert [default: %default]")
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
<div class=
"verbatim"><pre>
usage:
<yourscript
> [options] arg1 arg2
-h, --help show this help message and exit
-v, --verbose make lots of noise [default]
-q, --quiet be vewwy quiet (I'm hunting wabbits)
-m MODE, --mode=MODE interaction mode: novice, intermediate, or
expert [default: intermediate]
(If the help output is triggered by a help option,
<tt class=
"module">optparse
</tt> exits after
There's a lot going on here to help
<tt class=
"module">optparse
</tt> generate the best possible
the script defines its own usage message:
<div class=
"verbatim"><pre>
usage =
"usage: %prog [options] arg1 arg2"<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.
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 scriptdoesn't take any positional arguments.
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
options that take a value indicate this fact in their
automatically-generated help message, e.g. for the ``mode'' option:
<div class="verbatim
"><pre>
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
<div class="verbatim
"><pre>
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.
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>.
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<span class="release-info
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