The last feature that you will use in every script is
optparse's ability to generate help messages. All you have
to do is supply a help argument when you add an option. Let's
create a new parser and populate it with user-friendly (documented)
options:
If optparse encounters either -h or
--help on the command-line, or if you just call
parser.print_help(), it prints the following to stdout:
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)
-fFILE, --file=FILE write output to FILE
-mMODE, --mode=MODE interaction mode: one of 'novice', 'intermediate'
[default], 'expert'
There's a lot going on here to help optparse generate the
best possible help message:
the script defines its own usage message:
usage = "usage: %prog [options] arg1 arg2"
optparse expands "%prog" in the usage string to the name of the
current script, i.e. os.path.basename(sys.argv[0]). The
expanded string is then printed before the detailed option help.
If you don't supply a usage string, optparse uses a bland but
sensible default: "usage: %prog [options]", which is fine if your
script doesn't take any positional arguments.
every option defines a help string, and doesn't worry about
line-wrapping--optparse takes care of wrapping lines and
making the help output look good.
options that take a value indicate this fact in their
automatically-generated help message, e.g. for the ``mode'' option:
-mMODE, --mode=MODE
Here, ``MODE'' is called the meta-variable: it stands for the argument
that the user is expected to supply to
-m/--mode. By default, optparse
converts the destination variable name to uppercase and uses that for
the meta-variable. Sometimes, that's not what you want--for
example, the filename option explicitly sets
metavar="FILE", resulting in this automatically-generated
option description:
-fFILE, --file=FILE
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 formal syntax
``-fFILE'' 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.
When dealing with many options, it is convenient to group these
options for better help output. An OptionParser can contain
several option groups, each of which can contain several options.
Continuing with the parser defined above, adding an
OptionGroup to a parser is easy:
group = OptionGroup(parser, "Dangerous Options",
"Caution: use these options at your own risk. "
"It is believed that some of them bite.")
group.add_option("-g", action="store_true", help="Group option.")
parser.add_option_group(group)
This would result in the following help output:
usage: [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)
-fFILE, --file=FILE write output to FILE
-mMODE, --mode=MODE interaction mode: one of 'novice', 'intermediate'
[default], 'expert'
Dangerous Options:
Caution: use of these options is at your own risk. It is believed that
some of them bite.
-g Group option.
