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14.3.2.6 Generating help

`optparse`'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 `help` value for each option, and optionally a short usage message for your whole program. Here's an OptionParser populated with user-friendly (documented) options:

    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]")

If `optparse` encounters either `"-h"` or `"-help"` on the command-line, or if you just call `parser.print_help()`, it prints the following to standard output:

    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]

(If the help output is triggered by a help option, `optparse` exits after printing the help text.)

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 program, 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:

      -m MODE, --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:

      -f FILE, --filename=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 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 `%default` in the help string--`optparse` will replace it with `str()` of the option's default value. If an option has no default value (or the default value is `None`), `%default` expands to `none`.