14.3.2.6 Generating help

Python 2.5

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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.

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