6.21.3.3 Standard option actions

Python PEP

6.21.3.3 Standard option actions

The various option actions all have slightly different requirements and effects. Most actions have several relevant option attributes which you may specify to guide optparse's behaviour; a few have required attributes, which you must specify for any option using that action.

store [relevant: type, dest, nargs, choices]
The option must be followed by an argument, which is converted to a value according to type and stored in dest. If nargs > 1, multiple arguments will be consumed from the command line; all will be converted according to type and stored to dest as a tuple. See the ``Option types'' section below.
If choices is supplied (a list or tuple of strings), the type defaults to choice.
If type is not supplied, it defaults to string.
If dest is not supplied, optparse derives a destination from the first long option string (e.g., "-foo-bar" implies foo_bar). If there are no long option strings, optparse derives a destination from the first short option string (e.g., "-f" implies f).
Example:
```
parser.add_option("-f")
parser.add_option("-p", type="float", nargs=3, dest="point")
```
As it parses the command line
```
-f foo.txt -p 1 -3.5 4 -fbar.txt
```
optparse will set
```
options.f = "foo.txt"
options.point = (1.0, -3.5, 4.0)
options.f = "bar.txt"
```

store_const [required: const; relevant: dest]

The value const is stored in dest.

Example:

parser.add_option("-q", "--quiet",
                  action="store_const", const=0, dest="verbose")
parser.add_option("-v", "--verbose",
                  action="store_const", const=1, dest="verbose")
parser.add_option("--noisy",
                  action="store_const", const=2, dest="verbose")

If "-noisy" is seen, optparse will set

options.verbose = 2

store_true [relevant: dest]
A special case of store_const that stores a true value to dest.

store_false [relevant: dest]

Like store_true, but stores a false value.

Example:

parser.add_option("--clobber", action="store_true", dest="clobber")
parser.add_option("--no-clobber", action="store_false", dest="clobber")

append [relevant: type, dest, nargs, choices]
The option must be followed by an argument, which is appended to the list in dest. If no default value for dest is supplied, an empty list is automatically created when optparse first encounters this option on the command-line. If nargs > 1, multiple arguments are consumed, and a tuple of length nargs is appended to dest.
The defaults for type and dest are the same as for the store action.
Example:
```
parser.add_option("-t", "--tracks", action="append", type="int")
```
If "-t3" is seen on the command-line, optparse does the equivalent of:
```
options.tracks = []
options.tracks.append(int("3"))
```
If, a little later on, "-tracks=4" is seen, it does:
```
options.tracks.append(int("4"))
```
count [relevant: dest]
Increment the integer stored at dest. If no default value is supplied, dest is set to zero before being incremented the first time.
Example:
```
parser.add_option("-v", action="count", dest="verbosity")
```
The first time "-v" is seen on the command line, optparse does the equivalent of:
```
options.verbosity = 0
options.verbosity += 1
```
Every subsequent occurrence of "-v" results in
```
options.verbosity += 1
```
callback [required: callback; relevant: type, nargs, callback_args, callback_kwargs]
Call the function specified by callback. The signature of this function should be
```
func(option : Option,
     opt : string,
     value : any,
     parser : OptionParser,
     *args, **kwargs)
```
See section 6.21.4, Option Callbacks for more detail.
help
Prints a complete help message for all the options in the current option parser. The help message is constructed from the usage string passed to OptionParser's constructor and the help string passed to every option.
If no help string is supplied for an option, it will still be listed in the help message. To omit an option entirely, use the special value optparse.SUPPRESS_HELP.
optparse automatically adds a help option to all OptionParsers, so you do not normally need to create one.
Example:
```
from optparse import OptionParser, SUPPRESS_HELP

parser = OptionParser()
parser.add_option("-h", "--help", action="help"),
parser.add_option("-v", action="store_true", dest="verbose",
                  help="Be moderately verbose")
parser.add_option("--file", dest="filename",
                  help="Input file to read data from"),
parser.add_option("--secret", help=SUPPRESS_HELP)
```
If optparse sees either "-h" or "-help" on the command line, it will print something like the following help message to stdout (assuming sys.argv[0] is "foo.py"):
```
usage: foo.py [options]

options:
  -h, --help        Show this help message and exit
  -v                Be moderately verbose
  --file=FILENAME   Input file to read data from
```
After printing the help message, optparse terminates your process with sys.exit(0).
version
Prints the version number supplied to the OptionParser to stdout and exits. The version number is actually formatted and printed by the print_version() method of OptionParser. Generally only relevant if the version argument is supplied to the OptionParser constructor. As with help options, you will rarely create version options, since optparse automatically adds them when needed.

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