15.5. optparse — Parser for command line options

15.5.1.1. Terminology

argument

a string entered on the command-line, and passed by the shell to execl() or execv(). In Python, arguments are elements of sys.argv[1:] (sys.argv[0] is the name of the program being executed). Unix shells also use the term “word”.

It is occasionally desirable to substitute an argument list other than sys.argv[1:], so you should read “argument” as “an element of sys.argv[1:], or of some other list provided as a substitute for sys.argv[1:]”.

option

an argument used to supply extra information to guide or customize the execution of a program. There are many different syntaxes for options; the traditional Unix syntax is a hyphen (“-“) followed by a single letter, e.g. -x or -F. Also, traditional Unix syntax allows multiple options to be merged into a single argument, e.g. -x -F is equivalent to -xF. The GNU project introduced -- followed by a series of hyphen-separated words, e.g. --file or --dry-run. These are the only two option syntaxes provided by optparse.

Some other option syntaxes that the world has seen include:

• a hyphen followed by a few letters, e.g. -pf (this is not the same as multiple options merged into a single argument)

• a hyphen followed by a whole word, e.g. -file (this is technically equivalent to the previous syntax, but they aren’t usually seen in the same program)

• a plus sign followed by a single letter, or a few letters, or a word, e.g. +f, +rgb

• a slash followed by a letter, or a few letters, or a word, e.g. /f, /file

These option syntaxes are not supported by optparse, and they never will be. This is deliberate: the first three are non-standard on any environment, and the last only makes sense if you’re exclusively targeting VMS, MS-DOS, and/or Windows.

option argument

an argument that follows an option, is closely associated with that option, and is consumed from the argument list when that option is. With optparse, option arguments may either be in a separate argument from their option:

-f foo
--file foo

or included in the same argument:

-ffoo
--file=foo

Typically, a given option either takes an argument or it doesn’t. Lots of people want an “optional option arguments” feature, meaning that some options will take an argument if they see it, and won’t if they don’t. This is somewhat controversial, because it makes parsing ambiguous: if -a takes an optional argument and -b is another option entirely, how do we interpret -ab? Because of this ambiguity, optparse does not support this feature.

positional argument

something leftover in the argument list after options have been parsed, i.e. after options and their arguments have been parsed and removed from the argument list.

required option

an option that must be supplied on the command-line; note that the phrase “required option” is self-contradictory in English. optparse doesn’t prevent you from implementing required options, but doesn’t give you much help at it either.

For example, consider this hypothetical command-line:

prog -v --report report.txt foo bar

-v and --report are both options. Assuming that --report takes one argument, report.txt is an option argument. foo and bar are positional arguments.

15.5.1.2. What are options for?

Options are used to provide extra information to tune or customize the execution of a program. In case it wasn’t clear, options are usually optional. A program should be able to run just fine with no options whatsoever. (Pick a random program from the Unix or GNU toolsets. Can it run without any options at all and still make sense? The main exceptions are find, tar, and dd—all of which are mutant oddballs that have been rightly criticized for their non-standard syntax and confusing interfaces.)

Lots of people want their programs to have “required options”. Think about it. If it’s required, then it’s not optional! If there is a piece of information that your program absolutely requires in order to run successfully, that’s what positional arguments are for.

As an example of good command-line interface design, consider the humble cp utility, for copying files. It doesn’t make much sense to try to copy files without supplying a destination and at least one source. Hence, cp fails if you run it with no arguments. However, it has a flexible, useful syntax that does not require any options at all:

cp SOURCE DEST
cp SOURCE ... DEST-DIR

You can get pretty far with just that. Most cp implementations provide a bunch of options to tweak exactly how the files are copied: you can preserve mode and modification time, avoid following symlinks, ask before clobbering existing files, etc. But none of this distracts from the core mission of cp, which is to copy either one file to another, or several files to another directory.

15.5.1.3. What are positional arguments for?

Positional arguments are for those pieces of information that your program absolutely, positively requires to run.

A good user interface should have as few absolute requirements as possible. If your program requires 17 distinct pieces of information in order to run successfully, it doesn’t much matter how you get that information from the user—most people will give up and walk away before they successfully run the program. This applies whether the user interface is a command-line, a configuration file, or a GUI: if you make that many demands on your users, most of them will simply give up.

In short, try to minimize the amount of information that users are absolutely required to supply—use sensible defaults whenever possible. Of course, you also want to make your programs reasonably flexible. That’s what options are for. Again, it doesn’t matter if they are entries in a config file, widgets in the “Preferences” dialog of a GUI, or command-line options—the more options you implement, the more flexible your program is, and the more complicated its implementation becomes. Too much flexibility has drawbacks as well, of course; too many options can overwhelm users and make your code much harder to maintain.

15.5.2.1. Understanding option actions

Actions tell optparse what to do when it encounters an option on the command line. There is a fixed set of actions hard-coded into optparse; adding new actions is an advanced topic covered in section Extending optparse. Most actions tell optparse to store a value in some variable—for example, take a string from the command line and store it in an attribute of options.

If you don’t specify an option action, optparse defaults to store.

15.5.2.2. The store action

The most common option action is store, which tells optparse to take the next argument (or the remainder of the current argument), ensure that it is of the correct type, and store it to your chosen destination.

For example:

parser.add_option("-f", "--file",
                  action="store", type="string", dest="filename")

Now let’s make up a fake command line and ask optparse to parse it:

args = ["-f", "foo.txt"]
(options, args) = parser.parse_args(args)

When optparse sees the option string -f, it consumes the next argument, foo.txt, and stores it in options.filename. So, after this call to parse_args(), options.filename is "foo.txt".

Some other option types supported by optparse are int and float. Here’s an option that expects an integer argument:

parser.add_option("-n", type="int", dest="num")

Note that this option has no long option string, which is perfectly acceptable. Also, there’s no explicit action, since the default is store.

Let’s parse another fake command-line. This time, we’ll jam the option argument right up against the option: since -n42 (one argument) is equivalent to -n 42 (two arguments), the code

(options, args) = parser.parse_args(["-n42"])
print options.num

will print 42.

If you don’t specify a type, optparse assumes string. Combined with the fact that the default action is store, that means our first example can be a lot shorter:

parser.add_option("-f", "--file", dest="filename")

If you don’t supply a destination, optparse figures out a sensible default from the option strings: if the first long option string is --foo-bar, then the default destination is foo_bar. If there are no long option strings, optparse looks at the first short option string: the default destination for -f is f.

optparse also includes built-in long and complex types. Adding types is covered in section Extending optparse.

15.5.2.3. Handling boolean (flag) options

Flag options—set a variable to true or false when a particular option is seen —are quite common. optparse supports them with two separate actions, store_true and store_false. For example, you might have a verbose flag that is turned on with -v and off with -q:

parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose")

Here we have two different options with the same destination, which is perfectly OK. (It just means you have to be a bit careful when setting default values— see below.)

When optparse encounters -v on the command line, it sets options.verbose to True; when it encounters -q, options.verbose is set to False.

15.5.2.4. Other actions

Some other actions supported by optparse are:

"store_const"

store a constant value

"append"

append this option’s argument to a list

"count"

increment a counter by one

"callback"

call a specified function

These are covered in section Reference Guide, Reference Guide and section Option Callbacks.

15.5.2.5. Default values

All of the above examples involve setting some variable (the “destination”) when certain command-line options are seen. What happens if those options are never seen? Since we didn’t supply any defaults, they are all set to None. This is usually fine, but sometimes you want more control. optparse lets you supply a default value for each destination, which is assigned before the command line is parsed.

First, consider the verbose/quiet example. If we want optparse to set verbose to True unless -q is seen, then we can do this:

parser.add_option("-v", action="store_true", dest="verbose", default=True)
parser.add_option("-q", action="store_false", dest="verbose")

Since default values apply to the destination rather than to any particular option, and these two options happen to have the same destination, this is exactly equivalent:

parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose", default=True)

Consider this:

parser.add_option("-v", action="store_true", dest="verbose", default=False)
parser.add_option("-q", action="store_false", dest="verbose", default=True)

Again, the default value for verbose will be True: the last default value supplied for any particular destination is the one that counts.

A clearer way to specify default values is the set_defaults() method of OptionParser, which you can call at any time before calling parse_args():

parser.set_defaults(verbose=True)
parser.add_option(...)
(options, args) = parser.parse_args()

As before, the last value specified for a given option destination is the one that counts. For clarity, try to use one method or the other of setting default values, not both.

15.5.2.6. 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.

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)

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]

  Dangerous Options:
    Caution: use these options at your own risk.  It is believed that some
    of them bite.

    -g                  Group option.

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)

group = OptionGroup(parser, "Debug Options")
group.add_option("-d", "--debug", action="store_true",
                 help="Print debug information")
group.add_option("-s", "--sql", action="store_true",
                 help="Print all SQL statements executed")
group.add_option("-e", action="store_true", help="Print every action done")
parser.add_option_group(group)

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]

  Dangerous Options:
    Caution: use these options at your own risk.  It is believed that some
    of them bite.

    -g                  Group option.

  Debug Options:
    -d, --debug         Print debug information
    -s, --sql           Print all SQL statements executed
    -e                  Print every action done

15.5.2.7. Printing a version string

Similar to the brief usage string, optparse can also print a version string for your program. You have to supply the string as the version argument to OptionParser:

parser = OptionParser(usage="%prog [-f] [-q]", version="%prog 1.0")

%prog is expanded just like it is in usage. Apart from that, version can contain anything you like. When you supply it, optparse automatically adds a --version option to your parser. If it encounters this option on the command line, it expands your version string (by replacing %prog), prints it to stdout, and exits.

For example, if your script is called /usr/bin/foo:

$ /usr/bin/foo --version
foo 1.0

The following two methods can be used to print and get the version string:

OptionParser. print_version ( file=None )

Print the version message for the current program (self.version) to file (default stdout). As with print_usage(), any occurrence of %prog in self.version is replaced with the name of the current program. Does nothing if self.version is empty or undefined.

OptionParser. get_version ( )

Same as print_version() but returns the version string instead of printing it.

15.5.2.8. How optparse handles errors

There are two broad classes of errors that optparse has to worry about: programmer errors and user errors. Programmer errors are usually erroneous calls to OptionParser.add_option(), e.g. invalid option strings, unknown option attributes, missing option attributes, etc. These are dealt with in the usual way: raise an exception (either optparse.OptionError or TypeError) and let the program crash.

Handling user errors is much more important, since they are guaranteed to happen no matter how stable your code is. optparse can automatically detect some user errors, such as bad option arguments (passing -n 4x where -n takes an integer argument), missing arguments (-n at the end of the command line, where -n takes an argument of any type). Also, you can call OptionParser.error() to signal an application-defined error condition:

(options, args) = parser.parse_args()
...
if options.a and options.b:
    parser.error("options -a and -b are mutually exclusive")

In either case, optparse handles the error the same way: it prints the program’s usage message and an error message to standard error and exits with error status 2.

Consider the first example above, where the user passes 4x to an option that takes an integer:

$ /usr/bin/foo -n 4x
Usage: foo [options]

foo: error: option -n: invalid integer value: '4x'

Or, where the user fails to pass a value at all:

$ /usr/bin/foo -n
Usage: foo [options]

foo: error: -n option requires an argument

optparse-generated error messages take care always to mention the option involved in the error; be sure to do the same when calling OptionParser.error() from your application code.

If optparse’s default error-handling behaviour does not suit your needs, you’ll need to subclass OptionParser and override its exit() and/or error() methods.

15.5.2.9. Putting it all together

Here’s what optparse-based scripts usually look like:

from optparse import OptionParser
...
def main():
    usage = "usage: %prog [options] arg"
    parser = OptionParser(usage)
    parser.add_option("-f", "--file", dest="filename",
                      help="read data from FILENAME")
    parser.add_option("-v", "--verbose",
                      action="store_true", dest="verbose")
    parser.add_option("-q", "--quiet",
                      action="store_false", dest="verbose")
    ...
    (options, args) = parser.parse_args()
    if len(args) != 1:
        parser.error("incorrect number of arguments")
    if options.verbose:
        print "reading %s..." % options.filename
    ...

if __name__ == "__main__":
    main()

15.5.3.1. Creating the parser

The first step in using optparse is to create an OptionParser instance.

class optparse. OptionParser ( ... )

The OptionParser constructor has no required arguments, but a number of optional keyword arguments. You should always pass them as keyword arguments, i.e. do not rely on the order in which the arguments are declared.

usage (default: "%prog [options]")

The usage summary to print when your program is run incorrectly or with a help option. When optparse prints the usage string, it expands %prog to os.path.basename(sys.argv[0]) (or to prog if you passed that keyword argument). To suppress a usage message, pass the special value optparse.SUPPRESS_USAGE.

option_list (default: [])

A list of Option objects to populate the parser with. The options in option_list are added after any options in standard_option_list (a class attribute that may be set by OptionParser subclasses), but before any version or help options. Deprecated; use add_option() after creating the parser instead.

option_class (default: optparse.Option)

Class to use when adding options to the parser in add_option().

version (default: None)

A version string to print when the user supplies a version option. If you supply a true value for version, optparse automatically adds a version option with the single option string --version. The substring %prog is expanded the same as for usage.

conflict_handler (default: "error")

Specifies what to do when options with conflicting option strings are added to the parser; see section Conflicts between options.

description (default: None)

A paragraph of text giving a brief overview of your program. optparse reformats this paragraph to fit the current terminal width and prints it when the user requests help (after usage, but before the list of options).

formatter (default: a new IndentedHelpFormatter)

An instance of optparse.HelpFormatter that will be used for printing help text. optparse provides two concrete classes for this purpose: IndentedHelpFormatter and TitledHelpFormatter.

add_help_option (default: True)

If true, optparse will add a help option (with option strings -h and --help) to the parser.

prog

The string to use when expanding %prog in usage and version instead of os.path.basename(sys.argv[0]).

epilog (default: None)

A paragraph of help text to print after the option help.

15.5.3.2. Populating the parser

There are several ways to populate the parser with options. The preferred way is by using OptionParser.add_option(), as shown in section Tutorial. add_option() can be called in one of two ways:

• pass it an Option instance (as returned by make_option())

• pass it any combination of positional and keyword arguments that are acceptable to make_option() (i.e., to the Option constructor), and it will create the Option instance for you

The other alternative is to pass a list of pre-constructed Option instances to the OptionParser constructor, as in:

option_list = [
    make_option("-f", "--filename",
                action="store", type="string", dest="filename"),
    make_option("-q", "--quiet",
                action="store_false", dest="verbose"),
    ]
parser = OptionParser(option_list=option_list)

(make_option() is a factory function for creating Option instances; currently it is an alias for the Option constructor. A future version of optparse may split Option into several classes, and make_option() will pick the right class to instantiate. Do not instantiate Option directly.)

15.5.3.3. Defining options

Each Option instance represents a set of synonymous command-line option strings, e.g. -f and --file. You can specify any number of short or long option strings, but you must specify at least one overall option string.

The canonical way to create an Option instance is with the add_option() method of OptionParser.

OptionParser. add_option ( option )

OptionParser. add_option ( *opt_str, attr=value, ... )

To define an option with only a short option string:

parser.add_option("-f", attr=value, ...)

And to define an option with only a long option string:

parser.add_option("--foo", attr=value, ...)

The keyword arguments define attributes of the new Option object. The most important option attribute is action, and it largely determines which other attributes are relevant or required. If you pass irrelevant option attributes, or fail to pass required ones, optparse raises an OptionError exception explaining your mistake.

An option’s action determines what optparse does when it encounters this option on the command-line. The standard option actions hard-coded into optparse are:

"store"

store this option’s argument (default)

"store_const"

store a constant value

"store_true"

store a true value

"store_false"

store a false value

"append"

append this option’s argument to a list

"append_const"

append a constant value to a list

"count"

increment a counter by one

"callback"

call a specified function

"help"

print a usage message including all options and the documentation for them

(If you don’t supply an action, the default is "store". For this action, you may also supply type and dest option attributes; see Standard option actions.)

As you can see, most actions involve storing or updating a value somewhere. optparse always creates a special object for this, conventionally called options (it happens to be an instance of optparse.Values). Option arguments (and various other values) are stored as attributes of this object, according to the dest (destination) option attribute.

For example, when you call

parser.parse_args()

one of the first things optparse does is create the options object:

options = Values()

If one of the options in this parser is defined with

parser.add_option("-f", "--file", action="store", type="string", dest="filename")

and the command-line being parsed includes any of the following:

-ffoo
-f foo
--file=foo
--file foo

then optparse, on seeing this option, will do the equivalent of

options.filename = "foo"

The type and dest option attributes are almost as important as action, but action is the only one that makes sense for all options.

15.5.3.4. Option attributes

The following option attributes may be passed as keyword arguments to OptionParser.add_option(). If you pass an option attribute that is not relevant to a particular option, or fail to pass a required option attribute, optparse raises OptionError.

Option. action

(default: "store")

Determines optparse’s behaviour when this option is seen on the command line; the available options are documented here.

Option. type

(default: "string")

The argument type expected by this option (e.g., "string" or "int"); the available option types are documented here.

Option. dest

(default: derived from option strings)

If the option’s action implies writing or modifying a value somewhere, this tells optparse where to write it: dest names an attribute of the options object that optparse builds as it parses the command line.

Option. default

The value to use for this option’s destination if the option is not seen on the command line. See also OptionParser.set_defaults().

Option. nargs

(default: 1)

How many arguments of type type should be consumed when this option is seen. If > 1, optparse will store a tuple of values to dest.

Option. const

For actions that store a constant value, the constant value to store.

Option. choices

For options of type "choice", the list of strings the user may choose from.

Option. callback

For options with action "callback", the callable to call when this option is seen. See section Option Callbacks for detail on the arguments passed to the callable.

Option. callback_args

Option. callback_kwargs

Additional positional and keyword arguments to pass to callback after the four standard callback arguments.

Option. help

Help text to print for this option when listing all available options after the user supplies a help option (such as --help). If no help text is supplied, the option will be listed without help text. To hide this option, use the special value optparse.SUPPRESS_HELP.

Option. metavar

(default: derived from option strings)

Stand-in for the option argument(s) to use when printing help text. See section Tutorial for an example.

15.5.3.5. 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 Standard option types section.

  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"))
  

  The append action calls the append method on the current value of the option. This means that any default value specified must have an append method. It also means that if the default value is non-empty, the default elements will be present in the parsed value for the option, with any values from the command line appended after those default values:

  >>> parser.add_option("--files", action="append", default=['~/.mypkg/defaults'])
  >>> opts, args = parser.parse_args(['--files', 'overrides.mypkg'])
  >>> opts.files
  ['~/.mypkg/defaults', 'overrides.mypkg']
  

• "append_const" [required: const; relevant: dest]

  Like "store_const", but the value const is appended to dest; as with "append", dest defaults to None, and an empty list is automatically created the first time the option is encountered.

• "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, which is called as

  func(option, opt_str, value, parser, *args, **kwargs)
  

  See section 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
  
  # usually, a help option is added automatically, but that can
  # be suppressed using the add_help_option argument
  parser = OptionParser(add_help_option=False)
  
  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.

15.5.3.6. Standard option types

optparse has six built-in option types: "string", "int", "long", "choice", "float" and "complex". If you need to add new option types, see section Extending optparse.

Arguments to string options are not checked or converted in any way: the text on the command line is stored in the destination (or passed to the callback) as-is.

Integer arguments (type "int" or "long") are parsed as follows:

• if the number starts with 0x, it is parsed as a hexadecimal number

• if the number starts with 0, it is parsed as an octal number

• if the number starts with 0b, it is parsed as a binary number

• otherwise, the number is parsed as a decimal number

The conversion is done by calling either int() or long() with the appropriate base (2, 8, 10, or 16). If this fails, so will optparse, although with a more useful error message.

"float" and "complex" option arguments are converted directly with float() and complex(), with similar error-handling.

"choice" options are a subtype of "string" options. The choices option attribute (a sequence of strings) defines the set of allowed option arguments. optparse.check_choice() compares user-supplied option arguments against this master list and raises OptionValueError if an invalid string is given.

15.5.3.7. Parsing arguments

The whole point of creating and populating an OptionParser is to call its parse_args() method:

(options, args) = parser.parse_args(args=None, values=None)

where the input parameters are

args

the list of arguments to process (default: sys.argv[1:])

values

an optparse.Values object to store option arguments in (default: a new instance of Values) – if you give an existing object, the option defaults will not be initialized on it

and the return values are

options

the same object that was passed in as values, or the optparse.Values instance created by optparse

args

the leftover positional arguments after all options have been processed

The most common usage is to supply neither keyword argument. If you supply values, it will be modified with repeated setattr() calls (roughly one for every option argument stored to an option destination) and returned by parse_args().

If parse_args() encounters any errors in the argument list, it calls the OptionParser’s error() method with an appropriate end-user error message. This ultimately terminates your process with an exit status of 2 (the traditional Unix exit status for command-line errors).

15.5.3.8. Querying and manipulating your option parser

The default behavior of the option parser can be customized slightly, and you can also poke around your option parser and see what’s there. OptionParser provides several methods to help you out:

OptionParser. disable_interspersed_args ( )

Set parsing to stop on the first non-option. For example, if -a and -b are both simple options that take no arguments, optparse normally accepts this syntax:

prog -a arg1 -b arg2

and treats it as equivalent to

prog -a -b arg1 arg2

To disable this feature, call disable_interspersed_args(). This restores traditional Unix syntax, where option parsing stops with the first non-option argument.

Use this if you have a command processor which runs another command which has options of its own and you want to make sure these options don’t get confused. For example, each command might have a different set of options.

OptionParser. enable_interspersed_args ( )

Set parsing to not stop on the first non-option, allowing interspersing switches with command arguments. This is the default behavior.

OptionParser. get_option ( opt_str )

Returns the Option instance with the option string opt_str, or None if no options have that option string.

OptionParser. has_option ( opt_str )

Return true if the OptionParser has an option with option string opt_str (e.g., -q or --verbose).

OptionParser. remove_option ( opt_str )

If the OptionParser has an option corresponding to opt_str, that option is removed. If that option provided any other option strings, all of those option strings become invalid. If opt_str does not occur in any option belonging to this OptionParser, raises ValueError.

15.5.3.9. Conflicts between options

If you’re not careful, it’s easy to define options with conflicting option strings:

parser.add_option("-n", "--dry-run", ...)
...
parser.add_option("-n", "--noisy", ...)

(This is particularly true if you’ve defined your own OptionParser subclass with some standard options.)

Every time you add an option, optparse checks for conflicts with existing options. If it finds any, it invokes the current conflict-handling mechanism. You can set the conflict-handling mechanism either in the constructor:

parser = OptionParser(..., conflict_handler=handler)

or with a separate call:

parser.set_conflict_handler(handler)

The available conflict handlers are:

"error" (default)

assume option conflicts are a programming error and raise OptionConflictError

"resolve"

resolve option conflicts intelligently (see below)

As an example, let’s define an OptionParser that resolves conflicts intelligently and add conflicting options to it:

parser = OptionParser(conflict_handler="resolve")
parser.add_option("-n", "--dry-run", ..., help="do no harm")
parser.add_option("-n", "--noisy", ..., help="be noisy")

At this point, optparse detects that a previously-added option is already using the -n option string. Since conflict_handler is "resolve", it resolves the situation by removing -n from the earlier option’s list of option strings. Now --dry-run is the only way for the user to activate that option. If the user asks for help, the help message will reflect that:

Options:
  --dry-run     do no harm
  ...
  -n, --noisy   be noisy

It’s possible to whittle away the option strings for a previously-added option until there are none left, and the user has no way of invoking that option from the command-line. In that case, optparse removes that option completely, so it doesn’t show up in help text or anywhere else. Carrying on with our existing OptionParser:

parser.add_option("--dry-run", ..., help="new dry-run option")

At this point, the original -n/--dry-run option is no longer accessible, so optparse removes it, leaving this help text:

Options:
  ...
  -n, --noisy   be noisy
  --dry-run     new dry-run option

15.5.3.10. Cleanup

OptionParser instances have several cyclic references. This should not be a problem for Python’s garbage collector, but you may wish to break the cyclic references explicitly by calling destroy() on your OptionParser once you are done with it. This is particularly useful in long-running applications where large object graphs are reachable from your OptionParser.

15.5.3.11. Other methods

OptionParser supports several other public methods:

OptionParser. set_usage ( usage )

Set the usage string according to the rules described above for the usage constructor keyword argument. Passing None sets the default usage string; use optparse.SUPPRESS_USAGE to suppress a usage message.

OptionParser. print_usage ( file=None )

Print the usage message for the current program (self.usage) to file (default stdout). Any occurrence of the string %prog in self.usage is replaced with the name of the current program. Does nothing if self.usage is empty or not defined.

OptionParser. get_usage ( )

Same as print_usage() but returns the usage string instead of printing it.

OptionParser. set_defaults ( dest=value, ... )

Set default values for several option destinations at once. Using set_defaults() is the preferred way to set default values for options, since multiple options can share the same destination. For example, if several “mode” options all set the same destination, any one of them can set the default, and the last one wins:

parser.add_option("--advanced", action="store_const",
                  dest="mode", const="advanced",
                  default="novice")    # overridden below
parser.add_option("--novice", action="store_const",
                  dest="mode", const="novice",
                  default="advanced")  # overrides above setting

To avoid this confusion, use set_defaults():

parser.set_defaults(mode="advanced")
parser.add_option("--advanced", action="store_const",
                  dest="mode", const="advanced")
parser.add_option("--novice", action="store_const",
                  dest="mode", const="novice")

15.5.4.1. Defining a callback option

As always, the easiest way to define a callback option is by using the OptionParser.add_option() method. Apart from action, the only option attribute you must specify is callback, the function to call:

parser.add_option("-c", action="callback", callback=my_callback)

callback is a function (or other callable object), so you must have already defined my_callback() when you create this callback option. In this simple case, optparse doesn’t even know if -c takes any arguments, which usually means that the option takes no arguments—the mere presence of -c on the command-line is all it needs to know. In some circumstances, though, you might want your callback to consume an arbitrary number of command-line arguments. This is where writing callbacks gets tricky; it’s covered later in this section.

optparse always passes four particular arguments to your callback, and it will only pass additional arguments if you specify them via callback_args and callback_kwargs. Thus, the minimal callback function signature is:

def my_callback(option, opt, value, parser):

The four arguments to a callback are described below.

There are several other option attributes that you can supply when you define a callback option:

type

has its usual meaning: as with the "store" or "append" actions, it instructs optparse to consume one argument and convert it to type. Rather than storing the converted value(s) anywhere, though, optparse passes it to your callback function.

nargs

also has its usual meaning: if it is supplied and > 1, optparse will consume nargs arguments, each of which must be convertible to type. It then passes a tuple of converted values to your callback.

callback_args

a tuple of extra positional arguments to pass to the callback

callback_kwargs

a dictionary of extra keyword arguments to pass to the callback

15.5.4.2. How callbacks are called

All callbacks are called as follows:

func(option, opt_str, value, parser, *args, **kwargs)

where

option

is the Option instance that’s calling the callback

opt_str

is the option string seen on the command-line that’s triggering the callback. (If an abbreviated long option was used, opt_str will be the full, canonical option string—e.g. if the user puts --foo on the command-line as an abbreviation for --foobar, then opt_str will be "--foobar".)

value

is the argument to this option seen on the command-line. optparse will only expect an argument if type is set; the type of value will be the type implied by the option’s type. If type for this option is None (no argument expected), then