# `getopt` — C-style parser for command line options

Source code: Lib/getopt.py

Note

This module is considered feature complete. A more declarative and extensible alternative to this API is provided in the `optparse` module. Further functional enhancements for command line parameter processing are provided either as third party modules on PyPI, or else as features in the `argparse` module.

This module helps scripts to parse the command line arguments in `sys.argv`. It supports the same conventions as the Unix `getopt()` function (including the special meanings of arguments of the form ‘`-`’ and ‘`--`‘). Long options similar to those supported by GNU software may be used as well via an optional third argument.

Users who are unfamiliar with the Unix `getopt()` function should consider using the `argparse` module instead. Users who are familiar with the Unix `getopt()` function, but would like to get equivalent behavior while writing less code and getting better help and error messages should consider using the `optparse` module. See Choosing an argument parsing library for additional details.

This module provides two functions and an exception:

getopt.getopt(args, shortopts, longopts=[])

  Parses command line options and parameter list. args is the argument list to be parsed, without the leading reference to the running program. Typically, this means `sys.argv[1:]`. shortopts is the string of option letters that the script wants to recognize, with options that require an argument followed by a colon (`':'`) and options that accept an optional argument followed by two colons (`'::'`); i.e., the same format that Unix `getopt()` uses.

  Note

  Unlike GNU `getopt()`, after a non-option argument, all further arguments are considered also non-options. This is similar to the way non-GNU Unix systems work.

  longopts, if specified, must be a list of strings with the names of the long options which should be supported. The leading `'--'` characters should not be included in the option name. Long options which require an argument should be followed by an equal sign (`'='`). Long options which accept an optional argument should be followed by an equal sign and question mark (`'=?'`). To accept only long options, shortopts should be an empty string. Long options on the command line can be recognized so long as they provide a prefix of the option name that matches exactly one of the accepted options. For example, if longopts is `['foo', 'frob']`, the option `--fo` will match as `--foo`, but `--f` will not match uniquely, so `GetoptError` will be raised.

  If longopts is a string it gets treated as a list of a single element.

  The return value consists of two elements: the first is a list of `(option, value)` pairs; the second is the list of program arguments left after the option list was stripped (this is a trailing slice of args). Each option-and-value pair returned has the option as its first element, prefixed with a hyphen for short options (e.g., `'-x'`) or two hyphens for long options (e.g., `'--long-option'`), and the option argument as its second element, or an empty string if the option has no argument. The options occur in the list in the same order in which they were found, thus allowing multiple occurrences. Long and short options may be mixed.

  Changed in version 3.14: Optional arguments are supported.

getopt.gnu_getopt(args, shortopts, longopts=[])

  This function works like `getopt()`, except that GNU style scanning mode is used by default. This means that option and non-option arguments may be intermixed. The `getopt()` function stops processing options as soon as a non-option argument is encountered.

  If the first character of the option string is `'+'`, or if the environment variable `POSIXLY_CORRECT` is set, then option processing stops as soon as a non-option argument is encountered.

  If the first character of the option string is `'-'`, non-option arguments that are followed by options are added to the list of option-and-value pairs as a pair that has `None` as its first element and the list of non-option arguments as its second element. The second element of the `gnu_getopt()` result is a list of program arguments after the last option.

  Changed in version 3.14: Support for returning intermixed options and non-option arguments in order.

exception getopt.GetoptError

  This is raised when an unrecognized option is found in the argument list or when an option requiring an argument is given none. The argument to the exception is a string indicating the cause of the error. For long options, an argument given to an option which does not require one will also cause this exception to be raised. The attributes `msg` and `opt` give the error message and related option; if there is no specific option to which the exception relates, `opt` is an empty string.

exception getopt.error

  Alias for `GetoptError`; for backward compatibility.

An example using only Unix style options:

    >>> import getopt
    >>> args = '-a -b -cfoo -d bar a1 a2'.split()
    >>> args
    ['-a', '-b', '-cfoo', '-d', 'bar', 'a1', 'a2']
    >>> optlist, args = getopt.getopt(args, 'abc:d:')
    >>> optlist
    [('-a', ''), ('-b', ''), ('-c', 'foo'), ('-d', 'bar')]
    >>> args
    ['a1', 'a2']

Using long option names is equally easy:

    >>> s = '--condition=foo --testing --output-file abc.def -x a1 a2'
    >>> args = s.split()
    >>> args
    ['--condition=foo', '--testing', '--output-file', 'abc.def', '-x', 'a1', 'a2']
    >>> optlist, args = getopt.getopt(args, 'x', [
    ...     'condition=', 'output-file=', 'testing'])
    >>> optlist
    [('--condition', 'foo'), ('--testing', ''), ('--output-file', 'abc.def'), ('-x', '')]
    >>> args
    ['a1', 'a2']

Optional arguments should be specified explicitly:

    >>> s = '-Con -C --color=off --color a1 a2'
    >>> args = s.split()
    >>> args
    ['-Con', '-C', '--color=off', '--color', 'a1', 'a2']
    >>> optlist, args = getopt.getopt(args, 'C::', ['color=?'])
    >>> optlist
    [('-C', 'on'), ('-C', ''), ('--color', 'off'), ('--color', '')]
    >>> args
    ['a1', 'a2']

The order of options and non-option arguments can be preserved:

    >>> s = 'a1 -x a2 a3 a4 --long a5 a6'
    >>> args = s.split()
    >>> args
    ['a1', '-x', 'a2', 'a3', 'a4', '--long', 'a5', 'a6']
    >>> optlist, args = getopt.gnu_getopt(args, '-x:', ['long='])
    >>> optlist
    [(None, ['a1']), ('-x', 'a2'), (None, ['a3', 'a4']), ('--long', 'a5')]
    >>> args
    ['a6']

In a script, typical usage is something like this:

    import getopt, sys

    def main():
        try:
            opts, args = getopt.getopt(sys.argv[1:], "ho:v", ["help", "output="])
        except getopt.GetoptError as err:
            # print help information and exit:
            print(err)  # will print something like "option -a not recognized"
            usage()
            sys.exit(2)
        output = None
        verbose = False
        for o, a in opts:
            if o == "-v":
                verbose = True
            elif o in ("-h", "--help"):
                usage()
                sys.exit()
            elif o in ("-o", "--output"):
                output = a
            else:
                assert False, "unhandled option"
        process(args, output=output, verbose=verbose)

    if __name__ == "__main__":
        main()

Note that an equivalent command line interface could be produced with less code and more informative help and error messages by using the `optparse` module:

    import optparse

    if __name__ == '__main__':
        parser = optparse.OptionParser()
        parser.add_option('-o', '--output')
        parser.add_option('-v', dest='verbose', action='store_true')
        opts, args = parser.parse_args()
        process(args, output=opts.output, verbose=opts.verbose)

A roughly equivalent command line interface for this case can also be produced by using the `argparse` module:

    import argparse

    if __name__ == '__main__':
        parser = argparse.ArgumentParser()
        parser.add_argument('-o', '--output')
        parser.add_argument('-v', dest='verbose', action='store_true')
        parser.add_argument('rest', nargs='*')
        args = parser.parse_args()
        process(args.rest, output=args.output, verbose=args.verbose)

See Choosing an argument parsing library for details on how the `argparse` version of this code differs in behaviour from the `optparse` (and `getopt`) version.

See also

Module `optparse`

  Declarative command line option parsing.

Module `argparse`

  More opinionated command line option and argument parsing library.