15.6. "getopt" — C-style parser for command line options
********************************************************

**Source code:** Lib/getopt.py

======================================================================

Note: The "getopt" module is a parser for command line options whose
  API is designed to be familiar to users of the C "getopt()"
  function. Users who are unfamiliar with the C "getopt()" function or
  who would like to write less code and get better help and error
  messages should consider using the "argparse" module instead.

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.

This module provides two functions and an exception:

getopt.getopt(args, options[, long_options])

   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:]". *options* is
   the string of option letters that the script wants to recognize,
   with options that require an argument followed by a colon ("':'";
   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.

   *long_options*, 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 ("'='").  Optional arguments are not supported.  To accept
   only long options, *options* 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 *long_options* is "['foo',
   'frob']", the option "--fo" will match as "--foo", but "--f" will
   not match uniquely, so "GetoptError" will be raised.

   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.

getopt.gnu_getopt(args, options[, long_options])

   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.

   New in version 2.3.

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.

   Changed in version 1.6: Introduced "GetoptError" as a synonym for
   "error".

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']

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 str(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"
       # ...

   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
"argparse" module:

   import argparse

   if __name__ == '__main__':
       parser = argparse.ArgumentParser()
       parser.add_argument('-o', '--output')
       parser.add_argument('-v', dest='verbose', action='store_true')
       args = parser.parse_args()
       # ... do something with args.output ...
       # ... do something with args.verbose ..

See also:

  Module "argparse"
     Alternative command line option and argument parsing library.
