Python 3.6.5 Documentation >  "getopt" — C-style parser for command line options

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

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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, 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 ("':'";
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
("'='"). Optional arguments are not supported. 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.

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, 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.

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']

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"
# ...

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.