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getopt Module Complexity

The getopt module parses a command line the way the C getopt() function does: getopt() stops at the first non-option argument, and gnu_getopt() scans the whole line so options and positional arguments can be intermixed. It is pure Python and works on a list of strings you pass in, usually sys.argv[1:].

Arguments are the unit throughout. n is the arguments in args, p is the arguments the call steps over one at a time, s is the characters in shortopts, and k is the entries in longopts. For getopt(), p is the options and option values before the first non-option argument; for gnu_getopt(), it is every argument before a --, or before the first non-option in POSIX mode - when shortopts starts with +, or the POSIXLY_CORRECT environment variable is non-empty. Every step slices off the rest of the list, which is where the O(p·n) term comes from; setting up and handing back the remaining arguments cost at most one more such step, hence p + 1. The bounds treat each argument and each option name as a few characters long, as short-option clusters such as -abc are.

Complexity Reference

Parsing functions

Operation	Time	Space	Notes
getopt.getopt(args, shortopts, longopts=[])	O((p + 1)·(n + s + k))	O(n + k)	Arguments from the first non-option on cost no steps: they are returned as they stand
getopt.gnu_getopt(args, shortopts, longopts=[])	O((p + 1)·(n + s + k))	O(n + s + k)	Outside POSIX mode every argument before a -- is a step, so a long list of positional arguments costs O(n²)
Looking up one short option	O(s)	O(1)	A scan of shortopts
Looking up one long option	O(k)	O(k)	Every entry of longopts is tested as a prefix match

GetoptError

Operation	Time	Space	Notes
getopt.GetoptError(msg, opt='')	O(1)	O(1)	Raised for an unknown option, a missing argument, an argument given to an option that takes none, or an ambiguous long-option prefix
GetoptError.msg, GetoptError.opt	O(1)	O(1)	The message, and the option it concerns without its leading dashes
getopt.error	O(1)	O(1)	An alias of GetoptError

Parsing Command-Line Arguments

A short option taking an argument is followed by : in shortopts, and a long option by = in longopts. Each (option, value) pair comes back in the order given, with '' for an option that takes no argument.

import getopt

argv = ['-v', '-o', 'out.txt', '--level=3', 'input.txt']

opts, args = getopt.getopt(argv, 'vo:', ['level='])  # O((p + 1)·(n + s + k))
assert opts == [('-v', ''), ('-o', 'out.txt'), ('--level', '3')]
assert args == ['input.txt']

# Short options can be clustered, and an argument can follow its letter directly
opts, args = getopt.getopt(['-vofile'], 'vo:')
assert opts == [('-v', ''), ('-o', 'file')]

Long Option Prefixes

A long option matches any unambiguous prefix of one entry in longopts, so every lookup tests all k entries. An exact match wins over a longer entry that shares it as a prefix.

import getopt

longopts = ['foo', 'frob', 'foobar']

opts, _ = getopt.getopt(['--fr', '--foo'], '', longopts)  # O(k) per long option
assert opts == [('--frob', ''), ('--foo', '')]

try:
    getopt.getopt(['--f'], '', longopts)
except getopt.GetoptError as err:
    assert err.opt == 'f'
    assert 'not a unique prefix' in err.msg
else:
    raise AssertionError('an ambiguous prefix was accepted')

Where Scanning Stops

getopt() stops at the first argument that is not an option, and a bare - counts as one. What follows is returned as it stands, with no further steps. gnu_getopt() keeps going, collecting positional arguments as it meets them, and takes one step per argument until a --.

import getopt

argv = ['-a', 'file1', '-b', 'file2']

opts, args = getopt.getopt(argv, 'ab')  # stops at 'file1'
assert opts == [('-a', '')]
assert args == ['file1', '-b', 'file2']

opts, args = getopt.gnu_getopt(argv, 'ab')  # scans every argument
assert opts == [('-a', ''), ('-b', '')]
assert args == ['file1', 'file2']

# A leading '+' in shortopts restores getopt()'s behaviour
opts, args = getopt.gnu_getopt(argv, '+ab')
assert args == ['file1', '-b', 'file2']

# '--' ends option processing in both
opts, args = getopt.getopt(['-a', '--', '-b'], 'ab')
assert opts == [('-a', '')] and args == ['-b']

Long Argument Lists

Each step replaces the remaining arguments with a slice one element shorter, so a step costs O(n). That is harmless for a handful of options, but gnu_getopt() steps over positional arguments too: a command line carrying thousands of file names costs time quadratic in their number. A -- in front of them hands the rest over as one slice.

import getopt

files = [f'file{i}.txt' for i in range(1_000)]

# O(n²): every file name is a step, and every step copies the rest of the list
opts, args = getopt.gnu_getopt(['-v'] + files, 'v')
assert args == files

# O(n): the file names after '--' are taken in one slice
opts, args = getopt.gnu_getopt(['-v', '--'] + files, 'v')
assert opts == [('-v', '')] and args == files

# getopt() takes no step for them, since the first file name stops it
opts, args = getopt.getopt(['-v'] + files, 'v')
assert args == files

Optional Arguments

From Python 3.14, :: after a short option and =? after a long option make the argument optional. An optional argument must be attached - -ovalue or --color=red - because a separate word is never taken as the value.

import getopt

opts, args = getopt.getopt(['-ow', '-o', 'value'], 'o::')
assert opts == [('-o', 'w'), ('-o', '')]
assert args == ['value']

opts, args = getopt.getopt(['--color', '--color=red'], '', ['color=?'])
assert opts == [('--color', ''), ('--color', 'red')]

Keeping Arguments in Order

From Python 3.14, a shortopts that starts with - makes gnu_getopt() report the positional arguments it meets between options as (None, [arguments]) entries, so the caller sees options and arguments in their original order. Arguments after the last option are still returned as the second element, and the leading - takes precedence over POSIXLY_CORRECT.

import getopt

argv = ['a', '-x', 'b', 'c', '--y', 'd']

opts, args = getopt.gnu_getopt(argv, '-x', ['y'])
assert opts == [(None, ['a']), ('-x', ''), (None, ['b', 'c']), ('--y', '')]
assert args == ['d']

Handling Errors

Every parsing failure raises GetoptError. str(err) is its msg, and opt names the offending option without its dashes.

import getopt

try:
    getopt.getopt(['-o'], 'o:')
except getopt.GetoptError as err:
    assert err.opt == 'o'
    assert str(err) == err.msg == 'option -o requires argument'
else:
    raise AssertionError('a missing argument was accepted')

try:
    getopt.getopt(['-z'], 'o:')
except getopt.error as err:  # the same class
    assert err.opt == 'z'
    assert 'not recognized' in err.msg
else:
    raise AssertionError('an unknown option was accepted')

Performance Best Practices

✅ Do:

• Put -- in front of a long list of file names passed to gnu_getopt(), so they are taken in one slice instead of one O(n) step each

❌ Avoid:

• gnu_getopt() over thousands of positional arguments with no -- - the cost is quadratic in their number

Version Notes

• Python 3.14+: Optional arguments, with :: in shortopts and =? in longopts
• Python 3.14+: A leading - in gnu_getopt()'s shortopts returns options and positional arguments in order
• All Python 3: gnu_getopt() stops at the first non-option argument when the POSIXLY_CORRECT environment variable is non-empty

Related Modules

• argparse - declarative parsing with help and usage messages
• optparse - declarative option parsing that follows the same command-line conventions
• sys - sys.argv supplies the list you pass in