Python Basics: Parse Compact Option Lists with getopt

A small compatibility script may need conventional short and long options without a larger command-line interface. Python’s getopt separates options from operands while leaving value conversion to the caller. The fixture uses explicit argument lists so every result is reproducible.

Prepare a scratch directory

Tested on Linux with CPython 3.12.14. You need the named runtime, Bash, a text editor, and the standard mktemp, rm, and rmdir utilities. Run Python without -O so the assertions remain enabled. Any generated fixture files live inside an automatically cleaned temporary directory.

Keep using the same terminal. Stop if setup fails, before saving the source:

start_dir="$PWD"
work_dir="$(mktemp -d)" || exit 1
cd -- "$work_dir" || exit 1

Run the example

Save this as demonstrate-getopt.py:

import getopt

options, operands = getopt.getopt(
    ["-v", "--count=3", "input.txt"], "vc:", ["count="])
verbose = False
count = 1
for name, value in options:
    if name == "-v":
        verbose = True
    elif name in ("-c", "--count"):
        count = int(value)
assert count == 3 and verbose
assert operands == ["input.txt"]
print("Verbose:", verbose)
print("Count:", count)
print("Operands:", operands)

stopped, rest = getopt.getopt(["input.txt", "-v"], "v")
assert stopped == [] and rest == ["input.txt", "-v"]
print("After first operand:", rest)
_, literal = getopt.getopt(["--", "-v"], "v")
assert literal == ["-v"]
print("After option terminator:", literal)

try:
    getopt.getopt(["--unknown"], "v", ["count="])
except getopt.GetoptError as error:
    assert error.opt == "unknown"
    print("Unknown option: rejected")
else:
    raise AssertionError("Unknown option accepted")

Run it:

python3 -B demonstrate-getopt.py

Observed output:

Verbose: True
Count: 3
Operands: ['input.txt']
After first operand: ['input.txt', '-v']
After option terminator: ['-v']
Unknown option: rejected

Read the result

A colon after c and an equals sign after count declare required option arguments. Parsed values remain strings, so the program explicitly converts the count. Ordinary getopt stops at the first non-option operand. A standalone -- ends option processing even when the next operand begins with a hyphen.

Apply it carefully

For a larger new CLI, argparse usually provides better help, validation, and error reporting. getopt does not automatically enforce numeric ranges, reject duplicate options, or choose a duplicate-value policy. gnu_getopt has different scanning behavior; do not substitute it without checking the intended operand rules and environment. Catch conversion errors separately from option-syntax errors in a real command-line entry point.

The official reference describes the API and its options. The checks above cover the specific fixture, not every possible input or environment.

Clean up

Remove the source and then the empty practice directory:

rm -- demonstrate-getopt.py
cd -- "$start_dir" || exit 1
rmdir -- "$work_dir"
unset start_dir work_dir

If the directory is not empty, inspect the remaining files rather than deleting unknown contents.

Acknowledgements

This post was created with assistance from AI.