Use argparse BooleanOptionalAction for a local switch

By Batu · English technical notes

Also published in our Blogger archive.

A local on/off setting is often clearer when users can state either choice directly. argparse.BooleanOptionalAction creates the positive and negative long options from one declaration. Adding --cache with this action also accepts --no-cache; the first stores True and the second stores False.

The example sets default=True, which represents the application's chosen behavior when neither option appears. It then parses three fixed argument lists. An empty list keeps the default, --no-cache disables caching, and --cache explicitly enables it. The assertion checks that the resulting values are [True, False, True], and print(*values) produces the exact output True False True. Passing lists directly to parse_args() keeps the sample independent of the program that launches it.

This action is appropriate for a boolean setting, not a value-taking option. Avoid type=bool for text such as --cache False: Python considers non-empty strings truthy, so that spelling does not express the expected false value. BooleanOptionalAction also does not resolve conflicts between repeated flags; normal argument parsing processes them in order, so an interface should document whether repeated contradictory switches are allowed. The example makes no claim about cache implementation, persistence, or performance; it only parses a local switch.

BooleanOptionalAction was added in Python 3.9, so use Python 3.9 or newer for this exact declaration. AI assistance disclosure: this article was drafted with AI assistance.

See the official argparse documentation.

import argparse

parser = argparse.ArgumentParser(add_help=False)
parser.add_argument(
    "--cache",
    action=argparse.BooleanOptionalAction,
    default=True,
)

values = [
    parser.parse_args(arguments).cache
    for arguments in ([], ["--no-cache"], ["--cache"])
]

assert values == [True, False, True]
print(*values)

True False True