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Question

I would like to use argparse to parse boolean command-line arguments written as "--foo True" or "--foo False". For example:

my_program --my_boolean_flag False

However, the following test code does not do what I would like:

import argparse
parser = argparse.ArgumentParser(description="My parser")
parser.add_argument("--my_bool", type=bool)
cmd_line = ["--my_bool", "False"]
parsed_args = parser.parse(cmd_line)

Sadly, parsed_args.my_bool evaluates to True. This is the case even when I change cmd_line to be ["--my_bool", ""], which is surprising, since bool("") evalutates to False.

How can I get argparse to parse "False", "F", and their lower-case variants to be False?

Accepted Answer

I think a more canonical way to do this is via:

command --feature

and

command --no-feature

argparse supports this version nicely:

Python 3.9+:

parser.add_argument('--feature', action=argparse.BooleanOptionalAction)

Python < 3.9:

parser.add_argument('--feature', action='store_true')
parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)

Of course, if you really want the --arg <True|False> version, you could pass ast.literal_eval as the "type", or a user defined function ...

def t_or_f(arg):
    ua = str(arg).upper()
    if 'TRUE'.startswith(ua):
       return True
    elif 'FALSE'.startswith(ua):
       return False
    else:
       pass  #error condition maybe?

Answer 2

Yet another solution using the previous suggestions, but with the "correct" parse error from argparse:

def str2bool(v):
    if isinstance(v, bool):
        return v
    if v.lower() in ('yes', 'true', 't', 'y', '1'):
        return True
    elif v.lower() in ('no', 'false', 'f', 'n', '0'):
        return False
    else:
        raise argparse.ArgumentTypeError('Boolean value expected.')

This is very useful to make switches with default values; for instance

parser.add_argument("--nice", type=str2bool, nargs='?',
                        const=True, default=False,
                        help="Activate nice mode.")

allows me to use:

script --nice
script --nice <bool>

and still use a default value (specific to the user settings). One (indirectly related) downside with that approach is that the 'nargs' might catch a positional argument -- see this related question and this argparse bug report.

Answer 3

If you want to allow `--feature` and `--no-feature` at the same time (last one wins)

This allows users to make a shell alias with --feature, and overriding it with --no-feature.

Python 3.9 and above

parser.add_argument('--feature', default=True, action=argparse.BooleanOptionalAction)

Python 3.8 and below

I recommend mgilson's answer:

parser.add_argument('--feature', dest='feature', action='store_true')
parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)