Parsing boolean values with argparse


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?

Asked By: SuperElectric



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

command --feature


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')

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
       pass  #error condition maybe?
Answered By: mgilson

I was looking for the same issue, and imho the pretty solution is :

def str2bool(v):
  return v.lower() in ("yes", "true", "t", "1")

and using that to parse the string to boolean as suggested above.

Answered By: susundberg

There seems to be some confusion as to what type=bool and type='bool' might mean. Should one (or both) mean ‘run the function bool(), or ‘return a boolean’? As it stands type='bool' means nothing. add_argument gives a 'bool' is not callable error, same as if you used type='foobar', or type='int'.

But argparse does have registry that lets you define keywords like this. It is mostly used for action, e.g. `action=’store_true’. You can see the registered keywords with:


which displays a dictionary

{'action': {None: argparse._StoreAction,
  'append': argparse._AppendAction,
  'append_const': argparse._AppendConstAction,
 'type': {None: <function argparse.identity>}}

There are lots of actions defined, but only one type, the default one, argparse.identity.

This code defines a ‘bool’ keyword:

def str2bool(v):
  #susendberg's function
  return v.lower() in ("yes", "true", "t", "1")
p = argparse.ArgumentParser()
p.register('type','bool',str2bool) # add type keyword to registries
p.add_argument('-b',type='bool')  # do not use 'type=bool'
# p.add_argument('-b',type=str2bool) # works just as well
p.parse_args('-b false'.split())

parser.register() is not documented, but also not hidden. For the most part the programmer does not need to know about it because type and action take function and class values. There are lots of stackoverflow examples of defining custom values for both.

In case it isn’t obvious from the previous discussion, bool() does not mean ‘parse a string’. From the Python documentation:

bool(x): Convert a value to a Boolean, using the standard truth testing procedure.

Contrast this with

int(x): Convert a number or string x to an integer.

Answered By: hpaulj

In addition to what @mgilson said, it should be noted that there’s also a ArgumentParser.add_mutually_exclusive_group(required=False) method that would make it trivial to enforce that --flag and --no-flag aren’t used at the same time.

Answered By: foo
class FlagAction(argparse.Action):
    # From

    def __init__(self, option_strings, dest, default=None,
                 required=False, help=None, metavar=None,
                 positive_prefixes=['--'], negative_prefixes=['--no-']):
        self.positive_strings = set()
        self.negative_strings = set()
        for string in option_strings:
            assert re.match(r'--[A-z]+', string)
            suffix = string[2:]
            for positive_prefix in positive_prefixes:
                self.positive_strings.add(positive_prefix + suffix)
            for negative_prefix in negative_prefixes:
                self.negative_strings.add(negative_prefix + suffix)
        strings = list(self.positive_strings | self.negative_strings)
        super(FlagAction, self).__init__(option_strings=strings, dest=dest,
                                         nargs=0, const=None, default=default, type=bool, choices=None,
                                         required=required, help=help, metavar=metavar)

    def __call__(self, parser, namespace, values, option_string=None):
        if option_string in self.positive_strings:
            setattr(namespace, self.dest, True)
            setattr(namespace, self.dest, False)
Answered By: Robert T. McGibbon

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')

If you DON’T want to allow --feature and --no-feature at the same time

You can use a mutually exclusive group:

feature_parser = parser.add_mutually_exclusive_group(required=False)
feature_parser.add_argument('--feature', dest='feature', action='store_true')
feature_parser.add_argument('--no-feature', dest='feature', action='store_false')

You can use this helper if you are going to set many of them:

def add_bool_arg(parser, name, default=False):
    group = parser.add_mutually_exclusive_group(required=False)
    group.add_argument('--' + name, dest=name, action='store_true')
    group.add_argument('--no-' + name, dest=name, action='store_false')

add_bool_arg(parser, 'useful-feature')
add_bool_arg(parser, 'even-more-useful-feature')
Answered By: fnkr

A quite similar way is to use:


and if you set the argument –feature in your command

 command --feature

the argument will be True, if you do not set type –feature the arguments default is always False!

Answered By: dl.meteo

This works for everything I expect it to:

add_boolean_argument(parser, 'foo', default=True)
parser.parse_args([])                   # Whatever the default was
parser.parse_args(['--foo'])            # True
parser.parse_args(['--nofoo'])          # False
parser.parse_args(['--foo=true'])       # True
parser.parse_args(['--foo=false'])      # False
parser.parse_args(['--foo', '--nofoo']) # Error

The code:

def _str_to_bool(s):
    """Convert string to bool (in argparse context)."""
    if s.lower() not in ['true', 'false']:
        raise ValueError('Need bool; got %r' % s)
    return {'true': True, 'false': False}[s.lower()]

def add_boolean_argument(parser, name, default=False):                                                                                               
    """Add a boolean argument to an ArgumentParser instance."""
    group = parser.add_mutually_exclusive_group()
        '--' + name, nargs='?', default=default, const=True, type=_str_to_bool)
    group.add_argument('--no' + name, dest=name, action='store_false')
Answered By: Stumpy Joe Pete

Here is another variation without extra row/s to set default values. The boolean value is always assigned, so that it can be used in logical statements without checking beforehand:

import argparse
parser = argparse.ArgumentParser(description="Parse bool")
parser.add_argument("--do-something", default=False, action="store_true",
                    help="Flag to do something")
args = parser.parse_args()

if args.do_something:
     print("Do something")
     print("Don't do something")

print(f"Check that args.do_something={args.do_something} is always a bool.")
Answered By: Schaki

A simpler way would be to use as below.

parser.add_argument('--feature', type=lambda s: s.lower() in ['true', 't', 'yes', '1'])
Answered By: arunkumarreddy

I think the most canonical way will be:

parser.add_argument('--ensure', nargs='*', default=None)

ENSURE = config.ensure is None
Answered By: Andreas Maertens

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
        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.

Answered By: Maxim


parser.add_argument('--is_debug', default=False, type=lambda x: (str(x).lower() == 'true'))
Answered By: Evalds Urtans

Simplest way would be to use choices:

parser = argparse.ArgumentParser()

args = parser.parse_args()
flag = args.my_flag == 'True'

Not passing –my-flag evaluates to False. The required=True option could be added if you always want the user to explicitly specify a choice.

Answered By: gerardw

Quick and easy, but only for arguments 0 or 1:

parser.add_argument("mybool", default=True,type=lambda x: bool(int(x)))

The output will be “False” after calling from terminal:

python 0
Answered By: FEMengineer

Simplest & most correct way is:

from distutils.util import strtobool

parser.add_argument('--feature', dest='feature', 
                    type=lambda x: bool(strtobool(x)))

Do note that True values are y, yes, t, true, on and 1;
false values are n, no, f, false, off and 0. Raises ValueError if val is anything else.

Answered By: Akash Desarda

Simplest. It’s not flexible, but I prefer simplicity.

                      help='This is a boolean flag.',
                      choices=[True, False], 

EDIT: If you don’t trust the input, don’t use eval.

Answered By: Russell

Similar to @Akash but here is another approach that I’ve used. It uses str than lambda because python lambda always gives me an alien-feelings.

import argparse
from distutils.util import strtobool

parser = argparse.ArgumentParser()
parser.add_argument("--my_bool", type=str, default="False")
args = parser.parse_args()

if bool(strtobool(args.my_bool)) is True:
Answered By: Youngjae

As an improvement to @Akash Desarda ‘s answer, you could do

import argparse
from distutils.util import strtobool

parser = argparse.ArgumentParser()
    type=lambda x:bool(strtobool(x)),
    nargs='?', const=True, default=False)
args = parser.parse_args()

And it supports python --foo

(base) [[email protected] code]$ python
(base) [[email protected] code]$ python --foo 
(base) [[email protected] code]$ python --foo True
(base) [[email protected] code]$ python --foo False
Answered By: Costa Huang

just do the following , you can make --test = True by using

python filename –test

parser.add_argument("--test" , default=False ,help="test ?", dest='test', action='store_true')
Answered By: delan

Convert the value:

def __arg_to_bool__(arg):

        Convert string / int arg to bool
    :param arg: argument to be converted
    :type arg: str or int
    :return: converted arg
    :rtype: bool
    str_true_values = (
    str_false_values = (

    if isinstance(arg, str):
        arg = arg.upper()
        if arg in str_true_values:
            return True
        elif arg in str_false_values:
            return False

    if isinstance(arg, int):
        if arg == 1:
            return True
        elif arg == 0:
            return False

    if isinstance(arg, bool):
        return arg

    # if any other value not covered above, consider argument as False
    # or you could just raise and error
    return False


args = ap.parse_args()
my_arg = options.my_arg
my_arg = __arg_to_bool__(my_arg)

Answered By: scorpionipx

After previously following @akash-desarda ‘s excellence answer , to use strtobool via lambda, later, I decide to use strtobool directly instead.

import argparse
from distutils import util
parser.add_argument('--feature', type=util.strtobool)

Yes you’re right, strtobool is returning an int, not a bool. But strtobool will not returning any other value except 0 and 1, and python will get them converted to a bool value seamlessy and consistently.

>>> 0 == False
>>> 0 == True
>>> 1 == False
>>> 1 == True

While on receiving a wrong input value like

python --feature wrong_value

An argparse.Action with strtobool compared to lambda will produce a slightly clearer/comprehensible error message: error: argument --feature: invalid strtobool value: 'wrong_value'

Compared to this code,

parser.add_argument('--feature', type=lambda x: bool(util.strtobool(x))

Which will produce a less clear error message: error: argument --feature: invalid <lambda> value: 'wrong_value'
Answered By: Indra Ginanjar

This is actually outdated. For Python 3.7+, Argparse now supports boolean args (search BooleanOptionalAction).

The implementation looks like this:

import argparse

ap = argparse.ArgumentParser()

# List of args
ap.add_argument('--foo', default=True, type=bool, help='Some helpful text that is not bar. Default = True')

# Importable object
args = ap.parse_args()

One other thing to mention: this will block all entries other than True and False for the argument via argparse.ArgumentTypeError. You can create a custom error class for this if you want to try to change this for any reason.

Answered By: Zach Rieck

Expanding on gerardw’s answer

The reason parser.add_argument("--my_bool", type=bool) doesn’t work is that bool("mystring") is True for any non-empty string so bool("False") is actually True.

What you want is

import argparse

parser = argparse.ArgumentParser(description="My parser")

        choices=["False", "True"],

parsed_args = parser.parse_args()

my_bool = parsed_args.my_bool == "True"

$ python --my_bool False

$ python --my_bool True

$ python --my_bool true
usage: [-h] [--my_bool {False,True}] error: argument --my_bool: invalid choice: 'true' (choose from 'False', 'True')
Answered By: FiddleStix

I found good way to store default value of parameter as False and when it is present in commandline argument then its value should be true.

cmd command
when you want argument to be true:
python –csv

when you want your argument should be false:

import argparse
from ast import parse
import sys
parser = argparse.ArgumentParser(description='')
parser.add_argument('--csv', action='store_true', default = False 
,help='read from csv')
args = parser.parse_args()

if args.csv:
    print('reading from csv')
Answered By: lalit gangwar

You can create a BoolAction and then use it

class BoolAction(Action):
    def __init__(
            default: bool = False,
        if nargs is not None:
            raise ValueError('nargs not allowed')
        super().__init__(option_strings, dest, default=default, **kwargs)

    def __call__(self, parser, namespace, values, option_string=None):
        input_value = values.lower()
        b = input_value in ['true', 'yes', '1']
        if not b and input_value not in ['false', 'no', '0']:
            raise ValueError('Invalid boolean value "%s".)
        setattr(namespace, self.dest, b)

and then set action=BoolAction in parser.add_argument()

Answered By: reddy nishanth