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?

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', 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?