Parsing Boolean Values with argparse

In argparse, parsing boolean command-line arguments is a common task, but a common pitfall arises when attempting to parse values like "--foo True" or "--foo False" using the type=bool argument. Surprisingly, even when using an empty string as the argument (e.g., "--foo " "), the parsed value evaluates to True.

For correct Boolean parsing, argparse offers two recommended approaches:

Canonical Approach:

Use the '--feature' and '--no-feature' syntax, supported natively by argparse. In Python 3.9 and above:

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

In Python versions below 3.9:

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

With this approach, the presence of '--feature' sets the value to True, while '--no-feature' sets it to False. The absence of either argument defaults to True.

Optional Approach (Using Type Conversion):

If the "--arg " syntax is desired, custom type conversion functions can be employed. One example is ast.literal_eval:

parser.add_argument("--arg", type=ast.literal_eval)

Alternatively, a user-defined function can be created:

def true_or_false(arg):
    ua = str(arg).upper()
    if 'TRUE'.startswith(ua):
       return True
    elif 'FALSE'.startswith(ua):
       return False
    else:
       raise argparse.ArgumentTypeError('Invalid boolean value')