Is it possible to validate `argparse` default argument values?

Question

Is it possible to tell argparse to give the same errors on default argument values as it would on user-specified argument values?

For example, the following will not result in any error:

parser = argparse.ArgumentParser()
parser.add_argument('--choice', choices=['a', 'b', 'c'], default='invalid')

args = vars(parser.parse_args())            # args = {'choice': 'invalid'}

whereas omitting the default, and having the user specify --choice=invalid on the command-line will result in an error (as expected).

Reason for asking is that I would like to have the user to be able to specify default command-line options in a JSON file which are then set using ArgumentParser.set_defaults(), but unfortunately the behaviour demonstrated above prevents these user-specified defaults from being validated.

Update: argparse is inconsistent and I now consider the behavior above to be a bug. The following does trigger an error:

parser = argparse.ArgumentParser()
parser.add_argument('--num', type=int, default='foo')

args = parser.parse_args()      # triggers exception in case --num is not 
                                # specified on the command-line

I have opened a bug report for this: https://github.com/python/cpython/issues/100949

Answer 1

I took the time to dig into the source code, and what is happening is that a check is only happening for arguments you gave on the command line. The only way to enforce a check, in my opinion, is to subclass ArgumentParser and have it do the check when you add the argument:

class ValidatingArgumentParser(argparse.ArgumentParser):
    def add_argument(self, *args, **kwargs):
        super().add_argument(*args, **kwargs)
        self._check_value(self._actions[-1],kwargs['default'])

Answer 2

No. Explicit arguments need to be validated because they originate from outside the source code. Default values originate in the source code, so it's the job of the programmer, not the argument parser, to ensure they are valid.

(This is the difference between validation and debugging.)

(Using set_defaults on unvalidated user input still falls under the purview of debugging, as it's not the argument parser itself adding the default values, but the programmer.)