Add parameter `validator` to `argparse.ArgumentParser.add_argument`

[bjhend]

Feature or enhancement

Proposal:

I propose to add an optional parameter validator to argparse.ArgumentParser.add_argument. It should accept a callable with one argument of the type of the added parameter and return a boolean. If present the following should happen:

• On argument parsing the usual type conversion and value checks should be executed
• Then the validator callable is called with the parsed and type-converted argument
• If the result is True nothing happens and parsing is resumed
• If the result is False the value is rejected as invalid by raising an argparse.ArgumentError explaining the failed validation

In addition, if a default value is given it should be validated similarly by add_argument.

Maybe it would be useful to add a special exception ArgumentValidationError derived from ArgumentError that also receives the invalid parsed value.

Example

import argparse

def validate(val: int):
    return (val >= 10) and (val <= 100)

parser = argparse.ArgumentParser()
parser.add_argument('--foo', type=int, validator=validate, help="Set to value between 10 and 100")
args = parser.parse_args()
print(args.foo)

Now, if the program is called with --foo 5 an exception is raised pointing out the validation error.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

[sirosen]

Sorry, but I think opening an issue for this is premature.

If you look at the CPython devguide, we have a section about proposing new features. You should first suggest the idea in the Ideas category of the Python Discourse, and see if it gains traction -- in particular, if core developers are supportive.

I also think that in this particular case, you would need to explain why the existing type and action parameters for add_argument are not sufficient for this need.

I'm closing this as not-planned for now.