wish: explicitly support input validation

[markstos]

I see there is "coercion" now for input parameters, but it's not clear what happens if an invalid value is given that doesn't coerce.

For example, I have an option named --format that I want to restrict to values of csv or json.

If the coercion system already serves the purpose, the documentation should be updated to explain what happens when an argument is provided, but it fails to coerce.

[tj]

yeah good call, nothing specific IIRC, since null etc could be valid, so if it's really invalid the function should throw

[SomeKittens]

If you want to coerce, just throw/error in your coercing function - there are no plans to do that in .option

[cirosantilli]

Keyword: choices like in python argparser choices: https://docs.python.org/3/library/argparse.html#choices

[shadowspawn]

Thanks for link @cirosantilli , added to open issue #518