Stop writing CLI validation. Parse it right the first time

This kind of stuff is what makes me appreciate python's argparse.

It's a genuine pleasure to use, and I use it often.

If you dig a little deeper into it, it does all the type and value validation, file validation, it does required and mutually exclusive args, it does subargs. And it lets you do special cases of just about anything.

And of course it does the "normal" stuff like short + long args, boolean args, args that are lists, default values, and help strings.

Actually, I think argparse falls into the same trap that the author is talking about. You can define lots of invariants in the parser, and say that these two arguments can't be passed together, or that this argument, if specified, requires these arguments to also be specified, etc. But the end result is a namespace with a bunch of key-value pairs on it, and argparse doesn't play well with typing systems like mypy or pyright. So the rest of the tool has to assume that the invariants were correctly specified up-front.

The result is that you often still this kind of defensive programming, where argparse ensures that an invariant holds, but other functions still check the same invariant later on because they might have been called a different way or just because the developer isn't sure whether everything was checked where they are in the program.

What I think the author is looking for is a combination of argparse and Pydantic, such that when you define a parser using argparse, it automatically creates the relevant Pydantic classes that define the type of the parsed arguments.