Support positional args about args

[nex3]

I'm skeptical of adding such a complex and detailed validation API for positional arguments. In my experience, the handling of positional arguments is very idiosyncratic; either it's something very simple like "this takes two or three arguments" or it's something very complicated that isn't worth the hassle of trying to write an API to express. I'd still prefer the much simpler minPositional/maxPositional API I proposed originally in issue 20042.

[gerardsimons]

I would personally think I would enjoy more structure around the positional args. I find myself using options just so it is better self-documenting, better structured in the code and with more strict checks.

[seaneagan]

The above proposed API is not complex, addPositional(...)/addRest(...) are pretty much exactly like the existing addFlag(...)/addOption(...), and startOptionalPositionals() doesn't even take any arguments. Yet it covers any cases I'm aware of how positional arguments are passed to commands.

@nex3 can you give an example of a command-line interface which this simple API does not handle?

[nex3]

The above proposed API is not complex, addPositional(...)/addRest(...) are pretty much exactly like the existing addFlag(...)/addOption(...), and startOptionalPositionals() doesn't even take any arguments. Yet it covers any cases I'm aware of how positional arguments are passed to commands.

Clearly I disagree.

@nex3 can you give an example of a command-line interface which this simple API does not handle?

It doesn't handle things like "zero or two parameters" or "allow this flag only with a positional argument" or "the first positional should be named foo unless there's a second in which case it should be named bar". All of these require extra munging after the arguments have been parsed.

[seaneagan]

It doesn't handle things like "zero or two parameters"

Do you have a real world example? Surely there is a better design.

or "allow this flag only with a positional argument"

That's not specific to positional arguments, it's just a special case of "allow this argument only with this other argument".

or "the first positional should be named foo unless there's a second in which case it should be named bar".

So some_command foo or some_command bar baz? foo and bar are sub-commands then, not positional arguments.

All of these require extra munging after the arguments have been parsed.

Again, this is not specific to positional arguments, or even to command-line interfaces. It's just as common with (sets of) named arguments, and with programming language interfaces to have to do "precondition checks" after the arguments have passed the built-in standardized signature checks.