"Missing argument" message uses arg name, not metavar

[andy-maier]

If one defines the metavar different from the argument name, like in:

@adapter_group.command('list')
@click.argument('CPC-NAME', type=str, metavar='CPC')
@click.pass_obj
def adapter_list(cmd_obj, cpc_name):

the usage line in the help text correctly uses the metavar:

Usage: zhmc adapter list [OPTIONS] CPC

but the error message when invoked without arguments incorrectly uses the argument name:

Error: Missing argument "CPC-NAME".

I suggest that the metavar is used consistently in all display to the user.

[sirosen]

This is generated by a bit of a head scratcher, in the formatting code for a MissingParameter error.

If I'm reading this right, it's trying to be generic in its handling of Options and Arguments, so it takes param.opts as a relevant "name" for the parameter, and slash delimits all of the available names.
That gets nice help like Missing option "-f/--foo", and explains the issue here.

Argument.get_usage_pieces looks at the metavar correctly. Without tracing this out thoroughly, it seems like this is the key difference.

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Opinion Zone:
The real thing that's wrong here is that Argument has a non-empty opts value. I've seen this elsewhere in the code and it seems wrong -- the name doesn't help either, as it implies that it's a good detector of Option vs. Argument.
Fixing that would be a breaking change as completion and other advanced usage may depend upon this value.

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There are a bunch of ways of fixing this.
Probably the least invasive is to make MissingParameter formatting read param.get_usage_pieces and see if there's anything there. However, there's so much layering going on here, I don't really want to act without some kind of input from the maintainers.
Plus, although pragmatic, that fix is icky and may be abusing the meaning of get_usage_pieces.

Something that is common throughout is that a lot of the code handles Options and Argument equivalently. I'm not sure how much people are defining their own param types, and adding an attribute "option-style" vs. "argument-style" would probably resolve a lot of this confusion.

[sirosen]

Oh, idea which is not icky or messy and resolves a DRY violation: add a get_error_hint() method to Parameters, and fix MissingParameter and BadParameter to use it.
I'll do that and open a PR now.

[andy-maier]

@sirosen Thanks for solving this!!

[jwilson8767]

Until this is merged you can do something like:
raise click.MissingParameter(param=click.get_current_context().command.params[X])
Where X is the positional number of the parameter for this command (0 for first possible param, 1 for the next, and so on)