use `_default` instead of nothing as dest in complete_zsh

[tshu-w]

Hi, it will be much better if we use _default instead of nothing as dest in complete_zsh https://github.com/iterative/shtab/blob/master/shtab/__init__.py#L476 and https://github.com/iterative/shtab/blob/master/shtab/__init__.py#L486. User can get basic default competition without setting.

[casperdcl]

Hi @tshu-w - not sure I follow but would be happy to accept a PR :)

[tshu-w]

Hi, let me explain my thought further. As mentioned in omni-us/jsonargparse#108 (comment), I use shtab to enable shell completion for PythorchLightning CLI. However, It's hard for me to modify each option complete as they are many and complex. Therefore it is inconvenient for some options that can accept files or directories because by default there is no completion after enter this options. Here is an example:

run fit --model configs/model/default.yaml --data # I cannot got complete here

I would like to use _default instead of nothing as dest in complete_zsh, which mean I will change this line from

"({})".format(" ".join(map(str, opt.choices)))) if opt.choices else "",

to

"({})".format(" ".join(map(str, opt.choices)))) if opt.choices else "_default",

This would be incompatible and may not be what everyone expects, so for a formal PR, I think it would be a good idea to set a variable for the user to modify like:

DEFAULT_FUNCTIONS = {
    "bash": "_shtab_compgen_files",
    "zsh": "_default",
    "tcsh": ""
}

and

"({})".format(" ".join(map(str, opt.choices)))) if opt.choices else f"{DEFAULT_FUNCTIONS['zsh']}",

[casperdcl]

Hi sorry for the long delay.

Makes sense; interface could be shtab.add_argument_to(..., default_complete: dict[str, str]).

[tshu-w]

@casperdcl Yes, I'm looking forward to this enhancement.

[casperdcl]

actually... is this incompatible with #46?

[tshu-w]

Not really. What I want is a better default completion target in shell, #46 is to autofill the %(default)s in the help message.

What #46 wants is to look like:

# Basic syntax:
parser.add_argument("-t", "--threads", default=10, 
                    help="number of threads to use [default: %(default)s]")

# this will print help messages that look like:
-t THREADS, --threads THREADS
                        number of threads to use [default: 10]

[casperdcl]

Does #83 work for you?