CLI parsing—with `--help` text—`--foo bar`, how to give additional parameters to `bar`?

[Samuel Marks]

Previously I have solved related problems with explicit `-}` and `-{`
(or `-b`) as in `nginxctl`:
```
$ python -m nginxctl serve --temp_dir '/tmp' \
            -b 'server' \
              --server_name 'localhost' --listen '8080' \
              -b location '/' \
                --root '/tmp/wwwroot' \
              -} \
            -}
nginx is running. Stop with: /usr/local/bin/nginx -c /tmp/nginx.conf -s stop
```

To illustrate the issue, using `ml-params` and ml-params-tensorflow:
```
$ python -m ml_params --engine 'tensorflow' train --help

usage: python -m ml_params train [-h]
                                 --optimizer

{Adadelta,Adagrad,Adam,Adamax,Ftrl,Nadam,RMSprop}

Run the training loop for your ML pipeline.

optional arguments:
  -h, --help            show this help message and exit
  --optimizer {Adadelta,Adagrad,Adam,Adamax,Ftrl,Nadam,RMSprop}
                        The optimizer
```
Idea: preprocess `sys.argv` so that this syntax would work
`--optimizer Adam[learning_rate=0.01]`*

*square rather than round so as not to require escape characters or
quoting in `sh`

Unfortunately this approach wouldn't give nice `--help` text.

What's the right solution here?

Thanks,

Samuel Marks
Charity <https://sydneyscientific.org> | consultancy
<https://offscale.io> | open-source <https://github.com/offscale> |
LinkedIn <https://linkedin.com/in/samuelmarks>