Saving the chosen subgroup name somewhere on args

[lebrice]

Currently, when using subgroups, the value for the subgroup name is not saved anywhere on the args, since the subgroup overwrites the original argument in the second phase of parsing:

@dataclass
class Config:
    person: Person = subgroups({"bob": Bob, "alice": Alice}, default=Bob)

parser = ArgumentParser()
parser.add_arguments(Config, "config")

args = parser.parse_args()
print(args)

Recap of Subgroups:

(As a recap, here is what subgroups do:)

$ python test/test_issue_blab.py --help
usage: test_issue_blab.py [-h] [--person {bob,alice}] [--person.age int] [--person.cool bool]

optional arguments:
  -h, --help            show this help message and exit

Config ['config']:
  Configuration dataclass.

  --person {bob,alice}  (default: bob)

Bob ['config.person']:
  Person named Bob.

  --person.age int      (default: 32)
  --person.cool bool    (default: True)

$ python issue.py --person alice --help
usage: test_issue_blab.py [-h] [--person {bob,alice}] [--person.age int] [--person.popular bool]

optional arguments:
  -h, --help            show this help message and exit

Config ['config']:
  Configuration dataclass.

  --person {bob,alice}  (default: bob)

Alice ['config.person']:
  Person named Alice.

  --person.age int      (default: 13)
  --person.popular bool
                        (default: True)

The problem

$ python issue.py --person alice
Namespace(config=Config(person=Alice(age=13, popular=True)))

It would be useful to be able to extract the name of the chose subgroup. For instance, when using a subgroup for a choice between the HParams class of different models, we want to extract the choice of the model:

@dataclass
class Config:
     model: Model.HParams = subgroups({
         "simple": SimpleModel.HParams,
         "advanced": AdvancedModel.HParams,
         }, default=SimpleModel.HParams)

parser = ArgumentParser()
parser.add_arguments(Config, "config")
args = parser.parse_args()
config: Config = args.config
model_hparams: Model.HParams = config.model

# ehhh, what kind of Model was chosen, exactly?
# --> Currently no way of knowing, except by doing something sneaky
#       like this:
# Extract type of model from type of HParams:
model_type_str = type(model_hparams).__qualname__.rpartition(".")[0]
# Find matching type
model_type = [
    model_type for model_type in Model.__subclasses__()
    if model_type.__qualname__ == model_type_str
][0]

This however doesn't work, since different models might share the same HParams class! It's not a viable solution.

I'm thinking that having something like a subgroups dictionary in the args itself could be useful. Something like this:

$ python issue.py --person alice
Namespace(config=Config(person=Alice(age=13, popular=True)), subgroups={'config.person': 'alice'})