Usage of `parse_known_args()` before adding a dataclass containing subgroups results in error

[jonahsiekmannagility]

Describe the bug
Usage of parse_known_args() before adding a dataclass with add_arguments results in the following error when subsequently running parse_args().

To Reproduce

from dataclasses import dataclass
from simple_parsing import subgroups

@dataclass
class SubgroupBase:
    ...

@dataclass
class SubgroupMemberA(SubgroupBase):
    ...

@dataclass
class GroupPicker:
    subgroup_type: SubgroupBase = subgroups(
        {
            "a": SubgroupMemberA,
        },
        default_factory=SubgroupMemberA,
    )


if __name__ == "__main__":
    from simple_parsing import ArgumentParser

    parser = ArgumentParser()
    parser.add_argument("positional_argument", type=str)
   # Removing this line prevents the error
    args, _ = parser.parse_known_args()

    parser.add_arguments(GroupPicker, "type")
    # AttributeError: 'Namespace' object has no attribute 'type.subgroup_type'
    args = parser.parse_args()

Expected behavior
I expect parse_known_args and parse_args to be callable any number of times without throwing a stacktrace

Actual behavior
A clear and concise description of what is happening.

$ python issue.py
  File "[my script]", line 29, in <module>
    args = parser.parse_args()
  File ".../.local/share/uv/python/cpython-3.10.17-linux-x86_64-gnu/lib/python3.10/argparse.py", line 1833, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File ".../.venv/lib/python3.10/site-packages/simple_parsing/parsing.py", line 361, in parse_known_args
    parsed_args = self._postprocessing(parsed_args)
  File ".../.venv/lib/python3.10/site-packages/simple_parsing/parsing.py", line 580, in _postprocessing
    self._remove_subgroups_from_namespace(parsed_args)
  File ".../.venv/lib/python3.10/site-packages/simple_parsing/parsing.py", line 790, in _remove_subgroups_from_namespace
    chosen_value = getattr(parsed_args, dest)
AttributeError: 'Namespace' object has no attribute 'type.subgroup_type'

Desktop (please complete the following information):

• Version [e.g. 22]: Ubuntu 24.04, simple-parsing 0.1.7
• Python version: 3.10.17

Additional context
Add any other context about the problem here.

[lebrice]

Hey there, thanks for posting!

I think I've seen this bug before, and the workaround that was mentioned was to recreate the parser IIRC. Obviously this isn't ideal, we'd like to have parse_args/parse_known_args work in a stateless way, but it's a bit hard to do (I'd welcome some help on that front, by the way if you're interested).