[BUG] Python 3.9 has Subscripted Generics error in generated Union.py code

[wallagib]

Describe the bug
Python 3.9.18 has an error with the to_dict() function generated for a Union[]
Python 3.10 does not have an issue.

The generated code that errors out is:

def to_dict(self) -> Dict[str, Any]:
    task_id: Union[None, Union[int, str], Unset]
    if isinstance(self.task_id, Unset):
        task_id = UNSET
    elif isinstance(self.task_id, Union[int, str]):
        task_id = self.task_id
    else:
        task_id = self.task_id

The following code can be run with the openapi spec:

from todo_client.models.listable import Listable
x = Listable(task_id=1)
x.to_dict()

This is the error that shows in Python 3.9
TypeError: Subscripted generics cannot be used with class and instance checks

At the very least, the pyproject.toml should be updated to show that Python 3.9 is no longer completely supported. Ideally, we'd have a patch since 3.9 is not EOL yet.

OpenAPI Spec File
bad-openapi.json

Desktop (please complete the following information):

• OS: MacOS M1 14.3
• Python Version: 3.9.18 (originally seen on 3.9.6)
• openapi-python-client version 0.17.2

Additional Context
This does not happen in openapi-python-client version 0.13.2, which I was upgrading from.

[wallagib]

I'm not sure, but I think this is the template that gets expanded to provide the if/else. In the generated code, the elif and else blocks are the same in this case, so I'm hoping there's a patch to be had.

https://github.com/openapi-generators/openapi-python-client/blob/main/openapi_python_client/templates/property_templates/union_property.py.jinja#L63

[dbanty]

I think this is a duplicate of #958 which I'm about to merge a fix for, so this will get closed with that. If there's a slightly different issue, though, please reopen!

[wallagib]

Yes, confirmed that 0.17.3 fixes this bug. Thank @codebutler and @dbanty !