Issues with casting argument to bool when target variable has type Union[bool, str]

[thomasgaudelet]

🐛 Bug report

If an argument is typed as bool | str, when trying to set that argument to True from command line it will instead be set to string true.

To reproduce

Create a file test.py

from jsonargparse import ArgumentParser

def main_cli():
    parser = ArgumentParser()
    parser.add_argument("--plop", type=str|bool)
    cfg = parser.parse_args()
    print(cfg)


if __name__ == "__main__":
    main_cli()

From command line,
python test.py --plop true

Expected behavior

Expected behaviour would be to have cfg = {"plop": True}. Instead we get cfg = {"plop": "true"}.
It works as intended if parameter is set through a config file when using the CLI object.
But it always fails from command line.

Note, it works if the typing is bool|str

Environment

• jsonargparse version (e.g., 4.8.0): 4.24.0
• Python version (e.g., 3.9): 3.10
• How jsonargparse was installed (e.g. pip install jsonargparse[all]): pip install jsonargparse
• OS (e.g., Linux): Linux

[mauvilsa]

That is weird. I tried the code above with python 3.10 in ubuntu 22.04 and got:

$ ./issue_379.py --plop true
Namespace(plop=True)

[thomasgaudelet]

Ah wait, it depends on the order in the typing. If str|bool this fails as indicated. But it works as intended with bool|str. Amending message above for correctness.

[mauvilsa]

This is the expected behavior. For unions it goes from left to right checking if the value is valid for the subtype. It stops when the value is valid. So if str is first, it will stay as str.

[thomasgaudelet]

I see. I wonder if that could be amended as this can lead to silent failures. It feels like bool type should always be treated first when present.

[mauvilsa]

This behavior will not be changed. Explaining that all subtypes are checked left to right is easy to explain and understand. Some arbitrary priority that overrides the general rule only for some types would lead to more confusion.

My main question would be, why is the type str | bool? According to that, any string is accepted. Thus, "true" is a valid value. If only some strings are accepted, shouldn't the type hint be different? See for instance Lightning-AI/pytorch-lightning#18370 where the type was changed to Literal["all"] | bool.

[thomasgaudelet]

This is the exact package and parameter I have trouble with! Good to see the typing will be fixed.