Cannot instantiate a class from env variables

[rusmux]

🐛 Bug report

Cannot instantiate a class from env variables.

To reproduce

In main.py:

from jsonargparse import CLI


class Bar:

    def __init__(self, v: int) -> None:
        self._v = v


def main(b: Bar) -> None:
    print(b._v)


if __name__ == "__main__":
    CLI(main, default_env=True, env_prefix="FOO")

In shell:

export FOO_B__V=2
python -m main

Will result in the error:

usage: main.py [-h] [--config CONFIG] [--print_config[=flags]] [--b.help [CLASS_PATH_OR_NAME]] b
error: Validation failed: Key "b" is required but not included in config object or its value is None.

I can instantiate this class with:

export FOO_B="{v: 1}"
python -m main

But I cannot even override v:

export FOO_B__V=2
python -m main

Will still result in v being 1.

Expected behavior

I can instantiate a class from env variables. At least override fields.

Environment

• jsonargparse version: 4.37.0
• Python version: 3.12.8
• How jsonargparse was installed: uv pip install -U "jsonargparse[all]"
• OS: macOS

[mauvilsa]

Thank you for reporting!

Note that not every possible name for environment variables is supported. And this wouldn't be considered a bug. In principle you should look at the output of --help to know what environment variable names you can use.

In this particular case b is a subclass type. So Bar or a subclass of Bar is accepted. And there is no guarantee that all subclasses have v. For command line arguments now the base class is implicitly chosen when an init arg that matches is given. This case looks similar but for env vars. But env vars are different because they don't have an order. Right now I am not sure if FOO_B__V should be supported in this case.

[mauvilsa]

On a different note, since parameter b doesn't have a default, it becomes positional. But a subclass as positional is not really a use case. And I have noted that as positional, the output of --help doesn't show the env var FOO_B. Better to implement this as CLI(main, default_env=True, env_prefix="FOO", as_positional=False).