check_ar6_magicc7_version reports a missing openscm_runner when the real cause is a different ImportError

[karla-spiller]

Summary

check_ar6_magicc7_version in src/gcages/ar6/scm_running.py catches any ImportError from import openscm_runner.adapters and reports it as a missing optional dependency:

try:
    import openscm_runner.adapters  # noqa: PLC0415
except ImportError as exc:
    raise MissingOptionalDependencyError(
        "check_ar6_magicc7_version", requirement="openscm_runner"
    ) from exc

ModuleNotFoundError subclasses ImportError, so a failure inside openscm_runner is reported as openscm_runner being absent.

What it looks like

On Python 3.12+, openscm_runner.adapters imports the CICERO-SCM adapter, which imports distutils — removed from the standard library in 3.12 (openscm/openscm-runner#110). With openscm_runner installed and importable, we got:

MissingOptionalDependencyError: `check_ar6_magicc7_version` requires openscm_runner to be installed

The suggested fix is to install a package that is already installed. The actual missing module is distutils, which the message never mentions. The chained __cause__ carries it, but the top-level message is what gets read and acted on — it cost us a while to get past.

Suggested fix

Distinguish "the package is absent" from "the package failed to import". For example:

try:
    import openscm_runner.adapters
except ImportError as exc:
    if getattr(exc, "name", None) in {"openscm_runner", None}:
        raise MissingOptionalDependencyError(
            "check_ar6_magicc7_version", requirement="openscm_runner"
        ) from exc
    raise

Re-raising preserves the original message, which names distutils directly. Alternatively, keep MissingOptionalDependencyError but include str(exc) in the message.

Verified against the current release (v0.14.0).