openscm_runner.adapters cannot be imported on Python 3.12+ without setuptools (distutils removed) #110

Summary

import openscm_runner.adapters fails on Python 3.12 and later unless setuptools happens to be installed, because the CICERO-SCM adapter imports distutils, which was removed from the standard library in 3.12 (PEP 632).

Cause

adapters/__init__.py eagerly imports every adapter:

from .ciceroscm_adapter import CICEROSCM

and adapters/ciceroscm_adapter/ciceroscm_wrapper.py:10 does:

from distutils import dir_util

So importing the package at all pulls in distutils, regardless of which climate model the caller intends to use. On 3.12+ that only resolves through the _distutils_hack shim that setuptools installs.

Reproducing

Verified against v0.13.0 (current release) on Python 3.13:

>>> import openscm_runner.adapters
ModuleNotFoundError: No module named 'distutils'
  File ".../openscm_runner/adapters/__init__.py", line 6, in <module>
    from .ciceroscm_adapter import CICEROSCM
  File ".../ciceroscm_adapter/ciceroscm_wrapper.py", line 10, in <module>
    from distutils import dir_util

Why this bites in practice

setuptools is present in most environments by accident rather than by declaration, so the failure is environment-dependent and surfaces late. Tools such as uv do not install it into a project environment at all, so the same lockfile works on one machine and not another. In our case it made a downstream pipeline unable to run its climate stage on one checkout while an otherwise identical one worked, with nothing in either lockfile to explain the difference.

Possible fixes

• dir_util.copy_tree is the only usage as far as I can see, and shutil.copytree(..., dirs_exist_ok=True) covers it on all supported versions. shutil is already imported in that module.
• Alternatively, import the adapters lazily in adapters/__init__.py, so a distutils dependency in one adapter does not break users of the others.

Happy to open a PR for either if you have a preference.