Compatibility with Python>=3.13

[feltech]

What

Ensure OpenAssetIO builds and runs in Python version 3.13.

Why

There is a particular breaking change in that _Py_IsFinalizing has been renamed to Py_IsFinalizing, and the old name is still used in the codebase.

Acceptance Criteria

• OpenAssetIO continues to build and all tests pass in Python 3.11.
• OpenAssetIO builds and all tests pass in Python 3.13
• CI jobs to assert this.

[sh4shv4t]

Hi! I'd like to work on this issue. I'll investigate the _Py_IsFinalizing → Py_IsFinalizing change for Python 3.13 compatibility, update the code with the appropriate version guards if needed, and ensure the project builds and tests pass for both Python 3.11 and 3.13. Please let me know if there's any specific guidance I should base my work on.

[feltech]

That's great! Please check the contribution docs.

I suspect updating CI will be much more difficult that the actual fix(!).

I think .github/workflows/build.yml would be the best place to add a Python 3.13 test. I.e. add Python version to the build matrix and update theactions/setup-python call. Note that this is currently done in the .github/bootstrap_platform/action.yml utility, so you will need to pass a parameter to that specifying the Python version to use (and default to 3.11 if no parameter passed).

To develop/test, I suggest making a PR targeted against your own fork to check that CI passes, before making a PR targeting this repo. The main reason is that someone on our team will have to click to allow CI to run, so the feedback loop will be rather long.

I also suggest making the CI change first so you can see the expected build failure, and then apply the fix to the source code.

[feltech]

It's actually even more complex than I let on.

Since Python 3.13 is VFX Reference Platform CY2026, it also means our C++ libraries should be built with a different compiler toolchain. This is quite a complex CI change!

So far we've been lucky that our supported VFX Reference Platform versions, CY23 and CY24, use an almost identical toolchain - just Python 3.10 vs. 3.11, and CY25 is more-or-less identical to CY24 for our purposes.