Using tox-uv for cythonized packages

[eelcovv]

I have a package for which i have a setup.py in order to cythonize all my modules. This works fine.

Now I have a tox entry in my pyproject.toml which looks like :

[tool.tox]
requires = ["tox>4", "tox-uv>=1.11.3"]
isolated_build = true
env_list = ["py311", "py312", "fix", "dev", "type", "build"]

# === testenv to run pytest over all your environment ===
[tool.tox.env_run_base]

description = "Invoke pytest to run automated tests under {env_name} with coverage"
passenv = ["USERNAME"]
changedir = "{toxinidir}"
allowlist_externals = ["pytest", "uv"]
dependency_groups = ["dev"]
commands = [["pytest"]]

[tool.tox.env.build]
description = "Build the package using uv"
changedir = "{toxinidir}"
commands = [["uv",  "build"]]
allowlist_externals = ["uv"]

If i run tox r -e py311 everything runs fine: a py311 env is created, the cythonized package is installed (pyd files) and the unit test are run.

The problem is, if i add a py312 entry, the py312 environment created installs the pyd files build with python 311, and therefore the unit tests fail. The contents of the py312 directory for my package is :

    Directory: \.tox\py312\Lib\site-packages\mypackage


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----         3/29/2025  10:20 AM          51712 errors.cp311-win_amd64.pyd
-a----         3/29/2025  10:20 AM         110080 qchecks.cp311-win_amd64.pyd
-a----         3/29/2025  10:20 AM          49152 weak_crypto.cp311-win_amd64.pyd
-a----         3/29/2025  10:20 AM          34816 __init__.cp311-win_amd64.pyd

so the cp311 (python311) version is installed.

How can i make sure that also for the building step, the python version is changed to python 312 when i am running py312?

Any hints appreciated!