Replacing tox with UV

Recently I've been updating some of my libraries to Python 3.10+ after Python 3.9 has finally reached end of life.

The upgrade to Python 3.10 is a relatively simple one, but I thought it be a good idea to run tests on different versions of Python.

Existing Tooling

tox is a common tool for this use case, it allows you to define environments declaratively. 

requires = ["tox>=4"]
env_list = ["3.14", "3.13", "3.12", "3.11", "3.10"]

[env_run_base]
description = "run unit tests"
deps = [
    "pytest>=8",
    "pytest-sugar"
]
commands = [["pytest", { replace = "posargs", default = ["tests"], extend = true }]]

Similarly nox let's you do the same but in a more imperative way.

uv to rule them all

The thing is, tox and nox requires learning and using a new set of tools, whilst that's completely fine uv has spoiled us with the convenience of using a single tool.

There's already a lot that can be replaced by uv, for example:

python -m venv
uv venv

pip install ...
uv pip install ...

pyenv install ...
uv python install ...

I read an article from Simon Willison where he already worked out how to test a package with different Python versions with the -p flag.

uv run -p 3.10 pytest

This is pretty much job done here! But we can go a bit further ...

Extras

You can specify any extras and dependency groups via --extra and --group respectively:

uv run -p 3.10 --extra cli pytest

uv run -p 3.10 --group test pytest

Overriding Package Versions

A common use of tox is to test over different package versions, for example, if a package needs to support both pydantic==1.*.* and pydantic==2.*.*. We can also do this in uv with the --with or -w options:

uv run -w 'pydantic==1.*.*' pytest
uv run -w 'pydantic==2.*.*' pytest

This also works for a set of deps in a requirements.txt file:

uv run --with-requirements requirements-production.txt pytest

Simon's article also mentions using --with-editable to test with the local version of the code, which can come in handy:

uv run --with 'pydantic==1.*.*' --with-editable 'mylocal-package' pytest

Using an isolated venv

Simon also mentions using an isolated venv:

uv run --with 'pydantic==1.*.*' --isolated pytest

to avoid your test venv contaminating your development venv or vice versa.

I found this most useful when I needed to run tests in parallel, something like:

parallel "uv run -p {} --isolated pytest" ::: 3.{10..14}

This is a quick proof of concept but you can probably write a more robust bash or python script.

Conclusion

tox still remains useful especially with something like tox-uv. But if you're using uv anyway it may be useful to learn all the options mentioned above.

I've already had a lot of success using uv to quickly debug issues caused by version incompatibilities in pandas, something that just takes a bit more time to setup with tools like tox.

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