The most popular python tools each have a different place to put dev dependencies. I think they should just be optional.
Update (2024-07-29): Thanks to Shalev NessAiver who suggested a much better title for this article.
Update (2025-12-04): There's finally a standard place and I like it.
Both PEP 621 and the newer pyproject.toml Specification describe how a Python project should specify its dependencies which are required to run the project. They also describe optional-dependencies which are "extra" to the project and can be added for additional features. Groups of optional-dependencies are grouped under a common name.
There's another class of dependencies that are not discussed in the aforementioned standards and these are known as dev-dependencies: those packages which you need during development of the project, but not to use the project itself.
This is probably a flaw in the specification; compare that with package.json which has devDependencies and therefore the tools in the node ecosystem know where to put packages you add with --save-dev.
I generally concur with this comment about dependencies:
Why am I pointing this out? Because adding an actual dependency to my code is something I take very seriously. Importing lodash or react or whatever is a serious decision that I feel should be taken with extreme caution. Dependencies can lead to bloated code, exploits and bugs that are out of your control. If everything is on "devDependencies" or "dependencies", it's really hard to see how many of these dangerous imports a project has.
I tend to make it a personal goal to have as few dependencies as possible, but I couldn't care less for devDependencies.
I also make it a personal goal to reduce the number of dependencies my projects use. But I also care about my dev dependencies. These are the tools that make my projects possible and I care if they're slow or fast or have security issues. I take adding new dev dependencies seriously because it's overhead for anyone who wants to be so kind to work on my projects (usually some future version of myself). But I agree that it's a little bit different than a core dependency.
Back to Python. Because dev-dependencies are not part of the pyproject.toml specification, popular tools like pdm and poetry need to specify custom configuration to store this metadata.
But why? We could just treat dev-dependencies as optional-dependencies with the convention of calling the group dev (example). Then you'd be able to install them like:
python -m pip install pkg[dev]
# or, if you use uv
uv pip install pkg[dev]In this lengthy discussion on pdm, people seem to be worried about having the dev metadata published on PyPI. But again why? Are you worried Dependabot is going to tell you about security vulnerabilities in your dev dependencies? (Wait, that's a good thing.) Are you worried people will think your project is bloated? (They'd have to install your package with [dev] to notice.)
Sure it would have probably been better for pyproject.toml to have defined a dev-dependencies section. But until it's defined, why not use the standard mechanism for optional dependencies to define dev dependencies?
I can be convinced otherwise, but my current policy is to put dev dependencies in project.optional-dependencies.dev.