(as a maintainer of mypy)
For mypy, I’d be supportive of using embedded metadata to configure mypy for a single script. I think this would create a more consistent configuration experience for mypy. The set of CLI options or inline comments is not as expressive as what you can do with a config file (and you can’t check CLI options into the same file).
Like others, I was surprised by the “only use the embedded data if you’re looking at just the file containing it and not if it’s part of a directory”. This would solve some issues for mypy (there are some things you can only configure globally in a single mypy invocation), but will certainly cause surprises for users and limits the applicability to just a single script case. On net, I think this prescription is probably undesirable — in the code quality ecosystem, people seem to really like having a single invocation of a tool running on all the things, and integrations like pre-commit expect this.
I definitely like having more structured per-file configuration though. The current state of the art is special comments and I think this could work better. It’s often not expressive enough. It’s not special enough to avoid common mistakes, for instance, if you accidentally delete some code and end up with a module level # type: ignore, you’re going to be unhappy. Here’s a recent example of a similar issue in ruff: Ruff v0.0.281
(as a maintainer of Black)
I’m new to maintaining Black and Black is a project that actively dissuades its users from configuration, so I need to think more before I say anything.
(general comments)
Overall, I was surprised by how much people like putting tool-specific configuration in pyproject.toml. Reducing the number of configuration dialects and extra files seems to have been valuable; I’m not sure the authors of PEP 518 anticipated this. PEP 518 prescribes almost nothing, and certainly did not coordinate with non build tools in the manner we’re doing now, but it created a Schelling point and the community ran with it.
Since I already prefer the embedded TOML format for the core dependencies use case, I think it’s a bonus that PEP 723 allows for further serendipity in this space.