Make it possible to know about "unknown" keys that might be set in `[project]` table

[pradyunsg]

Inspired by https://github.com/pypa/flit/issues/505.

[FFY00]

Actually, we should go one step further and try to identify typos like in https://github.com/pypa/build/blob/96f9188ad181907fbd3e0efdf32dd3dc959d39c3/src/build/__init__.py#L177.

[pradyunsg]

As long as the caller is able to control how these would be presented, it'd satisfy my usecases!

The use case: In https://github.com/pradyunsg/sphinx-theme-builder, all output is piped through a ui.log function for stylizing and prefixing it, and all errors/warnings are presented as an instance of a rich-renderable exception).

[FFY00]

Well, I intended to raise a warning, like pypa/build. You can specify your own custom warning handler by overwriting warnings.showwarning to decide how the warnings are presented to the user (see https://github.com/pypa/build/blob/96f9188ad181907fbd3e0efdf32dd3dc959d39c3/src/build/__main__.py#L64). I think this provides all the control needed for your use-case, right?

[pradyunsg]

Sounds good to me!

[pradyunsg]

Thinking about this a bit more... I think it would be great if the warning object has the "unknown" keys accessible directly as an attribute.

[pradyunsg]

Based on a more recent usecase and "interaction with the real world", I would prefer that this be a validation error instead; since a build-backend should error out if it gets metadata here that's not declared properly.

If someone wants to allow for permissive parsing, I don't imagine they would be using this library for that.

[FFY00]

The main worry there would be due to backwards compatibility in [project] table, as it would prevent projects that use new keys that we currently don't know about from building.

Looking into this a bit more, I think a reasonable compromise would be to keep it a warning, but to add a filter to promote it to error by default. This way it would result in an error under normal situations, but still allow users to use -Wdefault or PYTHONWARNINGS=default (or ignore) to override it.

What do you think?

[rgommers]

That sounds reasonable to me. In most libraries adding a filter by default would be frowned, but it shouldn't matter here because usages of pyproject-metadata are quite short-lived.

It seems difficult to introduce new keys in [project] and hence fairly unlikely to happen any time soon; an escape hatch can't hurt though.

[henryiii]

IMO, unlike build, which has to process any backend, this is mostly used by backends. So if a key isn't recognized, then it's not being handled, because we are the ones handling it. For example, on the new PEP 639 license fields: we current don't handle it, so users (scikit-build-core, meson-python, and pdm.backend) can't use it in combination with pyproject-metadata. Setting license to string throws an error. And setting license-files to anything doesn't put it into the RFC metadata; it just silently does the wrong thing!

And typos are very common. I found thousands of typos in my download of all pyproject.tomls on PyPI.

If a build backend wants to support new fields, they can pre-process the input. That way they can manually handle some new field without having it

I think just making this an error is fine. Though we could have build backends opt-in by calling a validate_tables function or similar. We could have errors for the top level and for build-system as well. Unlike project, though, any future additions there could be unrelated to project metadata, while everything in project is supposed to affect the metadata for the project.

[henryiii]

Also, the current state is inconsistent: arbitrary keys like project.stuff pass, but project.readme.stuff does not; existing entries can be modified too (license was just changed to allow a string), so if this was just for future safety, then why are sub tables checked specifically?