Validate that `description` is single line

[FFY00]

[sigma67]

@FFY00 what is the rationale here?

This breaks all our project installs via pdm-backend (which was updated yesterday to 0.8.0rc1).

Since when are multiline descriptions illegal?

[henryiii]

The spec says they are one line:

https://packaging.python.org/en/latest/specifications/core-metadata/#summary

And putting multiple lines originally broke the format:

mesonbuild/meson-python#67 (comment)

But it looks like we were joining the newlines correctly since 0.6 in c74f4c5.

@frostming, thoughts?

[henryiii]

description (PEP 621) is the Summary (METADATA) field. readme (PEP 621) is Description (METADATA). See https://packaging.python.org/en/latest/specifications/pyproject-toml/#description and https://packaging.python.org/en/latest/specifications/pyproject-toml/#readme.

A multiline "description" would be readme.text in PEP 621.

[sigma67]

You are correct regarding the single line summary/description field

However due to the previous behavior it is effectively a breaking change

[henryiii]

@frostming can decide here (also @dnicolodi or @rgommers for meson-python). If it should be warning for a release, then error, or have an opt-in/out by backends, etc, happy to leave it to them. If I were deciding, I'd look at the numbers on PyPI of multiline project.descriptions. (I'll likely do that at some point anyway).

[rgommers]

Since there is already a report of breaking even with a pre-release, a deprecation rather than a hard error seems to be warranted.

For meson-python: we don't put an upper bound on pyproject-metadata, so this has the potential to break our users too. I've spot checked the most prominent users and those all use single line descriptions, but it's hard to be sure without spending a lot more time on it which packages would be broken.

[rgommers]

I've just created a milestone for 0.8.0 - it'd be useful to start tagging issues/PRs with a milestone, makes it easier to investigate potential issues later.

[henryiii]

even with a pre-release

This is vendored into pdm-backend, so it's actually a normal release of pdm-backend that found this, not a pre-release. Scikit-build-core will be vendoring in the next release too: scikit-build/scikit-build-core#703 - partially because we guarantee reproducible SDists, and those can be affected by changes in pyproject-metadata. Also I want to ship METADATA 2.2 soon, and don't want to wait for all distros to update.

[dnicolodi]

I'm not sure I follow. Is this about how the field is serialized in the METADATA file or is it about how it is specified in pyproject.toml? Clearly the former is possible as the format used for the METADATA file allows for splitting value into multiple lines (it just needs to be done properly, and it is actually required for very long field values). For the latter, the only thing that the specification says, is that description needs to be a string, thus, technically, newlines are allowed. However (unfortunately as most of the Python packaging specifications) the specification is very loose and lacking details on how the field is supposed to be used. I think that concatenating multiple lines into a single line is the correct thing to do. I'm not sure whether the multi-line support currently in pyproject-metadata is correct.