Incompatibility with PEP 621

[abravalheri]

Hi guys I am under the impression that pdm is not compatible with PEP 621 (I have been working in a pyproject.toml validator and this problem showed up when running it against some projects created using pdm).

In particular the problems lie in the following parts of the spec:

1. version - Format: string
2. Build back-ends MUST raise an error if the metadata specifies a field statically as well as being listed in dynamic.

PDM documentation indicate that users can do version = {use_scm = true} or version = {from = "pdm/__init__.py"}, which violates the spec in (1).

Regarding item (2), I understand that passing version = {use_scm = true} or version = {from = "pdm/__init__.py"} doesn't really is specifying a field statically, but this is still a very controversial usage/interpretation of the standard.

• I have searched the issue tracker and believe that this is not a duplicate.

Make sure you run commands with -v flag before pasting the output.

Steps to reproduce

1. Check a PDM's powered project that uses dynamic versions for its pyproject.toml
2. Validate the pyproject.toml file against PEP 621

Actual behavior

PDM accepts a inline-table version.

Expected behavior

PDM should error when version is not a string.
Additional tool-specific information needed for dynamic version discovery, should be stored in tool.pdm

[pawamoy]

I guess there's an issue with classifiers as well then. Too bad, it means we cannot specify some classifiers statically and let the build backend add some more dynamically...

[abravalheri]

Yeap, the 2 things cannot exist at the same time unless using a tool-specific table.

The PEP goes even further and says:

If the metadata does not list a field in dynamic, then a build back-end CANNOT fill in the requisite metadata on behalf of the user (i.e. dynamic is the only way to allow a tool to fill in metadata and the user must opt into the filling in).

So it is "not allowed" to implicitly add classifiers to the user-given list (i.e. add extra classifiers to PKG-INFO/METADATA without the field being listed in dynamic which in turn requires the user to leave the field completely unspecified).

[frostming]

Thanks, I've also noticed that and planned to change someday. version will be moved to tool table automatically, but classifiers, I think it is not intuitive to defined it in tool.pdm just for auto-filling, so maybe it is time to remove it from the dynamic fields and make it pure static... too bad, the same feeling as @pawamoy

[abravalheri]

Should we start a discussion on discord about this, at least for the classifiers? It is a very handy feature...

I don't know how the community would feel about creating another PEP to amend this one, though... It might be difficult to get people on board.

[frostming]

Sure, join the PDM discord channel which can be found in the README

[abravalheri]

Sorry, I mean a discussion with the packaging community about amending the PEP...

[frostming]

Okay, then https://discuss.python.org/c/packaging/14 should be the correct place, would you like to create a topic there?

[frostming]

But IMO according to the spirit of dynamic of PEP 665: dynamic fields shouldn't be listed as a static field at the same time, there is no space for the autofilling classifiers, unless users are okay(at least I am not) to have them written in the tool table. Or do you have better approach?

[abravalheri]

I understande @frostming, and I don't have a better approach. Maybe the best is to align with the PEP.

[oprypin]

So the ability to automatically specify Python version classifiers has been fully removed:

• Feature/strict-pep621 #759
• Move dynamic version to tool table pdm-backend#53

I feel like that was a huge overreaction. Surely there are ways to keep this functionality under a different config key. And I wasn't specifying any extra static classifiers anyway.

I'm once again in a situation where I'm lacking features compared to Poetry
https://python-poetry.org/docs/pyproject/#classifiers

This, along with the fact that features are being removed like this just on a whim, is again a showstopper for me.

[pawamoy]

You mean marking classifiers as dynamic, and specifying classifiers in [tool.pdm] (or not), so that PDM can also add dynamic ones?

[pawamoy]

Well in the end, I like declaring my classifiers as static, because it forces me to actually add some meaningful ones 😁
Not sure if classifiers other than supported Python versions are really that useful though 😅