(fix): add __version__

[LDeakin]

No description provided.

[ilan-gold]

We haven't settled on a release process, but I think when you do releases with git tags, there is a _version.py that is updated automatically. Phil definitely knows more than I do about this.

i.e., https://github.com/scverse/anndata/blob/af6480e44a3fa84b7c2dc7195e59d2ce443b06d0/src/anndata/__init__.py#L13

I believe this is done via hatch-vcs

[LDeakin]

Ok, we'll need to tie that in with Maturin in some way. I think the single source of truth is currently the Cargo.toml

[ilan-gold]

https://docs.python.org/3/library/importlib.metadata.html#importlib.metadata.version It's possible this somehow works magically, but I honestly don't know (and probably have no way of knowing from that docstring)

[ilan-gold]

Ok I should have just tried this. It does work, and must be coming from the build process but don't really understand how.

[flying-sheep]

importlib.metadata.version() gets the version from an installed package’s metadata. It uses the import machinery to find the installed package’s “.dist-info/METADATA” file that contains all metadata like name, version, license, dependencies, …
So once you hatch run ... or maturin develop or pip install -e ., you get a special “editable” version of your package+metadata installed into a virtualenv. And calling importlib.metadata.version() inside of the package, the code is executed from that venv where it’s installed, and doing so finds that metadata and returns its value.

To explain everything else:

To build a Python package, one needs a build backend.
We currently use maturin directly as a build backend.
Having VCS versioning requires the build backend to support that.
Maturin doesn’t, and currently you can’t use maturin as a plugin for another build backend (PyO3/maturin#1419), so you can’t leverage their capabilities (e.g. VCS versioning)

So we currently can’t do that. But:

Maturin derives the Python package’s version number from Cargo.toml. And for Rust packages exists very nice relase automation in the form of release-plz.

[LDeakin]

Since it all depends on the Cargo.toml package version anyway, I changed this to just use the CARGO_PKG_VERSION. This is what polars does.

[flying-sheep]

That’s 300μs faster at import time which is nothing. But since it’s also not complicated either is fine.