Specifying the minimum supported python version

[konstin]

Currently, there is no way to specify minimal supported python version, and compiling against a too low version or fail with an error about missing symbols (or worse, at runtime due to some failed import). With maturin, I'd like to build abi3 wheels for the lowest supported python version, so there's only one wheel and that supports all versions. I'm currently thinking about how to best specify this. Ideally, pyo3's build script or at least setuptools-rust should also check for this version and exit with an understandable error message about the wrong python version (I can write the pull request).

I currently have three different ideas:

• In the cargo metadata section. The big plus is that user already configure a Cargo.toml.
• In pyproject.toml. We could start in the tools section and switch over the official python-requires when PEP 621 is accepted.
• Feature flags. Add py36-abi3, py37-abi3, etc. features

@kngwyu @davidhewitt What do you think?

[davidhewitt]

Is it possible to use metadata from the package itself in pyo3's build.rs? Even if it is I'm not enthusiastic about doing that. OTOH Maturin could definitely check the cargo metadata, and setuptools-rust the pyproject.toml.

I wonder if we can add macros like rustversion ?

[kngwyu]

I think the combination of feature flags (for build scripts) + toml config (for maturin or setuptools) is good, though we have to invert unstable-api flags to do so.

I wonder if we can add macros like rustversion ?

We already have #[cfg(Py_3_7)] or so for that purpose.

[davidhewitt]

We already have #[cfg(Py_3_7)] or so for that purpose.

#[cfg(Py_3_7)] etc. only works inside of the pyo3 code though; downstream crates don't see these flags unless we give them a way to add them in their own build scripts.

[konstin]

I checked that only setting the cfg's in build.rs works nicely with abi3. I've just checked with this hacky snippet:

fn main() -> Result<()> {
    if env::var_os("CARGO_FEATURE_ABI3").is_some() {
        let msg = r#"cargo:rustc-cfg=Py_3_5
cargo:rustc-cfg=Py_3_6
cargo:rustc-cfg=Py_3_7
cargo:rustc-cfg=Py_3
cargo:rustc-cfg=py_sys_config="WITH_THREAD"
cargo:python_flags=FLAG_WITH_THREAD=1,CFG_Py_3_5,CFG_Py_3_6,CFG_Py_3_7"#;
        println!("{}", msg);
        return Ok(());
    }

The really cool part about this is that the targeted python version doesn't need to be installed on the build machine anymore, which means e.g. faster and easier CI.

I also realized that one big disadvantage of the toml solutions would be that we would need to add a toml parser as build dependency.

[kngwyu]

I think we can employ basically the same strategy as @konstin 's snippet, but what I'm not sure is whether we should support the combination of abi3 and minimum required Python version (e.g., abi3 build, but does not support Python 3.6).
Any idea?
cc: @alex

[alex]

I'm not sure I understand the question. Can you try rephrasing it?

[kngwyu]

Ah, sorrry, but I noticed that my original question meant nothing so please ignore it.

[kngwyu]

@konstin

The really cool part about this is that the targeted python version doesn't need to be installed on the build machine anymore, which means e.g. faster and easier CI.

You mean we can build py35-abi3 using Python 3.6 or 3.7?

[alex]

Being able to build an ABI3 wheel which targets a lower ABI than the Python you're building with would be cool. (e.g. Using Python 3.8 to build an abi3 wheel with 3.5 as the minimum version) However, it's not a hard requirement for me. Not sure what the best API for it would be.

[konstin]

Being able to build an ABI3 wheel which targets a lower ABI than the Python you're building with would be cool. (e.g. Using Python 3.8 to build an abi3 wheel with 3.5 as the minimum version) However, it's not a hard requirement for me. Not sure what the best API for it would be.

Even better, with abi3 you should be able to build without any python version present! That's what I tried to do with the snippet I posted above, where I only set rust config and don't read or link any python.

[davidhewitt]

@konstin now that #1263 is merged, would be interesting to hear how this works for you in maturin!

[konstin]

Thank you @kngwyu and @davidhewitt for all the help and work!

Being able to build an ABI3 wheel which targets a lower ABI than the Python you're building with would be cool. (e.g. Using Python 3.8 to build an abi3 wheel with 3.5 as the minimum version) However, it's not a hard requirement for me. Not sure what the best API for it would be.

As a follow-up, on linux/mac you can build now without any python version, while on windows you need a python interpreter for linking with its python3.lib, where the python version shouldn't matter.