pyo3(get, set) attribute not found when using cfg_attr

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🌍 Environment

Your operating system and version: Arch Linux x86_64 5.7.4-arch1-1
Your python version: Python 3.8.3
How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: pacman
Your Rust version (rustc --version): rustc 1.46.0-nightly (6bb3dbfc6 2020-06-22)
Your PyO3 version: 0.10.1
Have you tried using latest PyO3 master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?: No

💥 Reproducing

Please provide a minimal working example. This means both the Rust code and the Python.

This compiles

#[cfg_attr(feature="pyo3", pyclass)]
#[derive(Debug, Default)]
pub struct Foo {
    #[pyo3(get, set)]
    bar: bool,
}

However, this doesn't

#[cfg_attr(feature="pyo3", pyclass)]
#[derive(Debug, Default)]
pub struct Foo {
    #[cfg_attr(feature="pyo3", pyo3(get, set))]
    bar: bool,
}

Because it can't find the pyo3 attribute

error: cannot find attribute `pyo3` in this scope
  --> src/material/texture/mod.rs:45:32
   |
45 |     #[cfg_attr(feature="pyo3", pyo3(get, set))]
   |                                ^^^^

Please also write what exact flags are required to reproduce your results.

This is basically a duplicate of #780.

Based on #780 (comment) I wonder if we can resolve this by changing #[pyclass] expansion to (among other things) add a #[derive(PyClassImpl)] or something like that. Where the PyClassImpl derive macro would apparently be called with all cfg_attr attributes resolved.

This is kind of a hack, though, so I wonder if really we should be asking the compiler team if it's reasonable to expand all #[cfg_attr] clauses before calling custom attributes (as it already appears to do that for derive).

Looks like the compiler team may indeed be doing that rust-lang/rust#83824

Is there a suggested workaround for this issue until that compiler feature lands? Thanks for the help!

Is there a suggested workaround for this issue until that compiler feature lands? Thanks for the help!

You can manually implement getters/setters, and guard that block with a cfg:

#[cfg_attr(feature = "pyo3", pyclass)]
struct Foo{
    bar: u8
}

#[cfg(feature = "pyo3")]
#[pymethods]
impl Foo{
    // not named "bar" because there might already a rust getter for it
    #[getter(bar)]
    fn py_bar(&self) -> u8{
        self.bar
    }
}

I've had to do this for structs with hundreds of fields - not fun. Note that you may need to apply .clone() for fields that are not Copy.

@mejrs

@mejrs get_all is awesome, thank you very much! If I hadn't scrolled until here, I would not have known. Maybe this should be documented somewhere more easy to find?

It is listed at https://pyo3.rs/v0.18.3/class.html#customizing-the-class I'm not sure where else this information should be. Where were you looking for it?

oh sorry, I thought the "pyo3(get,set) doesn't work with cfg_attr" problem was mentioned in the FAQs - if it is included in the future I'd recommend a sentence that suggests get_all as a workaround.

For those stumbling upon this, here is a super explicit example with get_all and cfg_attr

#[cfg_attr(feature = "pyo3", pyclass(get_all))]
pub struct MyStruct {
    pub my_field_a: f32,
    pub my_field_b: f32
}

Revert "remove cg-if dependency"

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`pyo3(get, set)` attribute not found when using `cfg_attr` #1003

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pyo3(get, set) attribute not found when using cfg_attr #1003

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birkenfeld commented on Jun 26, 2020

davidhewitt commented on Aug 30, 2020

davidhewitt commented on Apr 23, 2021

cgranade commented on May 18, 2021

mejrs commented on May 19, 2021

Trolldemorted commented on Apr 15, 2023

mejrs commented on Apr 17, 2023

Trolldemorted commented on Apr 21, 2023

hugohadfieldwayve commented on Apr 27, 2023

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`pyo3(get, set)` attribute not found when using `cfg_attr` #1003