Cannot create slot (__len__) methods with staticmethod annotation

[willstott101]

I was just exploring the expanded macro source of the library I am working on and was curious if I could increase the performance of, say __len__ by avoiding the reference to &self. For context this is a Vector3(x, y, z) whose __len__ will always return 3.

So to start with I have this:

#[pymethods]
impl Vector3 {
    fn __len__(&self) -> usize {
        3
    }
}

which is fine, but there's a load of code generated around this call.
If I switch to an associated function (still within #[pymethods]):

    fn __len__() -> usize {
        3
    }

pyo3 asks me to mark it as a #[staticmethod]:

error: static method needs #[staticmethod] attribute                                                                                                  
   --> src/vec3.rs:140:5
    |
140 |     fn __len__() -> usize {
    |     ^^

However if I add that annotation:

error[E0061]: this function takes 0 arguments but 1 argument was supplied                                                                                    
   --> src/vec3.rs:40:6
    |
39  |   #[pymethods]
    |   ------------ argument of type `*mut pyo3::ffi::PyObject` unexpected
40  |   impl Vector3 {
    |  ______^
41  | |     #[new]
42  | |     fn new(x: Option<f64>, y: Option<f64>, z: Option<f64>) -> Self {
43  | |         Vector3(na::Vector3::new(
...   |
140 | |     #[staticmethod]
141 | |     fn __len__() -> usize {
    | |______________^
    |
note: associated function defined here
   --> src/vec3.rs:141:8
    |
141 |     fn __len__() -> usize {
    |        ^^^^^^^
help: remove the extra argument

As you can see the generated code is wrong:

               unsafe fn __pymethod___len____(_py: _pyo3::Python<'_>,
                    _raw_slf: *mut _pyo3::ffi::PyObject)
                    -> _pyo3::PyResult<_pyo3::ffi::Py_ssize_t> {
                    let _slf = _raw_slf;
                    _pyo3::callback::convert(_py, Vector3::__len__(_slf))
                }

So long story short, I think there's a bug here... specifically that slot methods that are also staticmethods try to pass an unsafe object pointer into the static method (at least for __len__).

[birkenfeld]

I just tested and surprisingly, defining __len__ as a staticmethod works in pure Python...

[davidhewitt]

Looks like this also works even for things like __add__ (although I can't really see why you'd want to do that).

I see no reason why we should prevent this pattern, so I'll have a play and see how easily we can support it. Certainly the current state of the implementation is a bug.

[willstott101]

I think I found the problem: #3055