[0.22] How to fix *mut pyo3::Python<'static> cannot be shared between threads safely when using with_gil and allow_threads

[relf]

I have implemented an optimizer in Rust and use it from Python. I pass a Python function and use pyo3-log to get some info during the optimization process. With the previous API, it works fine, I got something like:

   fn minimize(&self, py: Python, fun: PyObject) -> PyResult<OptimResult> {
        let fun = fun.to_object(py);
        let obj = move |x: &ArrayView2<f64>| -> Array2<f64> {
            Python::with_gil(|py| {
                let args = (x.to_owned().into_pyarray_bound(py),);
                let res = fun.call1(py, args).unwrap();
                let pyarray: &PyArray2<f64> = res.extract(py).unwrap();
                pyarray.to_owned_array()
            })
        };
        let optimizer = builder::optimize(obj).build();

        let res = py.allow_threads(|| {
            optimizer.run().expect("optimize the objective function")
        });
        ...

now I am trying to upgrade to the new API (PyO3 0.22), I've tried with the following code but I get a compilation error :

fn minimize(&self, py: Python, fun: PyObject) -> PyResult<OptimResult> {
  let fun = fun.bind(py);
  let obj = move |x: &ArrayView2<f64>| -> Array2<f64> {
      Python::with_gil(|py| {
          let args = (x.to_owned().into_pyarray_bound(py),);
          let res = fun.call1(args).unwrap();
          let pyarray = res.downcast_into::<PyArray2<f64>>().unwrap();
          pyarray.to_owned_array()
      })
  };
  let optimizer= builder::optimize(obj).build();

  let res = py.allow_threads(|| {  #  Compilation error `*mut pyo3::Python<'static>` cannot be shared between threads safely...
      optimizer.run().expect("optimize the objective function")
  });
  ...

I've seen there are some developments in progress around allow_threads in #3646 and some discussions #3788 but I do not see how to fix it. Thanks for your help and this awesome project!

[mejrs]

fun has a Python token, so the closure that captures does too. (This is also explained in the compiler error).

[relf]

Ok so do you know how should I write this to make it work? I mean the old version works like a charm what is the translation using the new API?

[mejrs]

I think you can simply delete the let fun = fun.bind(py); line.

[relf]

Thanks for your reply! I just had to put the fun.bind(py) call within the closure.