Need help in debugging the below compiler error

[psvri]

Hello,

This is a simplified version of the code I am trying to implement.

use pyo3::exceptions::PyTypeError;
use pyo3::prelude::*;
use pyo3::types::PyString;
use std::borrow::Cow;

#[pyclass(frozen)]
#[derive(Clone)]
struct Number(usize);

fn into_cow(obj: &PyAny) -> Cow<Number> {
    if obj.is_instance_of::<Number>() {
        Cow::Borrowed(obj.extract::<&Number>().unwrap())
    } else if obj.is_instance_of::<PyString>() {
        Cow::Owned(Number(obj.extract::<&str>().unwrap().parse().unwrap()))
    } else {
        panic!("invalid type {:?}", obj.get_type())
    }
}

// impl FromPyObject<'_> for &Number {
//     fn extract(obj: &'_ PyAny) -> PyResult<&Number> {
//         if obj.is_instance_of::<PyString>() {
//             PyResult::Ok(&Number(obj.extract::<&str>()?.parse().unwrap()))
//         } else {
//             PyResult::Err(PyTypeError::new_err(format!(
//                 "Cannot convert type {} into Number ",
//                 obj.get_type()
//             )))
//         }
//     }
// }

When I compile this I get

error[E0277]: the trait bound `&Number: PyClass` is not satisfied
   --> src\lib.rs:12:27
        Cow::Borrowed(obj.extract::<&Number>().unwrap())
                           ^^^^^^^ the trait `PyClass` is not implemented for `&Number`

    = help: the trait `PyClass` is implemented for `Number`
    = note: required for `&Number` to implement `pyo3::FromPyObject<'_>`
note: required by a bound in `pyo3::PyAny::extract`

    pub fn extract<'a, D>(&'a self) -> PyResult<D>
           ------- required by a bound in this associated function
     where
         D: FromPyObject<'a>,
          ^^^^^^^^^^^^^^^^ required by this bound in `PyAny::extract`

Since the above error message says to implement required for '&Number' to implement 'pyo3::FromPyObject<'_> . So when I uncomment the above lines I get the below error.

conflicting implementations of trait `PyFunctionArgument<'_, '_>` for type `&Number`
 --> src\lib.rs:6:1
  |
6 | #[pyclass(frozen)]
  | ^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `pyo3`:
          - impl<'a, 'py, T> PyFunctionArgument<'a, 'py> for T
            where T: pyo3::FromPyObject<'py>, T: 'a;
  = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)

Can anyone help me on how to solve the above.

[davidhewitt]

Instead of

    if obj.is_instance_of::<Number>() {
        Cow::Borrowed(obj.extract::<&Number>().unwrap())
    }

try

    if let Ok(c) = obj.downcast::<PyCell<Number>>() {
        Cow::Borrowed(c.get())
    }

[psvri]

Thank you, it worked like a charm.

Out of curiosity do you know why the compiler giving a bad recommendation ?

[davidhewitt]

FromPyObject for &Number is currently not user-implementable, which is why that creates a conflicting implementation for a different trait. Maybe we can resolve that in the future.

The first error you got is because we have a blanket implementation of FromPyObject for T: PyClass + Clone. It creates a lot of confusing error messages, so I'd be tempted to remove it sometime.

[psvri]

Okay, thanks a lot for the help