Pyo3：没有为 `&Py<PyAny>` 实现特征 `PyClass`答案

【问题标题】：Pyo3: The trait `PyClass` is not implemented for `&Py<PyAny>`Pyo3：没有为 `&Py<PyAny>` 实现特征 `PyClass`
【发布时间】：2021-05-18 23:07:21
【问题描述】：

我正在学习 rust 并尝试使用 pyo3 和 maturin 制作一个非常简单的 python 模块。不过，我在 rust 代码上遇到了问题，

Cargo.toml

[package]
name = "lenrs"
version = "0.1.0"
authors = ["matheusfillipe"]
edition = "2018"

[dependencies.pyo3]
version = "0.13.2"
features = ["extension-module"]

[lib]
crate-type = ["cdylib"]
name = "lenrs"

src/lib.rs

extern crate pyo3;

use pyo3::exceptions::PyTypeError;
use pyo3::prelude::*;
use pyo3::wrap_pyfunction;

#[pyfunction]
fn length(py: Python, obj: &PyObject) -> PyResult<PyObject> {
    if let Ok(s) = obj.extract::<String>(py) {
        return Ok(s.len().to_object(py));
    }
    if let Ok(s) = obj.extract::<Vec<String>>(py) {
        return Ok(s.len().to_object(py));
    }
    Err(PyTypeError::new_err("Not Supported"))
}

#[pymodule]
fn lenrs(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_wrapped(wrap_pyfunction!(length))?;
    Ok(())
}

lenrs/init.py

from .lenrs import *

构建输出

$ cargo build                 
   Compiling lenrs v0.1.0 (/home/matheus/projects/learn-rust/lenrs)
error[E0277]: the trait bound `&Py<PyAny>: PyClass` is not satisfied
 --> src/lib.rs:7:1
  |
7 | #[pyfunction]
  | ^^^^^^^^^^^^^ the trait `PyClass` is not implemented for `&Py<PyAny>`
  |
  = note: required because of the requirements on the impl of `pyo3::FromPyObject<'_>` for `&Py<PyAny>`
  = note: required because of the requirements on the impl of `ExtractExt<'_>` for `&Py<PyAny>`
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `lenrs`

To learn more, run the command again with --verbose.

我发现问题出在#[pyfunction] 上。当我有一个仅适用于字符串的更简单版本并且length 函数将返回PyResult<()> 时，一切都运行良好，而我最终只会返回Ok(())，但现在我不确定该怎么做如果不支持该类型，此函数将引发 python 错误。

【问题讨论】：

标签： python rust rust-cargo pyo3

【解决方案1】：

我刚刚发现 obj 不应该是引用，将函数签名更改为：fn length(py: Python, obj: PyObject) -> PyResult<PyObject> 使其工作：

extern crate pyo3;

use pyo3::exceptions::PyTypeError;
use pyo3::prelude::*;
use pyo3::wrap_pyfunction;

#[pyfunction]
fn length(py: Python, obj: PyObject) -> PyResult<PyObject> {
    if let Ok(s) = obj.extract::<String>(py) {
        return Ok(s.len().to_object(py));
    }
    if let Ok(s) = obj.extract::<Vec<String>>(py) {
        return Ok(s.len().to_object(py));
    }
    Err(PyTypeError::new_err("Not Supported"))
}

#[pymodule]
fn lenrs(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_wrapped(wrap_pyfunction!(length))?;
    Ok(())
}

【讨论】：