Implementing `__richcmp__` makes object unhashable

[kevinheavey]

Bug Description

If you try to call Python's hash function on an object for which you've implemented __richcmp__, it fails with unhashable type. The only fix I've found is to also implement __hash__.

This also happens if you implement __eq__ though I've heard __eq__ isn't supported anyway.

Steps to Reproduce

use pyo3::{basic::CompareOp, prelude::*};

#[pyclass]
#[derive(PartialEq, PartialOrd, Debug, Default)]
pub struct DoesNotHash(u32);

#[pymethods]
impl DoesNotHash {
    #[new]
    pub fn new() -> Self {
        Self(3)
    }
    
    // Uncomment the below to break hashing.
    // fn __richcmp__(&self, other: &Self, op: CompareOp) -> bool {
    //     match op {
    //         CompareOp::Eq => self == other,
    //         CompareOp::Ne => self != other,
    //         CompareOp::Lt => self < other,
    //         CompareOp::Gt => self > other,
    //         CompareOp::Le => self <= other,
    //         CompareOp::Ge => self >= other,
    //     }
    // }
}

/// A Python module implemented in Rust.
#[pymodule]
fn hash_example(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_class::<DoesNotHash>()?;
    Ok(())
}

Backtrace

No response

Your operating system and version

Ubuntu 20.04.3 LTS

Your Python version (python --version)

Python 3.9

Your Rust version (rustc --version)

rustc 1.59.0-nightly (7abab1efb 2021-12-17)

Your PyO3 version

0.15.1

How did you install python? Did you use a virtualenv?

Installed in fresh conda environment.

Additional Info

No response

[davidhewitt]

Thanks for reporting. As the default hash implementation comes from CPython, this is most likely by design and simply an oversight in our documentation. Just needs some investigation to confirm.

[davidhewitt]

Yep looks like it's expected that the default object equality and hashing are not provided if any of these methods are implemented: https://github.com/python/cpython/blob/a1444f43584af0f7a0af72aa06ba0a86ae5a87a2/Objects/typeobject.c#L6011