Support __eq__ for pyclass

[Lunarmagpie]

I don't think is possible to create a class like this in pyo3. I couldn't find a way to write an equality method in the docs.

class SomeClass:
    def __eq__(self, other):
        return True

I need to be able to write an implementation for this so I can make a python wrapper around rust enums. I found that there wasn't a way to do an equality check. I'm considered writing an is_equal_to method then making another class in python that knows how to use is_equal_to when you call __eq__. I want to avoid this because I think its clunky and requires a lot of boilerplate. Forcing an end user to use a method like is_equal_to is jarring so I can't do that.

[davidhewitt]

At the moment you should implement __richcmp__, which is the C api where all these operators are defined. See #1625 (comment) for a current example, unfortunately the docs aren't great for it right at this moment.

Let's leave this issue open, because I think with the new #[pymethods] magic methods there might be a way to enable users to write the usual Python operations like __eq__. I just need to find a chance to have a play with enabling this.

[Lunarmagpie]

Thanks for explaining!

[ssokolow]

To me, it feels like the most ergonomic thing would be to offer something like #[derive(Eq, Hash, PartialEq, PyEq, PyHash)].

[davidhewitt]

@ssokolow there's been suggestions previously to have #[pyclass(dataclass)] to do exactly that, see #1375 (comment)

Just not got around to implementing!

[ssokolow]

I don't necessarily want dataclass support... just the ability to proxy implementations through easily.

(My most recent example would have been fine with #[pyclass(dataclass)] since it's just porting an IntEnum to Rust as part of migrating as much as possible of a PyQt codebase into its Rust backend, but I've had cases where I would want to easily expose derives as the corresponding dunder methods without having to make all fields public or refactor the thing to put the private methods in a field containing an opaque object.)

(Even then, since I was also porting from rust-cpython to PyO3 at the same time and this is my first use of PyO3, I wound up turning Roles.guess(name) into a guess_role(name: &str) -> Role free function because I'm probably going to refactor things enough in future passes anyway that I didn't feel like front-loading my understanding of how class methods work in PyO3.)

[davidhewitt]

Ah, interesting. I will have a think about whether there's a nice way to do this - it might solve some papercuts I've had too!

[alex]

Being able to define __eq__, instead of __richcmp__, would be a big QoL improvement. The specific advantages are: Not needing to open-code a not-equal or order-comparison implementations, since those could just offer standard behaviors.

[davidhewitt]

Finally pushed #3203 to implement this!