Add types for None, Ellipsis, and NotImplemented

[rytheo]

Fixes #3162

[davidhewitt]

Thank you! Looks like a merge conflict and also needs a newsfragment 3408.added.md please

[rytheo]

The singletons don't expose their type objects in the C API. Is it necessary for these types to implement PyTypeInfo?

[davidhewitt]

Ah yes, looks like they are not part of the stable API and will even be removed in Python 3.13 python/cpython#107030

So the way to implement for None and NotImplemented should be like we implement PySequence and PyMapping. (You're right, we can't implement PyTypeInfo for them.)

[davidhewitt]

Thanks, this implementation looks correct to me. Would you be willing to add some tests please?

I think it should be sufficient to check that some arbitrary value like empty dict doesn't downcast to these types (e.g. PyDict::new(py).downcast::<PyEllipsis>().is_err()) and also that downcasting the singletons succeeds (e.g. py.Ellipsis().as_ref(py).downcast().unwrap().is(&py.Ellipsis())).

[mejrs]

Should we deprecate/remove Py::None in favour of this?

[davidhewitt]

That's a good question; I'd prefer to leave the py.None() etc APIs unchanged for 0.20 because they'll probably all change anyway in 0.21. Probably we do want to remove py.None() as it's untyped.

So for now, this PR looks good to me, thanks @rytheo!

[davidhewitt]

Ah that's unfortunate, it looks like PyPy doesn't expose PyEllipsis_Type. Probably the most practical solution is to change the PyEllipsis implementation here to expose PyTryFrom instead of PyTypeInfo as well, then the three implementations work the same. (An alternative would be to use #[cfg] to have different CPython and PyPy implementations, but I don't think that's as nice for either maintainers or users.)

[rytheo]

@davidhewitt the coverage-ubuntu CI failure was caused by some unterminated character literal error. It doesn't seem to be related to my changes. Any ideas?

[davidhewitt]

Looks like a possible bug in cargo llvm-cov, let's see if rerunning fixes

[davidhewitt]

Please rebase and CI should be working again.