Add support for type hinting objects

[IvanIsCoding]

Related to #4849

This PR adds the pyo3-ffi bindings to type hinting objects
and then exposes them in pyo3.

I gated this for Python 3.9+ but I am not confident this is the appropriate way to doing so in PyO3. Is this the idiomatic way? I prefered to gate the whole module instead of individual methods.

[IvanIsCoding]

@ngoldbaum can you have a look at this before the other maintainers?

[ngoldbaum]

I've purposely avoided learning much about Python type hints so I'm not sure why you'd want to use this, although I didn't read the test closely to figure that out.

There is guide/src/python-typing-hints.md in the narrative docs - can you maybe expand the content there to describe how you might use this?

[IvanIsCoding]

I've purposely avoided learning much about Python type hints so I'm not sure why you'd want to use this, although I didn't read the test closely to figure that out.

There is guide/src/python-typing-hints.md in the narrative docs - can you maybe expand the content there to describe how you might use this?

I will address the comments and try to update other PyO3 documentations related to type hints. But the core motivation is making implementing __class_getitem__ easier.

[IvanIsCoding]

After reading guide/src/python-typing-hints.md, I think David's comment in #4849 (comment) becomes more relevant.

__class_getitem__ is more of an implementation details, so I'd rather defer the documentation updates once in a separate PR we add a macro allowing #[pyclass(generic)] to generate __class_getitem__ for the user.

That way, we can just document that #[pyclass(generic)] is the closest we have to inheriting from typing.Generic or doing class MyExampleClass[T] on the modernized form.

[ngoldbaum]

Fair enough about the docs! Let’s let another maintainer who knows a bit more about type hint plumbing in Python take a look at this but the FFI bits look correct to me.

[davidhewitt]

Thanks! FFI bindings look good, I have some suggestions for the safe wrapper 👍

[davidhewitt]

The API documents that it may fail, so I think we should use PyResult return value here.

[IvanIsCoding]

Thanks! FFI bindings look good, I have some suggestions for the safe wrapper 👍

I think it should handle errors now. Also, I will sitck with PyGenericAlias::new accepting PyObject instead of PyType, let's make Python handle the errors on this one.

[davidhewitt]

Thanks, just one final suggestion then.