Multiple inheritance with slotted parent

[aminalaee]

My knowledge of Python data model is pretty limited, but I was expecting the following would work:
https://docs.python.org/3/reference/datamodel.html#slots

#[pyclass(subclass)]
#[derive(Clone, Debug)]
struct A {}

#[pymodule]
fn my_module(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_class::<A>()?;
    Ok(())
}

from my_module import A

class B:
    __slots__ = ('example',)


class C(A, B): ...

This fails with:

>       class C(A, B):
E       TypeError: multiple bases have instance lay-out conflict

But the rust class A doesn't define the slots, or even if I set it to empty, this still fails but I expected multiple inheritance should work.
If A was defined in Python, the same thing would work.

Can anyone point me to the right direction of what is happening?

[davidhewitt]

So the cause here is that PyO3 still adds some extra space to struct A's Python layout to do our refcell tracking. You can turn this off with #[pyclass(frozen)]. (We've discussed making frozen the default in the past.)

Testing locally, I indeed find this works:

#[pyclass(subclass, frozen)] // <--- frozen added here
#[derive(Clone, Debug)]
struct A {}

#[pymodule]
fn my_module(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_class::<A>()?;
    Ok(())
}

Note that if you make A non-empty then again you'll have a conflict. That's just a limitation of Python multiple inheritance, there's not much that can be done about that.

[aminalaee]

Thank you for the answer.