Is it possible to initialise a subclass of a class defined in rust?

[samuelcolvin]

I assume this is impossible, but since it would be a massive win for pydantic, I thought it worth asking.

Let's say I have:

#[pyclass(subclass)]
struct MyClass {
    vec: Vec<Whatever>,
}

impl MyClass {
    fn with_capacity() -> Self {
        ....
    }
}

#[pymethods]
impl MyClass {
    #[new]
    fn new() -> Self {
        ...
    }

    pub fn method(&self) -> PyResult<usize> {
        ...
    }
}

Because MyClass is marked with #[pyclass(subclass)] I can subclass it in python.

from my_pyo3_library import MyClass

class MySubclass(MyClass):
    ...

Is it possible to pass MySubclass back to pyo3/rust code and create instances of MySubclass using the with_capacity method or similar.

E.g. is it somehow possible to tell rust that I have a variant of MyClass with some kind of pointer pointing to it's subclass but that it's a MyClass and therefore has a with_capacity method?

So something like

from my_pyo3_library import MyClass, do_magic

class MySubclass(MyClass):
    ...

do_magic(MySubclass)

fn do_magic(subclass_of_my_class: &PyAny) {
    let MyClassVariant = magic(subclass_of_my_class)
    MyClassVariant::with_capacity()

Or equivilantly, is it possible to create a MyClass instance, then cast it to a MySubclass?

[samuelcolvin]

Ping. Anyone able to give and feedback on this?

Would be great to know even if the answer is "no, you're crazy, that's impossible".

[samuelcolvin]

MyClass is accessible in python fine, since it's marked with #[pyclass(subclass), but that's not really the question.

I also don't want to use with_capacity in python, but rather in rust, on a subclass of MyClass.

[birkenfeld]

Both of these points were clear to me when I wrote the response :)

[samuelcolvin]

Then how are you not sure what I'm asking? :)

[birkenfeld]

Good question. However, a better question, did my reply not help at all?

[samuelcolvin]

Well, it made me slightly more confident that the trick I was trying to accomplish is indeed impossible.

But I wanted to be sure you had understood the question before giving up on this, hence trying to clarify what I meant.

[birkenfeld]

With the additional info I could glean from your recent comment in #2347, let me have one more go:

It is not possible to create an instance of MySubclass in Rust without going through the Python machinery, i.e., calling MySubclass using call() or similar.

When you have an instance of MySubclass in Rust (as a PyAny etc) it is possible to get at a PyRef<MyClass> using extract.

[samuelcolvin]

Thanks for this, makes sense.