Extend Python Class from Rust

[davidhewitt]

A user asked a question on Gitter just now about extending a Python class from Rust.

While I think it's possible to do this by hand with a lot of unsafe code by implementing PyTypeInfo for the base type manually, it's pretty complicated.

This is an issue to think one day about how to make this easier. Possibly it's good enough to document the current process, or maybe there are design changes we can make internally.

[nekitdev]

I had an initial idea of calling PyType_Type, just like from python, in order to construct new classes with multiple inheritance:

class A:
    a = 13

class B:
    b = 42

C = type("C", (A, B), {})

[davidhewitt]

Interesting. You might find #1152 interesting which I think might use that or something similar (haven't had a chance to review it yet)

[chinedufn]

Possibly it's good enough to document the current process

Are there any existing examples that you can point me to of how this is done now?

Thanks!

[LegNeato]

I recently hit this and asked in gitter too

"How do I use abstract base classes / make my Rust code with the same api as a built in class pass an isinstance check? I
can't seem to find it in the docs or any examples. I specifically want my Rust class to pass isinstance(i, uuid.UUID) where i is a instance created by my Rust code.
(if it isn't supported, happy to put up a PR if someone will point me in the right direction and it is reasonable for a pyo3 beginner)"

David Hewitt @davidhewitt Feb 20 22:50
@LegNeato you're looking at a special case of #991. Unfortunately this is not supported at the moment. It's quite a hard issue to solve, but if you're interested in implementing it I can discuss the design and implementation steps with you on that issue.

@davidhewitt I likely won't have time soon due to work so don't sweat it, but if you find some cycles and can throw some information here I can possibly take a crack at it in the future when work settles down.

[LegNeato]

Potentially relevant: https://www.python.org/dev/peps/pep-0253/