Bound types with __instancecheck__/__subclasscheck__

[bethebunny]

I have a number of types that are implementing essentially an interface for other bound types. The C++ code knows which types "implement" those interfaces, so I can easily bind a function which typechecks them.

I'd like to use a normal pattern to compare these in python, eg. if isinstance(o, MyInterface):, which would also hint to the type checker that o is a legal MyInterface for usage. However, __instancecheck__ must be defined on the metaclass in Python.

I don't think there's any easy way to actually implement this in nanobind today. I'm wondering if there's something tricky I can try with nb::type_slots, or if anyone has any other good ideas.

[wjakob]

nanobind does not permit metaclass customization. This is intentional and has come up a few times before (please see the issue/PR/discussion history). Based on your questions (also in separate threads), I think that you require a level of customization that will not be possible with stock nanobind, and you might consider to maintain your own fork.