Extract nanobind multiple inheritance support to separate project?

[virtuald]

I embarked on an adventure to port multiple inheritance support from pybind11 to nanobind via gpt-5.3-codex. I got pretty far, and it occurred to me that probably one could implement this without actually modifying nanobind at all (or at least, in a very limited way).

As I was contemplating nb_type_get, I started searching for other projects and found your polymorphic type caster implementation and the related MI helpers.... so I guess I was right, one can do that. :)

MI is a really common need, so it seems to me that extracting the code into a separate pypi project would be a pretty reasonable thing to do. Are you interested in doing this? If not, I'll probably swipe some of it and do it myself.

[jslee02]

Late reply, but thanks for raising this.

Version scope: this code is from current DART main / DART 7 dartpy. It is not a DART 6 LTS API commitment.

You are right that the current DART helper is effectively a small compatibility layer around nanobind's single-inheritance model. In DART we use it for C++ hierarchies such as BodyNode / EndEffector, where the same object needs to cross Python as Frame, JacobianNode, and the concrete derived type. The current implementation is still fairly DART-shaped: it has a per-base polymorphic caster registry and custom type_caster specializations for the DART base classes.

I think extracting the idea could be useful, but I would prefer not to publish the current code as-is under a DART-owned PyPI package until the API is made generic and tested outside DART. It also relies on nanobind internals such as nb_type_get, nb_type_put_p, and nb_type_info, so the maintenance story should be thought through carefully. My instinct is that the best long-term home is either upstream nanobind support or a small external package that is explicitly versioned against nanobind internals.

That said, the DART code is BSD-2-Clause, and I am fine with you using it as a reference or starting point. If you spin up a separate project, I would be happy to look at it and point out the DART cases it needs to preserve.