[QUESTION] Working with a C++ library that has boost::python bindings

Problem Descriptiopn

Hello there, I'm actually using a 3dparty library that provides python bindings... but sadly they didn't use pybind11 😢 ...

I need to interact with the python objects created by the library(from Python) with a C++ API. So far I made it work but I don't like at all how it's looking:

• It's too hacky
• It doesn't support type checking, my wrapper accepts a PyObject*, basically anything created from a python session
• I did some "manual" type checking, but, again, it's ugly
• It adds boost::pyton as an extra dependency

Solution provided so far

template <typename ClassType>
typename ClassType::Ptr convert_to_class_type(py::object py_obj) {
    boost::python::extract<typename ClassType::Ptr> x(py_obj.ptr());
    if (x.check()) {
        return x();
    } else {
        throw std::invalid_argument("ClassType not supported");
    }
}

template <typename ClassType>
void Wrapper(py::object py_obj) {
    auto obj = convert_to_class_type<ClassType>(py_obj);
    // do some stuff with the obj..
}

PYBIND11_MODULE(dummy_module, m) {
    m.def("ClassTypeWrapper", &Wrapper<CustomLibraryType>);
}

What I would like to write:

template <typename ClassType>
void Wrapper(ClassType obj) {
    // do some stuff with the obj... no conversion needed
}

PYBIND11_MODULE(dummy_module, m) {
    m.def("ClassTypeWrapper", &Wrapper<CustomLibraryType>);
}

An extra example

So, I use a lot of the Open3D library. And whenever I write C++ code it just works out of the box with Open3D types:

#include <open3d/geometry/PointCloud.h>
#include <pybind11/pybind11.h>

namespace py = pybind11;

void Process(const o3d::geometry::PointCloud& point_cloud) {}

PYBIND11_MODULE(cloud_module, m) { m.def("process", &Process); }

Whenever I build that small module I can run the following code

import cloud_module
import open3d as o3d

pcd = o3d.geometry.PointCloud()
cloud_module.process(pcd)

If I don't use the right o3d.geometry.PointCloud() type, then there will be a runtime error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-4f2097197c74> in <module>
      3 
      4 mesh = o3d.geometry.TriangleMesh()
----> 5 cloud_module.process(mesh)

TypeError: process(): incompatible function arguments. The following argument types are supported:
    1. (arg0: open3d::geometry::PointCloud) -> None

Invoked with: TriangleMesh with 0 points and 0 triangles.

Which is the expected behavior.

Final Question

I've checked the Custom type caters but doesn't seem to be the right solution.

Is there any easy way to achieve this?

Thanks in advance!!!

Start with the custom type casters documentation and the PYBIND11_MODULE/Wrapper entry points shown in the issue; compare them with the typed binding behavior demonstrated by Open3D. Determine whether pybind11 supports this third-party boost::python-bound object use case without the extra dependency, and document the supported outcome.