cannot exchange objects from python <-> C++. The object is from a third library

Discussed in https://github.com/pybind/pybind11/discussions/4357

^{Originally posted by saroj31 November 23, 2022}
I have a very basic question. I say basic because I am new to pybind11 and this is my first attempt to use the library to make python bindings for my C++ code. So here I am using a C++ class which internally uses a Realsense class. When I try to contruct a constructor for it then it gives me a TypeError.
Can someone explain me what I am doing wrong in PYbind11 persepctive? Definitely I can see that there is a type mismatch here but Still I beleive this is a general problem which many of you would have faced. I want to solve this puzzle. Read all documentation but the cutom type caster example also uses an internal pybnd11 template internally. I don't think mine is a similar thing. I want to somhow pass the info from C++ side to python side and vice versa. If you can direct me to some right examples or documentation then it will be awesome.

My C++ Class in Devicemanager.h/cpp :

include <librealsense2/rs.hpp>
class DeviceManager {

	public:
		explicit DeviceManager(rs2::context& ctx);
		td::size_t getNumOfConnectedDevices();
}

My Binder.cpp code

#include <pybind11/pybind11.h>
#include "rsWrap/DeviceManager.h"
#include <librealsense2/rs.hpp>
#include <librealsense2/hpp/rs_context.hpp>


#define STRINGIFY(x) #x
#define MACRO_STRINGIFY(x) STRINGIFY(x)

int add(int i, int j) {
    return i + j;
}

namespace py = pybind11;
using namespace RealSenseWraLib;

PYBIND11_MODULE(qco_py_wrapper, m) {


    py::class_<DeviceManager>(m, "DeviceManager")
            .def(py::init<rs2::context &>())
            .def("getNumOfConnectedDevices", &DeviceManager::getNumOfConnectedDevices);

    m.def("add", &add, R"pbdoc(
            Add two numbers
            Some other explanation about the add function.
        )pbdoc");

}

The python Code that I want to make work is:

import pyrealsense2 as rs
import qco_py_wrapper as qco

dmanager =  qco.DeviceManager(rs.context());
print(dmanager)

Error That I get is this:

Traceback (most recent call last):
  File " *<my Directory>*/build/python_wrapper/test.py", line 5, in <module>
    dmanager =  qco.DeviceManager(rs.context());
TypeError: __init__(): incompatible constructor arguments. The following argument types are supported:
    1. qco_py_wrapper.DeviceManager(arg0: rs2::context)

Invoked with: <pyrealsense2.pyrealsense2.context object at 0x7f3cc3c16d30>

When I do a help on qco_py_wrapper then the following appears:

Help(qco_py_wrapper):
`Help on module qco_py_wrapper:

NAME
    qco_py_wrapper

CLASSES
    pybind11_builtins.pybind11_object(builtins.object)
        DeviceManager
    
    class DeviceManager(pybind11_builtins.pybind11_object)
     |  Method resolution order:
     |      DeviceManager
     |      pybind11_builtins.pybind11_object
     |      builtins.object
     |  
     |  Methods defined here:
     |  
     |  __init__(...)
     |      __init__(self: qco_py_wrapper.DeviceManager, arg0: rs2::context) -> None
     |  
     |  getNumOfConnectedDevices(...)
     |      getNumOfConnectedDevices(self: qco_py_wrapper.DeviceManager) -> int
     |  
     |  ----------------------------------------------------------------------
     |  Static methods inherited from pybind11_builtins.pybind11_object:
     |  
     |  __new__(*args, **kwargs) from pybind11_builtins.pybind11_type
     |      Create and return a new object.  See help(type) for accurate signature.

`

Start with the DeviceManager.h/cpp constructor and Binder.cpp binding shown in the issue, then compare the rs.context object from pyrealsense2 with the exposed rs2::context type. Review the linked discussion and relevant pybind11 type-caster documentation; the work is complete when the required cross-module object exchange is clearly supported or its limitation is documented.