How to bind a Singleton across libraries?

[jordanlevy96]

I wrote up a detailed StackOverflow post: https://stackoverflow.com/questions/77764718/how-to-bind-a-singleton-to-python-in-pybind-across-libraries

Also a repo with minimal reproduction: https://github.com/jordanlevy96/pybind-singleton

TLDR: How to Singleton with pybind?

I've tried binding a simple Singleton class a few different ways:

PYBIND11_MODULE(my_module, m)
{
    py::class_<Singleton>(m, "Singleton")
        .def_static(
            "print_test", []()
            { Singleton::GetInstance().PrintTest(); },
            py::return_value_policy::reference)
        .def_static(
            "get_test_val", []()
            { 
                Singleton &singleton = Singleton::GetInstance();
                return singleton.test; },
            py::return_value_policy::reference)
        .def_static(
            "get_instance", &Singleton::GetInstance, py::return_value_policy::reference)
        .def_readwrite("test", &Singleton::test);
}

But Python is always creating a new instance of the Singleton. It may have something to do with my build process; I have shared libraries set up in CMake:

cmake_minimum_required(VERSION 3.12)
project(pybind-singleton)

cmake_policy(SET CMP0148 NEW)
set(CMAKE_CXX_STANDARD 17)

find_package(pybind11 CONFIG REQUIRED)
find_package(Python3 COMPONENTS Interpreter Development REQUIRED)

add_library(my_lib SHARED
    src/Singleton.cpp
)

message(STATUS "Python include dirs: " ${PYTHON_INCLUDE_DIRS})
message(STATUS "pybind include dirs: " ${pybind11_INCLUDE_DIRS})
target_include_directories(my_lib PUBLIC include ${PYTHON_INCLUDE_DIRS} ${pybind11_INCLUDE_DIRS})
target_link_libraries(my_lib PUBLIC Python3::Python)

pybind11_add_module(my_module src/binding.cpp)
target_link_libraries(my_module PUBLIC my_lib)


add_executable(${CMAKE_PROJECT_NAME} src/main.cpp)
target_link_libraries(${CMAKE_PROJECT_NAME} PUBLIC my_lib)

[Debith]

I added similar question:
#5054

In my case, the singleton works functionally but it is not behaving like a nice Python object (I can't define functions for it)