How to make a Python callback funtion match (i.e. become compatible with) a pybind11 C++ class constructor signature?

[TrondMelen]

I try to make a Python interface to an existing API represented by a C++ class where internal events trigger the execution of registered callbacks. I would be grateful if someone can help. Thanks in advance!

The simplified case below demonstrates my problem. The C++ 'My_class' constructor expects one argument of type '(void (data_s const&))', but to Python this is incompatible with '<function my_data_handler at 0x7fef4d1fc180>'. The class instantiation fails with the Python error message "TypeError: __init__(): incompatible constructor arguments." I guess I must either relax the C++ type or tighten the Python type. Right?

The C++ file 'My_class.hpp' defining (header only) 'My_class':

#pragma once

// a struct
struct data_s {
    unsigned int sequence_counter;
    unsigned int byte_count;
    unsigned char array[100];
};

// a function signature taking the struct as argument
typedef void (*data_handler_t)(const struct data_s &);

// class with a constructor and one method/function
class My_class {
public:
    My_class(data_handler_t data_handler) :
        the_data_handler(data_handler) {}

    void run() {
        data_s data;                      // struct allocation
        (the_data_handler)(data);         // the callback is executed here
    }
private:
    data_handler_t the_data_handler{};    // the callback is stored here
};

The pybind11 C++ file 'binder.cpp':

#include <pybind11/pybind11.h>

namespace py = pybind11;

#include "../my_cpp_class/My_class.hpp"

PYBIND11_MODULE(my_py_module, m, py::mod_gil_not_used()) {
    py::class_<My_class>(m, "My_class")
        .def(py::init<data_handler_t>())
        .def("run", &My_class::run);
}

The Python program 'main.py' that fails:

import my_py_module

class data_s:
    def __init__(self):
        self.counter = 0
        self.byte_count = 0
        self.bytearray(100)

def my_data_handler(data: data_s):
    print(f"my_data_handler(): counter is {data.counter}\n")

if __name__ == "__main__":
    api = my_py_module.My_class(my_data_handler)
    api.run()

The failing interactive Python session:

$ python
Python 3.13.5 (main, Jun 25 2025, 18:55:22) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exec(open("../my_py_module/main.py").read())
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    exec(open("../my_py_module/main.py").read())
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<string>", line 15, in <module>
TypeError: __init__(): incompatible constructor arguments. The following argument types are supported:
    1. my_py_module.My_class(arg0: void (data_s const&))

Invoked with: <function my_data_handler at 0x7fef4d1fc180>
>>> 

Pybind11 is version 2.13.6