Customization for wrapping class members

[cnyeh]

Summary

Currently, both C2PY_IGNORE/C2PY_WRAP and match_names/reject_names don't seem to allow users to customize the wrapping of individual members within a class. It would be very helpful to have the ability to selectively wrap specific members of a C++ class when exposing it to Python.

For example, in the following MF class, bound to my_module:

// mf.cpp
#include <c2py/c2py.hpp>

struct MF {
  MF(int _nkpts, int _nsin):
    nkpts(_nkpts), nspin(_nspin) {}
    
  int nkpts;
  C2PY_IGNORE int nspin;
};

# CMakeLists.txt
cmake_minimum_required(VERSION 3.20 FATAL_ERROR)
project(mpi_context VERSION 3.2.0 LANGUAGES C CXX)
set(CMAKE_CXX_STANDARD 20)

# Find Python, c2py and clair
find_package(Python COMPONENTS Interpreter Development NumPy)
find_package(c2py REQUIRED)
find_package(clair REQUIRED)

Python_add_library(my_module MODULE mf.cpp)

# Link to low level c2py and add the -fplugin option using a cmake target. 
target_link_libraries(my_module PRIVATE c2py::c2py clair::c2py_plugin)

I would like to expose only nkpts in Python, while keeping nspin hidden:

# text.py
import my_module

my_mf = my_module.MF(10, 2)

print(my_mf.nkpts)  # 10
print(my_mf.nspin)  # Current: 2
                    # Desired: AttributeError: 'my_module.MF' object has no attribute 'nspin'

Motivation

There are many cases where binding a class but selectively exposing only certain members to Python is desired. This is particularly useful when a member, such as nspin, is not a standard type, allowing users to avoid the need to write a customized py_converter for it.