Adding bound methods at runtime

[ianhbell]

I have a class to which I need to write a function in pybind11 to add dynamic methods. These methods expose low-level methods that are particular to the type contained within itself. In normal Python, we could do like so: https://stackoverflow.com/a/65571141/1360263

But I haven't found a way to do this in pybind11. My desired approach looks something like this:

double vdW_get_a(py::object& o){
    return std::get<vdWEOS1>(o.cast<const teqp::cppinterface::AbstractModel *>()->get_model()).get_a();
}

// You cannot know at runtime what is contained in the model so you must iterate
// over possible model types and attach methods accordingly
void attach_model_specific_methods(py::object& obj){
    using namespace teqp::cppinterface;
    const auto& model = obj.cast<AbstractModel *>()->get_model();
    auto setattr = py::getattr(obj, "__setattr__");
    if (std::holds_alternative<vdWEOS1>(model)){
        setattr("_a", py::object(&vdW_get_a));
    }
};

But this doesn't compile. I would prefer to bind a lambda function since I have quite a few of these functions to add.

How can one add runtime methods to an instance, dynamically?

[ianhbell]

This link talks about it in detail for Python: https://stackoverflow.com/questions/972/adding-a-method-to-an-existing-object-instance

With pybind11, the 0-th method doesn't work (py::cpp_function doesn't have the __get__ method), but the first method works for me. It's verbose, and unpleasant to work with, but most importantly, it functions properly.

Snippet:

// You cannot know at runtime what is contained in the model so you must iterate
// over possible model types and attach methods accordingly
void attach_model_specific_methods(py::object& obj){
    using namespace teqp::cppinterface;
    const auto& model = obj.cast<AbstractModel *>()->get_model();
    auto setattr = py::getattr(obj, "__setattr__");
    auto MethodType = py::module_::import("types").attr("MethodType");
    
    if (std::holds_alternative<vdWEOS1>(model)){
        py::cpp_function get_a([](py::object& o){
            return std::get<vdWEOS1>(o.cast<const teqp::cppinterface::AbstractModel *>()->get_model()).get_a();
        });
        setattr("get_a", MethodType(get_a, obj));
        py::cpp_function get_b([](py::object& o){
            return std::get<vdWEOS1>(o.cast<const teqp::cppinterface::AbstractModel *>()->get_model()).get_b();
        });
        setattr("get_b", MethodType(get_b, obj));
    }
};