How to create a functional singleton similar to py::none?

[Debith]

Hi,

I am trying to get functional singleton but I am not able to get it working quite the way I wish. The singleton behaviour work actually, so importing the class Undefined in the python code and assert Undefined() == Undefined() works fine.

However, I want to be able to do:

from x import Undefined

assert Undefined == Undefined  # no parenthesis
assert repr(Undefined) == "Undefined" # This is now <capsule object at 0x...>

Can anyone help with this, please?

Below is the c++ code.

static py::object sentinel_object() {
  static py::object sentinel;

  if (sentinel.ptr() == nullptr) {
    sentinel = py::capsule(&sentinel);
    sentinel.inc_ref();
  }

  return sentinel;
}


class Undefined : public py::object {
public:
    PYBIND11_OBJECT(Undefined, object, [](py::handle h) { return h.is(sentinel_object()); });
    Undefined() : py::object(sentinel_object(), stolen_t{}) {}
    ~Undefined() = default;

    static py::object instance() {
        static const py::object undefined = Undefined();
        return undefined;
    }
};

Binding

PYBIND11_MODULE(x, m)
    // Ideally here I would like to do
    // m.attr("Undefined", Undefined::instance());
    py::class_<Undefined>(m, "Undefined")   // FIXME: wouldn't want to define this here.
        .def_static("__new__", [](const py::object &) {
            return Undefined::instance(); })
        .def(py::init<>([](const Undefined& self) { return self; }))
        .def("__repr__", [](py::handle self) { return py::str("<Undefined>"); });  // THIS DOES NOT SEEM TO WORK?!