Binding a custom C++ class to existing Python type

[jiawen]

I'm pretty familiar with how Pybind11 works. But I have a funny situation: I have a simple C++ struct for a Rational type and various functions that return it:

class Rational {
  int n;
  int d;
};

Rational my_function();

Rather than using py::class_<Rational> to define a new class, is it possible to get Pybind11 to bind Rational to a Python fraction? Do I have to use a custom type caster?

I want to then bind my_function as m.def("my_function", &my_function) and have it automatically return a py::object that is an instance of fraction.