Embedding nanobind

[smurfix]

The following snippets seem reasonably straightforward to me, so why not document that — instead of telling people that nanobind doesn't support embedding?

PyObject *init_ttd()
{
    // magic incantation
    static PyModuleDef ttd_def;
    py::detail::init(nullptr);
    py::module_ m = py::steal<py::module_>(py::detail::module_new("_ttd",&ttd_def));

    /* here, add the usual nanobind m.def() and m.class() etc. code */

    // magic incantation, part 2
    return m.release().ptr();
}

void run_python() {
    /* See the Python docs for all the other details on setting up an embedded interpreter */
    PyImport_AppendInittab("ttd", &init_ttd);
    /* Py_InitializeFromConfig() goes here */

    /* instead of eval and friends, just load a module and call "ttd._main.main()" */
    nanobind::module_ ttd = nanobind::module_::import_("ttd._main");
    ttd.attr("main")();

[wjakob]

There is more to it than just a few lines of code. You can look through the pybind11 GitHub tracker to >100 complicated issues that people have run into when using embedding. I don't really understand/care about these complexities and prefer to keep nanobind focused on the binding use case.