Solution to https://stackoverflow.com/q/78378063/3962537

#include <pybind11/pybind11.h>
#include <pybind11/embed.h>
#include <iostream>

namespace py = pybind11;

PYBIND11_EMBEDDED_MODULE(testmodule, m)
{
    m.def("start", [](py::module& mod) {
        using namespace pybind11::literals;
        auto import_fn = py::globals()["__builtins__"].attr("__import__");

        auto bar = import_fn(mod.attr("__name__")
            , "fromlist"_a=py::make_tuple("bar")
            ).attr("bar");

        py::print(bar.attr("test"));
    });
}

int main()
{
    py::scoped_interpreter guard{};

    try {
        py::exec(R"(\
import testmodule
import foo

testmodule.start(foo)
)");
    } catch (py::error_already_set& e) {
        std::cerr << e.what() << '\n';
    }

    return 0;
}