MCVE for 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& foo) {
        try {
            py::module bar = foo.attr("bar");
            std::cout << "ok\n";
        } catch (py::error_already_set const& e) {
            std::cerr << "error" << e.what() << '\n';
        }
    });
}

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;
}