[QUESTION] Why does `py::eval_file` not block and not return in QT?

I connect a button to bellowing function call:

void EvalFile(const std::string& py_file) {
    std::lock_guard<std::mutex> guard(g_python_mutex_);
    py::eval_file(py_file, scope);//  here will not block qt main thread, but also not return
}

First, click the button once, then the main thread will call this function but not step out of it. If paused, it always stops at

PyObject *result = PyRun_FileEx(f, fname_str.c_str(), start, global.ptr(), local.ptr(), closeFile);

which is a sub block of py::eval_file. The python file evaluated will open a UI, and I will not close it.

Then, click the button once again. The main thread could also step into this function.

Why does not this function block?

Start at the py::eval_file entry point and the PyRun_FileEx call shown in the report, then reproduce the behavior from a Qt button callback with a Python file that opens a UI. Trace whether control returns only after that UI closes and document the observed interaction between the Qt main thread and Python execution.