Question

Let’s say I have a simple C++ class:

class Widget
{
public:
    Widget() :
            x(1)
    { }

    void sleep()
    {
        sleep(x);
    }

private:
    int x;
};

Widget::sleep blocks, so I would like to free up the GIL so that Python can do some other stuff, so I wrapped Widget::sleep in a function like this:

static void pyWidgetSleep(Widget& self)
{
    PyThreadState* state = PyEval_SaveThread();
    self.sleep();
    PyEval_RestoreThread(state);
}

For completeness, the binding code looks like this:

BOOST_PYTHON_MODULE(libnative)
{
    boost::python::class_<Widget>("Widget")
        .def("sleep", &pyWidgetSleep);
}

I have a simple Python script that looks like this:

import libnative
import threading

class C:
    def __init__(self):
        self.x = libnative.Widget()
        self.sleeping = False

    def foo(self):
        self.sleeping = True
        self.x.sleep()

c = C()
t = threading.Thread(target=c.foo)
t.start()
while not c.sleeping:
    pass
del c.x

Do all participants in an expression get their ref count incremented for the duration of the expression?
The only referant to c.x when used in C.foo is c, which the line after t.start() conveniently deletes.
Since pyWidgetSleep releases the GIL, del c.x might decrement the last reference to the Widget instance, causing undefined behavior.

---

I can’t make this break on my machine, which seems to be a good indication that it works as I would expect, but reference-counting isn’t documented to this degree of clarity
(or, at the very least, I can’t seem to find it). The relevant portion of CPython appears to be PyEval_EvalCodeEx, which looks like it applies Py_INCREF to all arguments
involved in a function call, but I could be totally off on that one.

Answer 1

The answer to this question is yes, it is safe. This happens as a consequence of how member functions are called in Python. Consider if Widget was implemented in Python:

class Widget:
    def __init__(self):
        self.x = 1

    def sleep(self):
        os.sleep(self.x)

Notice how the first parameter to Widget.sleep is self? The reference count is incremented for the duration of the call! It seems obvious in retrospect…