Calling detaching a window when importing a module doesn't work

[mrzv]

Not sure what's causing it, but when I try to call mainloop(detach = ...) during the initialization of my own module exposed using pybind11, not only does it not detach, but it hangs even after I close the window. Here's an example code:

struct MyScreen: public ng::Screen
{
    MyScreen():
        ng::Screen({ 1200, 800 }, "Test")           {}
};

struct Environment
{
    Environment(py::module m, py::module nanogui): nanogui_(nanogui)
    {
        py::object ng_init     = nanogui_.attr("init");
        py::object ng_mainloop = nanogui_.attr("mainloop");

        ng_init();

        auto* viewer = new MyScreen;
        viewer->drawAll();
        viewer->setVisible(true);

        py::object viewer_obj = py::cast(viewer);
        m.attr("viewer") = viewer_obj;

        py::tuple args;
        py::dict kwargs;
        kwargs["detach"] = viewer_obj;

        handle_ = ng_mainloop(*args, **kwargs);
    }

    ~Environment()
    {
        py::object ng_shutdown = nanogui_.attr("shutdown");
        py::object handle_join = handle_.attr("join");
        handle_join();
        ng_shutdown();
    }

    py::module nanogui_;
    py::object handle_;
};

PYBIND11_PLUGIN(mymod)
{
    py::module m("mymod", "mymod bindings");

    auto nanogui = py::module::import("nanogui");
    py::object screen = nanogui.attr("Screen");

    py::class_<MyScreen>(m, "MyScreen", screen);

    py::class_<Environment>(m, "Environment");

    m.attr("_env") = py::cast(new Environment(m,nanogui));

    return m.ptr();
}

[mrzv]

Same effect can be achieved with the following Python code:

# mymod.py

import nanogui as ng

ng.init()

s = ng.Screen(ng.Vector2i(1200,800), "Test")
s.drawAll()
s.setVisible(True)

h = ng.mainloop(detach = s)

Then import mymod causes the same problem as the C++ code above.

[wjakob]

I get

 % python2.7 
Python 2.7.10 (default, Oct 23 2015, 19:19:21)
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import mymod
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: not holding the import lock
>>>

Apparently a thread that imports a Python module must also hold a special import lock. This of course wreaks havoc when switching threads beneath Python.

http://stackoverflow.com/questions/4754319/what-is-import-lock-in-python
http://bugs.python.org/issue18122

I think this is a bit too weird of an issue to consider a pybind11/nanogui bug.

[wjakob]

(The solution is to import your module, and to postpone the mainloop call to a method that is invoked following the import.)

[mrzv]

Actually, it seems to also be some weird interaction with IPython. In plain Python, I also get "not holding import lock", but actually everything seems to function fine. In IPython, I don't get the RuntimeError, but I do get the hanging behavior.

Python has some weird quirks. But I agree with you, it's not worth considering a pybind11/nanogui bug.