App.main_loop() never returns, atexit functions are never called, excepthook replaced

[areslagae]

Hi all,

Is this intended behavior (macOS)?

1/ App.main_loop() never returns
2/ atexit handlers are not run
3/ the uncaught exception handler is replaced but does not call the one it is replacing

Thanks,

import toga
import atexit
import sys

def my_atexit():
    print(f'my atexit is never called')

original_excepthook = sys.excepthook
def my_exception_handler(type, value, traceback):
    print(f'my uncaught exception handler is never called')
    original_excepthook(type, value, traceback)

class App(toga.App):
    def startup(self):
        self.main_window = toga.MainWindow()
        self.button = toga.Button('exit', on_press=self._button_on_press)
        self.main_window.content = self.button
        self.main_window.show()
    def _button_on_press(self, button):
        # raise RuntimeError()
        self.app.exit()

sys.excepthook = my_exception_handler    
# raise RuntimeError()
atexit.register(my_atexit)
app = App('App', 'org.beeware.toga.app')
app.main_loop()
print(f'This is never printed.')

[mhsmith]

The app does indeed terminate before main_loop returns, because of CocoaLifecycle.stop.

I'm not sure what the reason for this is, and I agree it's not what you would expect from the documentation. @freakboy3742: what do you think?

[freakboy3742]

The underlying reason is that [NSApplication terminate:] (which is what [CocoaLifeCycle stop] is invoking) is triggering a low-level C exit, which is terminating the Python interpreter without going through the full Py_Finalize process that would trigger your Python atexit handlers.

I think the fix here is that the call to [NSApplication terminate:] should actually be [NSApplication stop:] - that stops the event loop without fully terminating the app. I've opened beeware/rubicon-objc#745 to track the issue (and beeware/rubicon-objc#746 is a prospective fix).