[PyQt] QMessagebox in slot terminates application

[Darryl Wallace]

Hello,

> Although now I'm a little confused as to why the app is even returning 
> a value when the MessageBox is closed. Even if you take sys.exit() out 
> and just print out the value the app returns, it will still exit. 
> Basically the whole app acts like a DialogBox that gets triggered by 
> the MessageBox.
You need to start a QApplication because it starts the eventloop for the 
GUI application to run in.  See the documentation for 
QApplication::exec.  The program exits with code 0 because the 
application exited normally.
>
> I'm not sure I fully understand what all the exec() function does.
I tested this out in the systray example that comes with PyQt.  I 
connected the maximize action to a method I made called "showMessage" 
which then just starts a msg box.  If the Systray object  is visible on 
the screen (via restore) then the QMessageBox does not kill the 
program.  If the Systray object is hidden it does not kill the program.

The QMessageBox inherits QDialog which has the function exec(): "Shows 
the dialog as a modal dialog, blocking until the user closes it. " 

Though, my guess is that when exec() is called via the QMessageBox, it 
is stealing the event loop from the Systray (or something along those 
lines), since its parent (the Systray) is not visible.  So when the 
messagebox closes it is the end of the application.  Furthermore, the 
closeEvent for the Systray object is not called when the QMessageBox is 
called.  This indicates, to me, that the QMessageBox is the end.

I'm not sure if this is an accurate explanation.  But that is what I 
think is happening.  Perhaps Phil or a person more experienced with the 
inner workings can provide a better explanation.

A work-around would be to create a QMessageBox manually (i.e. not one of 
those static public members) fill it with what you want and call .show() 
depending on the state of the Systray widget (i.e. if 
systray.isVisible() call exec_()  else, call .show()).

darryl