PySide widgets using .ui files being garbage-collected unexpectedly

Question

I have a PySide QMainWindow that I'm running in Nuke. Some widgets used by the application use .ui files created in Qt Designer.

Until recently, the QMainWindow class was not given a parent. Because of this, when Nuke was minimized or changed focus, the QMainWindow did not minimize or gain focus with it.

To fix that issue, when creating the QMainWindow, I used the QApplication.activeWindow() method to get an object to feed the QMainWindow as a parent.

parent = QApplication.activeWindow()
window = MyMainWindow(parent)

If I do this, the QMainWindow will minimize and change focus with Nuke. However, when accessing subwidgets of any widget created with .ui files, it will raise an Exception

Traceback (most recent call last):
    ...
RuntimeError: Internal C++ object (PySide.QtGui.QPushButton) already deleted.

I'm using a method very similar to this to load the .ui files onto my QWidget classes

Why are the C++ objects being deleted (garbage-collected)? Why does the behavior change when I specify a parent for the QMainWindow? Is there another way to parent the QMainWindow to Nuke so that in minimizes and focuses correctly or a different way to load the .ui files without experiencing this garbage collection issue?

Can you show how the parent is being passed to the ui loading method and the code for the ui loader if it differs from the one in the link. I suspect there is a bug in how the parent of the widgets from the ui file is being specified I'm the ui loader, causing them to be garbage collected. — three_pineapples, Jan 16 '16 at 11:26
I don't have an answer to your question, but in general, I would recommend using an approach that emulates the `uic.loadUiType` function from PyQt (similar to [this example](http://stackoverflow.com/questions/4442286/python-code-genration-with-pyside-uic/14195313#14195313)). In other words, parsing your `.ui` files once when a module is imported and generating Python classes that your custom widgets inherit (or don't, depending on how much setup you need to do). — nrusch, Feb 02 '16 at 19:04

score 3 · Accepted Answer · answered Feb 02 '16 at 19:45

The problem occurs when one of the parent widgets gets garbage-collected because no python object reference exists. For instance:

def create_window():
    parent = QApplication.activeWindow()
    window = MyMainWindow(parent)
    return window

When the function returns, the parent object goes out of scope and the C++ pyobject it represents will eventually get garbage collected. It is strange that this is only a problem for widgets created with .ui files. The solution is to just keep a reference to all the parent objects. I'm using a global variable to store references to them.

GC_PROTECT = []

def create_window():
    parent = QApplication.activeWindow()
    GC_PROTECT.append(parent)
    window = MyMainWindow(parent)
    return window

score 0 · Answer 2 · answered Nov 03 '16 at 21:43

if the window is declared as a Global, it won't get destroyed until the window itself is closed. So you want the reference to the window/widget itself to continue existing somewhere, not necesarily the parent (as mentioned in a previous answer)

def create_window():
    parent = QApplication.activeWindow()
    # not necesarily the best practice to declare it here, but comfortable
    Global window
    window = MyMainWindow(parent)
    return window

PySide widgets using .ui files being garbage-collected unexpectedly

2 Answers2