Garbage collection warning after transition to PySide6

[jamo]

Hello, I've recently transitioned an application from PySide2 to PySide6 and most works fine as it did before, BUT: when I close the application the following warning is printed gc:0: ResourceWarning: gc: 1 uncollectable objects at shutdown; use gc.set_debug(gc.DEBUG_UNCOLLECTABLE) to list them. I applied the hint and this is the result:

...
gc:0: ResourceWarning: gc: 1 uncollectable objects at shutdown
      [<PySide6.QtCore.Property object at 0x000001FE00BE8730>]

After a lot digging I could trace it back to an interaction with matplotlib. Importing any of the qt backends will yield this issue. Funnily, it seems to be resolved in Python 3.9. I attached a minimum example below. I ran this with PySide6 6.4.0.1 and matplotlib 3.6.2 in Python 3.8.10 and 3.9.13, and the warning only appears for 3.8.

So yea, I guess the simple solution would be to switch my app's minimum python version from 3.8 to 3.9 but I'm still curious why this was not an issue with PySide2 or if there is a setting for the matplotlib that I must apply.

import gc
gc.set_debug(gc.DEBUG_UNCOLLECTABLE)

from PySide6 import QtWidgets

from matplotlib.backends import backend_qt5agg

import worker

class Dialog(QtWidgets.QDialog):

    def __init__(self):
        super(Dialog, self).__init__()
        self.resize(882, 700)
        self.setModal(True)


if __name__ == "__main__":
    app = QtWidgets.QApplication()
    mw = Dialog()
    mw.show()
    app.exec()

[jamo]

So as a follow up, since this bug still appeared for Python 3.9 when running the "complex version", I tried to look for more error sources.

And turns out when I run my complex app like

if __name__ == "__main__":
    app = QtWidgets.QApplication()
    mw = MainWindowAssembly()
    mw.show()
    app.exec()

everything is fine.

If instead, I run it like

def run():
    app = QtWidgets.QApplication()
    mw = MainWindowAssembly()
    mw.show()
    app.exec()

if __name__ == "__main__":
    run()

I get the "uncollectable object" message. Curiously, as long as mw appeared in the "main" part all is good. For example, this works:

def run():
    app = QtWidgets.QApplication()
    mw = MainWindowAssembly()
    mw.show()
    app.exec()
    return mw


if __name__ == "__main__":
    mw = run()

This reminds me of the typical Qt beginner's mistake of not assigning your Qt object to a variable once it's created (or to self if it's part of a class). But in this context it does not make any sense to me.

I can't reproduce it with a simple example and I cannot share the complex code. The only insight I can give on MainWindowAssembly is that it is a subclass of (QtWidgets.QMainWindow, Ui_MainWindow, AbstractMainWindow) where Ui_MainWindow is code generated via pyside6-uic and AbstractMainWindow is an abstract class with a Qt metaclass.

class AbstractQObject(type(QtCore.QObject), abc.ABCMeta):
    pass

class AbstractMainWindow(metaclass=AbstractQObject):
    ...

However, I dont think it has something to do with this abstract construct since the error still occurs when I remove the inheritance from the abstract class.

[SGaist]

Hi and welcome to devnet,

Do you have the same issue if you use:

def run():
    app = QtWidgets.QApplication(sys.argv)
    mw = MainWindowAssembly()
    mw.show()
    sys.exit(app.exec())

if __name__ == "__main__":
    run()

?