Pyside6 how to disconnect connection?

[eyllanesc]

@jeremy_k The question is the following: in bindings such as PySide or PyQt, the ownership of the items is held by the scene, so when you remove the item from the scene then the item has no ownership and is eventually removed by the python GC (python is not C++), and at the time the GC removes the python object it also removes its connections.

[Dariusz]

@eyllanesc said in Pyside6 how to disconnect connection?:

@jeremy_k The question is the following: in bindings such as PySide or PyQt, the ownership of the items is held by the scene, so when you remove the item from the scene then the item has no ownership and is eventually removed by the python GC (python is not C++), and at the time the GC removes the python object it also removes its connections.

Unles signal holds a pointer to object and thus it will never get deleted... since scene didn't disconnect the signal since scene.removeItem() does not delete the item, nor item is QObject so its treated as lambda function (I guess?) as @jeremy_k suggested...

[eyllanesc]

@Dariusz To provide what I indicate I can provide a MWE, where every 1 second a signal is emitted and that is connected to an item, in the fifth second the item is removed but it does not cause any error.

from PySide6 import QtCore, QtWidgets


class FooItem(QtWidgets.QGraphicsEllipseItem):
    def foo_slot(self):
        print("slot")


class Scene(QtWidgets.QGraphicsScene):
    fooSignal = QtCore.Signal()


class View(QtWidgets.QGraphicsView):
    def __init__(self, parent=None):
        super().__init__(parent)

        scene = Scene(self)
        self.setScene(scene)

        timer = QtCore.QTimer(self, interval=1000)
        timer.timeout.connect(self.scene().fooSignal)
        timer.start()

        item = FooItem()
        self.scene().addItem(item)

        self.scene().fooSignal.connect(item.foo_slot)

        QtCore.QTimer.singleShot(5000, self.remove_item)

    def remove_item(self):
        items = self.scene().items()
        if items:
            item = items[0]
            self.scene().removeItem(item)


if __name__ == "__main__":
    app = QtWidgets.QApplication([])
    w = View()
    w.show()
    app.exec_()

Output:

slot
slot
slot
slot
slot

[Dariusz]

@eyllanesc You are right, this works as it should. I must have more issues elsewhere and the "disconnect" signal solution I found must not be fixing the larger issue I have :- (

[jeremy_k]

The code I had issues with, using PyQt5, is:

from PyQt5.QtCore import QObject
obj1 = QObject()
obj2 = QObject()
obj1.destroyed.connect(lambda: obj2.deleteLater())
del obj2
del obj1

Here, the capture is explicit, and the punishment is swift:

Traceback (most recent call last):
File "<stdin>", line 1, in <lambda>
NameError: name 'obj2' is not defined
Abort trap: 6

[eyllanesc]

@Dariusz That is why I say that it provides an MRE, you will not expect the community to be crazy throwing papers on the wall waiting for one to stick.

[eyllanesc]

@jeremy_k First of all GC only removes the objects that have no references.

The problem is very different, in principle when deleteLater is called it deletes the C ++ object and then a reference to the python object is eliminated, if it no longer has references the object would be eliminated as I already indicated, but in your particular case the lambda method has a scope so it has a reference and therefore the python object (the wrapper) will not be eliminated but the C ++ object

[Dariusz]

@eyllanesc said in Pyside6 how to disconnect connection?:

@Dariusz That is why I say that it provides an MRE, you will not expect the community to be crazy throwing papers on the wall waiting for one to stick.

Yes I understand your point, however, the app I'm working on has 150+ files and few thousand lines of code... Doing simple example usually "solve my issues", but my issues are not simple and solving them is quite "hard"... so I'm trying to ask for a "simple" answer to the question like... how to properly disconnect a signal if I have to do it manually and I don't want to rely on automatic qt logic... I guess I'm "old fashion" I like to manage my memory myself and python is a bit... "leme do it memememe" and I never know if he does it or not :- )

[eyllanesc]

This post is deleted!

[eyllanesc]

@Dariusz Since you use thousands of files I suppose you use some system that manages versions like GIT and then rollback to the point that it does not generate problems, then implement the basic functionality in a minimal project where you can handle all the tests, then make a branch of the original project and at that time you just implemented the new functionality already tested.

[Dariusz]

@eyllanesc Yes thats true. I would have to roll back 5 years and try to unpack the crazy system I build while learning how to code :- ). Well actually its what I'm doing now... a grand refactor rebuilding every piece of the app... Just trying to wrap my head around some odd crashes that I get with graphics views. Maybe I'm overdoing my memory managment and I should just... let it all be. hmmmm

[eyllanesc]

@Dariusz You say: I build while learning how to code, you are crazy because no one should refactor code if they do not know the technology since you are going to make the same mistake as the previous programmer. Goodbye since your case is a hopeless case that will not have a solution since the forum is a site that has little space for a problem as big as the one you have.

[SGaist]

Hi,

@jeremy_k said in Pyside6 how to disconnect connection?:

The code I had issues with, using PyQt5, is:
from PyQt5.QtCore import QObject
obj1 = QObject()
obj2 = QObject()
obj1.destroyed.connect(lambda: obj2.deleteLater())
del obj2
del obj1

One of the main issue here is that you destroy obj2 before obj1. So it's likely garbage collected before obj1 is deleted hence the call to deleteLater will fail.

[jeremy_k]

I guess I provided inadequate subtext for that example. It's documentation in a project I work on, explaining that qObject.signal.connect(lambda: anotherQObject.function(arg)) is unsafe in PyQt if anotherQObject might be destroyed before qObject. The same would happen in C++ with QObject::connect(qObject, &MyClass::signal, [&]() { anotherQObject->function(arg); });

In C++, one possible solution is to use QObject::Connect(sender, signal, context object, functor). To the best of my knowledge In PyQt5, and I suspect in PySide2/Qt for Python, there is no equivalent. You can write one by taking the connection object returned by object.signal.connect() and using it in a slot connected to anotherQObject.destroyed.