Using Signals in QGraphicsItem

[JonB]

@adrien-lsh
So you are claiming if you inherit from QObject, define a signal and emit it without a slot attached it crashes?

Try swapping the inheritance order to make QObject the first class inherited from (i.e. (QObject, QGraphicsLineItem). This would be required for moc from C++, I don't know if it might be required from Python. I am (then) also slightly unsure about plain super() calls from Python, I don't know if you need to qualify that to indicate which of the two super classes to use.

You have not said what version of Qt you are using and whether PySide or PyQt. If you produce a minimal, standalone example which I can copy and paste I will test here.

[adrien-lsh]

@JonB
Yes that's what is happening. I am using PySide6 and here is an example:

from PySide6.QtCore import QObject, QPointF, Signal
from PySide6.QtGui import QPen, Qt
from PySide6.QtWidgets import QApplication, QGraphicsItem, QGraphicsView, QGraphicsScene, QGraphicsLineItem


class GraphicsView(QGraphicsView):
    def __init__(self, parent=None):
        super().__init__(parent)
        scene = QGraphicsScene()
        self.line = CroppingLine(0, 0, 100, 100)
        scene.addItem(self.line)
        self.setScene(scene)


class CroppingLine(QGraphicsLineItem, QObject):
    moved = Signal(object, QPointF)

    def __init__(self, x1: float, y1: float, x2: float, y2: float):
        super().__init__(x1, y1, x2, y2)
        self.setPen(QPen(Qt.GlobalColor.blue, 3))
        self.setFlags(QGraphicsItem.GraphicsItemFlag.ItemIsMovable)

    def mouseMoveEvent(self, event):
        super().mouseMoveEvent(event)
        self.moved.emit(self, self.scenePos())

if __name__ == '__main__':
    import sys
    app = QApplication(sys.argv)
    window = GraphicsView()
    window.show()
    sys.exit(app.exec())

About the swap of the superclasses, I dont think it would work because the super().__init__() would not call QGraphicsLineItem init function but QObject's.

[JonB]

@adrien-lsh
Before you make me try this, I did ask you put QObject first, that is at least required using C++/moc so I'd like you to test it. As I said, you should look up how super() works in Python, I think you can call something like super(QGraphicsLineItem).__init__() (actually it might be super(QGraphicsLineItem, self).__init__()?) if you need to, but you are the Python programmer!

[adrien-lsh]

@JonB
Nope it does not work with super(QGraphicsLineItem, self).__init__(...) because Python is trying to call the super class of QGraphicsLineItem which is QGraphicsItem and my class does not directly inherit from it so it doesnt work.

I did achieve the inheritance swap using super(QObject, self).__init__(...) tho and my line displays correctly but it still crashes :/

[JonB]

@adrien-lsh
Since you seem to have provided a nice, standalone, complete example I can copy and paste --- unlike so many other people --- I will give it a play now....

[adrien-lsh]

@JonB
Thanks! Let me know if you need any more information.

[JonB]

@JonB
Your code above, copied and pasted, works fine for me. Ubuntu 24.04, Qt 6.4.2, PySide6. Or at least I get an open window with a line in it. Could you be very specific about what you do to make it "crash"?

[adrien-lsh]

@JonB
Try to move the line.

[JonB]

@adrien-lsh
Yeah, that's easier said than done! It's so thin, and I can't tell when I have actually grabbed it or not! Anyway I have reproed now, it does a SIGSEGV from

0x00007ffff711fc14 in PySide::SignalManager::emitSignal(QObject*, char const*, _object*) ()
   from /home/jon/.venv/pyside6/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.7
(gdb) bt
#0  0x00007ffff711fc14 in PySide::SignalManager::emitSignal(QObject*, char const*, _object*) ()
    at /home/jon/.venv/pyside6/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.7

Looking into that now...

[adrien-lsh]

@JonB
Yeah I should've set a bigger width ^^. Thank you.

[JonB]

@adrien-lsh
I think it is to do with the initialisation, requiring both inherited classes to be initialised. You were only doing so for the QGraphicsLineItem, not initialising the QObject. I find the following does work without crashing:

class CroppingLine(QObject, QGraphicsLineItem):
    moved = Signal(object, QPointF)

    def __init__(self, x1: float, y1: float, x2: float, y2: float):
        #super(QGraphicsLineItem, self).__init__(x1, y1, x2, y2)
        QObject.__init__(self)
        QGraphicsLineItem.__init__(self, x1, y1, x2, y2)

There may be other ways of doing this with super() and/or not swapping the inheritance order, I just know this works fine.

[adrien-lsh]

@JonB
Of course ! It never initialized the QObject so signals would never work.
Thank you for your help !

[SGaist]

Hi,

It's Python3, remove the class from your super calls. This should allow Python to do proper initialisation of the chain of classes (though sometimes it might fail for complex cases).

[Edit: we are in such a complex case]

[JonB]

@SGaist
But the OP started with just

super().__init__(x1, y1, x2, y2)

and, for whatever reason, that segments (later, on signal emit) because the QObject part has not been initialized, apparently. That is why I tried splitting it into two separate explicit initializations, which makes it work. So what exactly are you proposing?

[SGaist]

@JonB I misread the code !

We are in the territory where your solution (using __init__) is the way to go. Multiple inheritance in Python is quite tricky...

QObject expects an optional parent parameter which is not given here but it's not expecting the parameters for the QGraphicsLineItem constructor hence the failure. You can try to use the kwargs trick however it won't work either as it's not part of any of the method signature and thus can't work either.

[Nlight]

This post is deleted!