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Forum Update on Monday, May 27th 2025

Using Signals in QGraphicsItem

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  • A Offline
    A Offline
    adrien-lsh
    wrote on last edited by adrien-lsh
    #9

    @JonB
    I tried all of that already, when the signal is emitted (even without or with different parameters) it crashes: the line does not move, it waits a bit, then the window closes.
    It crashes on the Signal.emit().

    btw sorry I cannot respond faster I have the 10m new guy cooldown

    JonBJ 1 Reply Last reply
    1
    • A adrien-lsh

      @JonB
      I tried all of that already, when the signal is emitted (even without or with different parameters) it crashes: the line does not move, it waits a bit, then the window closes.
      It crashes on the Signal.emit().

      btw sorry I cannot respond faster I have the 10m new guy cooldown

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by
      #10

      @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.

      1 Reply Last reply
      0
      • A Offline
        A Offline
        adrien-lsh
        wrote on last edited by adrien-lsh
        #11

        @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.

        JonBJ 1 Reply Last reply
        0
        • A adrien-lsh has marked this topic as unsolved on
        • A 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.

          JonBJ Offline
          JonBJ Offline
          JonB
          wrote on last edited by JonB
          #12

          @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!

          A 1 Reply Last reply
          0
          • JonBJ 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!

            A Offline
            A Offline
            adrien-lsh
            wrote on last edited by
            #13

            @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 :/

            JonBJ 1 Reply Last reply
            0
            • A 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 :/

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on last edited by
              #14

              @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....

              A JonBJ 2 Replies Last reply
              0
              • JonBJ 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....

                A Offline
                A Offline
                adrien-lsh
                wrote on last edited by
                #15

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

                1 Reply Last reply
                0
                • JonBJ 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....

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by
                  #16

                  @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"?

                  A 1 Reply Last reply
                  0
                  • JonBJ 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"?

                    A Offline
                    A Offline
                    adrien-lsh
                    wrote on last edited by
                    #17

                    @JonB
                    Try to move the line.

                    JonBJ 1 Reply Last reply
                    0
                    • A adrien-lsh

                      @JonB
                      Try to move the line.

                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by
                      #18

                      @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...

                      A 1 Reply Last reply
                      0
                      • JonBJ 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...

                        A Offline
                        A Offline
                        adrien-lsh
                        wrote on last edited by
                        #19

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

                        JonBJ 1 Reply Last reply
                        0
                        • A adrien-lsh

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

                          JonBJ Offline
                          JonBJ Offline
                          JonB
                          wrote on last edited by JonB
                          #20

                          @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.

                          A 1 Reply Last reply
                          1
                          • A adrien-lsh has marked this topic as solved on
                          • JonBJ 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.

                            A Offline
                            A Offline
                            adrien-lsh
                            wrote on last edited by
                            #21

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

                            1 Reply Last reply
                            0
                            • SGaistS Offline
                              SGaistS Offline
                              SGaist
                              Lifetime Qt Champion
                              wrote on last edited by SGaist
                              #22

                              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]

                              Interested in AI ? www.idiap.ch
                              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                              JonBJ 1 Reply Last reply
                              0
                              • SGaistS 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]

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #23

                                @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?

                                SGaistS 1 Reply Last reply
                                0
                                • JonBJ 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?

                                  SGaistS Offline
                                  SGaistS Offline
                                  SGaist
                                  Lifetime Qt Champion
                                  wrote on last edited by SGaist
                                  #24

                                  @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.

                                  Interested in AI ? www.idiap.ch
                                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                                  1 Reply Last reply
                                  0
                                  • N Offline
                                    N Offline
                                    Nlight
                                    wrote on last edited by Nlight
                                    #25
                                    This post is deleted!
                                    1 Reply Last reply
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