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Using Signals in QGraphicsItem

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  • 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!

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    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
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    • 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
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      • 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....

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        adrien-lsh
        wrote on last edited by
        #15

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

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        • 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"?

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          • 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"?

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            adrien-lsh
            wrote on last edited by
            #17

            @JonB
            Try to move the line.

            JonBJ 1 Reply Last reply
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            • A adrien-lsh

              @JonB
              Try to move the line.

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              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
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              • 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...

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                adrien-lsh
                wrote on last edited by
                #19

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

                JonBJ 1 Reply Last reply
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                • 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.

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                  • 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.

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                    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 !

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                    • 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
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                      • 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
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                        • 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

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                            Nlight
                            wrote on last edited by Nlight
                            #25
                            This post is deleted!
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