Q_ARG missing in python/ how to use invokeMethod ?

Jim Gir

Hi,
I'm trying to use QMetaObject.invokeMethod wich needs some PySide2.QtCore.QGenericArgument wich has to be provided by the Q_ARG macro (https://doc.qt.io/qtforpython-5.12/PySide2/QtCore/QGenericArgument.html#PySide2.QtCore.QGenericArgument)

Actually I can't find this Q_ARG macro in QtCore or anywhere else. where is it ?

More generally how to use invokeMethod without it ?
Jimmy

Jim Gir

I already saw it.
it import Q_ARG from QtCore but it doesn't exist in PySide2. I can't import it

JonB

@Jim-Gir
Have you read e.g. https://stackoverflow.com/questions/23538904/qmetaobjectinvokemethod-doesnt-find-methods-with-parameters and other hits discussing this from Googling pyside2 q_arg?

SGaist

Hi,

Well, it looks like there's indeed something off here with Q_ARG, did you already check the bug report system ? If you don't find anything there please open a report providing a minimal runnable sample that reproduces the problem.

JonB

@Jim-Gir
I also meant to draw your attention to https://stackoverflow.com/questions/53296261/usage-of-qgenericargument-q-arg-when-using-invokemethod-in-pyside2. Which says there still isn't any Q_ARG in PySide2 and you can't do what you with no workaround :(

As they point out in the answer that is a bug, it does not matter if it is QString or str, which still remains in PySide2 and unfortunately there is no workaround – eyllanesc Nov 14 '18 at 15:35

numaelisis

someone reported the bug?
the problem is from pyside qt4
For the moment I resolved like this:

in qml, file.qml:
ApplicationWindow{
  id:root
  property var argsTemp
  Item{
       id: myitem
       objectName:"myitem"
 
       function myFunctionBrige (){
                   myFunction(argsTemp[0]);
                   argsTemp=[];
       }
 
       function myFunction(arg){
                  console.log("myargLost",arg);
       }
}

in python:
engine.load(QUrl.fromLocalFile('file.qml'))
root =  engine.rootObjects()[0]
root.setProperty("argsTemp",["arg1"])
object_qml = root.findChild(QObject, "myitem")
QMetaObject.invokeMethod(object_qml, "myFunctionBrige")

ideaplus

Use pyqt5 instead of pyside2
or Use signal slots instead of invokeMethod
@Jim-Gir

numaelisis

but when are many functions? ...
and if you need to invoke from native code to qml?

using qt5 with c ++ no problem,
but PySide2, the Q_ARG macro is missing

JonB

@numaelisis
Is this a question? Yes, PySide2 does not have any support for Q_ARG.

ideaplus

https://bugreports.qt.io/browse/PYSIDE-1262

Alex Wilkinson

For myself, I came across this when trying to use an example written in PyQt5 and invokeMethod in PySide2.
The example was using invokeMethod to return a QImage from a separate thread that had done some processing.

An alternative solution that (with minimal testing) works is to create a signal object, and connect it to a slot via a queued connection.

from PySide2 import QtCore, QtGui
class MyClass(QtCore.QObject):
   imageReady = QtCore.Signal(QtGui.QImage)


  def __init__(self):
   super()
   self.imageReady.connect(self.setImage, QtCore.Qt.QueuedConnection)

  def processNewImageOnThread(image):
     threading.Thread(target=self.process_image, args=(image)).start()

  def process_image(image):
    # Process image here
    self.imageReady.emit(image)

  @QtCore.Slot(QtGui.QImage)
  def setZoomedImage(self, image):
        # set this image on the viewmodel, which needs to be done on the GUI thread.
        VM.zoomedImage = image

I believe this example can be expanded to any set of arguments.