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I tried to do some operation (setParent) on an object in Python (an instance of a class which inherits from a different class - to be specific, QtGui.QLabel), but during runtime the above error was raised. The object itself has had some fields with actual content (verified on debug), but from some reason I couldn't "use" it. What does the error mean and how can I fix it? For some additional information, I shall say that the object was returned from a static method before I tried to do this operation on it.

The subclass has a __init__() function of its own:

def __init__(self, image, father):
        super(AtomicFactory.Image, self).__init__(father)
        self.raw_attributes = image.attributes
        self.attributes = {}
        father.addChild(self)
        self.update()

Now I wrote a similar code, a simple one, that had the same error on the line widget.setParent(mw) when the window was moved.

#!/usr/bin/env python
import sys
import copy
from PyQt4 import QtCore, QtGui

if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    main_widget=QtGui.QWidget()
    widget = QtGui.QPushButton('Test')
    widget.resize(640, 480)
    widget.setParent(main_widget)
    widget.move(0, 0)
    widget2=QtGui.QPushButton('Test2')
    widget2.i=0
    widget2.resize(600, 200)
    widget2.setParent(main_widget)
    widget2.move(640, 0)
    def onResize(event):
        print event
        mw=copy.deepcopy(main_widget)
        widget.setParent(mw)
        widget2.setParent(mw)
        widget.move(0, 0)
        widget2.move(640, 480)
        main_widget_width=main_widget.width()
        widget_width=widget.width()
        width2=main_widget_width-widget_width
        height2=widget2.height()
        widget2.resize(width2, height2)
        widget2.move(640, 0)
    main_widget.resizeEvent=onResize
    def onClick():
        size=(widget2.width(), widget2.height())
        if(widget2.i%2==0):
            widget2.resize(int(size[0]/2), int(size[1]/2))
        else:
            widget2.resize(size[0]*2, size[1]*2)
        widget2.i+=1
    QtCore.QObject.connect(widget, QtCore.SIGNAL('clicked()'), onClick)
    main_widget.show()
    sys.exit(app.exec_())

What was the problem?

Bakuriu
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Tomer
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  • If your derived class `MyDerived` has its own `__init__` try adding the line `super(MyDerived, self).__init__()`. – halex Sep 05 '12 at 11:36
  • It does. The object was actually created before and was used fine. I could have also used setParent() without problems in the initial stage. It was then inserted into a list and the static method I described above "found" it within the list, and then returned it to another static method. When the latter tried to use setParnet() on it, the error was returned. Anyways, the __init__() custom function of the subclass is: – Tomer Sep 05 '12 at 11:40
  • def __init__(self, image, father): super(AtomicFactory.Image, self).__init__(father) self.raw_attributes = image.attributes self.attributes = {} father.addChild(self) self.update() – Tomer Sep 05 '12 at 11:43
  • I kind-of doubt the static method has anything to do with this, but I might be wrong. – Martijn Pieters Sep 05 '12 at 11:53
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    What is your subclass? Is it `AtomicFactory.Image`, or something else? – Daniel Roseman Sep 05 '12 at 12:04
  • Yes, AtomicFactory.Image is the subclass – Tomer Sep 05 '12 at 13:59
  • No. It can't be `AtomicFactory.Image`. In the `super` call you must put the name of that class. If you put the name of a different class, or of an ancestor class you might break things. Obviously you can't have: `class AtomicFactory.Image(QLabel, ...): ...`, because `AtomicFactory.Image` is not a valid identifier. – Bakuriu Sep 05 '12 at 17:03
  • The class is named `Image`, and it's defined under a class named `AtomicFactory`... That's not the problem, as I said, it did work some of the time. – Tomer Sep 05 '12 at 17:13
  • In your last sample code I _think_ that the problem is that `copy.deepcopy` will copy the python object, but not the underlying C++ object. By the way: what does it mean "it's defined under a class named AtomicFactory"? Does it mean that you defined a class inside a class? – Bakuriu Sep 05 '12 at 17:20
  • Yes. What do you mean by underlying C++ object? – Tomer Sep 05 '12 at 17:31
  • When you create a `QObject` or `QLabel`, the object you create is actually a proxy to the C++ `QObject` or `QLabel`. The underlying C++ object is the actual `QObject` that is linked to the python proxy. Anyway it seems like you can't deepcopy a `QObject`(see [here](http://www.riverbankcomputing.com/pipermail/pyqt/2009-April/022896.html)). So find an other way of copying objects. Also, are you sure you need a class defined in a class? I think that kind of things simply complicates code. – Bakuriu Sep 05 '12 at 17:42

1 Answers1

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If you want to inherit QObject (or QWidget), you must always call the super-class __init__:

class MyObject(QObject):
    def __init__(self, *args, **kwargs):
        super(MyObject, self).__init__(arguments to parent class)
        #other stuff here

You may also call the parent's class's __init__ after some instructions, but you can't call QObject methods or use QObject attributes until you did so.

Edit: In your case you are trying to deepcopy a QWidget, but this is not possible. Python may be able to copy the wrapper of the QWidget, but the QWidget itself is a C++ object that python cannot handle with the default implementation of copy.deepcopy, hence whenever you call a method of the copied instance you get the RuntimeError because the underlying C++ object wasn't initialized properly.

The same is true for pickling these objects. Python is able to pickle the wrapper, not the C++ object itself, hence when unpickling the instance the result is a corrupted instance.

In order to support deepcopy() the QWidget class should implement the __deepcopy__ method, but it does not do that.

If you want to copy widgets you'll have to implement by yourself all the mechanism by hand.

Bakuriu
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  • Please see the answer I wrote to Martijn Pieters above. I don't know how to share my code in a more clear way though... – Tomer Sep 05 '12 at 11:44