pyqt5 and multiple inheritance

Question

I'd like to create a new class that inherits two subclasses of QWidget. I know multi-inheritance isn't possible in pyqt, but how could I manage to have the properties of both parent classes in one subclass?

What I wish I could do is as follows:

class A(QWidget):

    def __init__(self, widget, parent=None):
        widget.destroyed.connect(self.destroy_handler)

    @pyqtSlot()
    def destroy_handler(self):
        pass

class B (A, QStatusBar):
    def __init__(self, widget, parent=None):
        A.__init__(self, widget)
        QStatusBar.__init__(self, parent)

    @pyqtSlot()
    def destroyed_handler(self):
        print("Destroyed")

Spirine · Accepted Answer · 2015-04-19 17:32:05Z

7

I finally found how to do it: first of all, the problems came from A and QStatusBar inheriting QWidget. We can't change QStatusBar, so we must changer A. A shouldn't inherit QWidget: so let's create another class, AInterface, like that:

class AInterface(QObject):
    def __init__(self, a, parent=None)
        super().__init__(parent=parent)
        self.a = a
        self.connect_signal()

    def connect_signal(self, widget):
        widget.destroyed.connect(self.handler)

    @pyqtSlot()
    def handler(self):
        self.a.handler()

A has now the following implementation:

class A:
    def __init__(self, widget):
        a.widget = widget
        a.interface = AInterface(self)

    def handler(self):
        pass

Thus, now we can create subclasses inheriting not only A but also any QObject, like this:

class B(QStatusBar, A):
    def __init__(self, widget, parent=None):
        QStatusBar.__init__(self, parent=parent, wiget=widget)
        A.__init__(self, widget)

    def handler(self):
        self.show('Destroyed', 3000)

Notice the widget=widget in the constructor of QStatusBar: if we don't specify it, a TypeError is thrown...

answered Apr 19, 2015 at 17:32

Spirine

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2 Comments

krassowski Over a year ago

I have a slightly different case and it looks like some issue with QtObjects swallowing arguments of all sibling classes when using multiple inheritance. EDIT: this seems to be intended: pyqt.sourceforge.net/Docs/PyQt5/multiinheritance.html

Roberto Over a year ago

Ran into this as well. I was extending a basic widget: class SpecialLabel(QLabel, OtherClass) and kept getting errors for missing arguments. Turns out that QLabel tried to call the constructor for OtherClass. Fixed it by going along with it and naming the arguments. In SpecialLabel.__init__: super().__init__(other_arg=other_arg, *args, **kwargs). This called both QLabel.__init__ and OtherClass.__init__ correctly.

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