parent() or super() to get a parent object?

Question

I have two classes :

The first one (the parent class) instantiates the child class in a method. I am trying to modify parent objects properties inside the child class. (These objects are PyQT QWidget s).

Here is the begining of my Parent and Child classes :

Parent :

class ParentClass(QtGui.QMainWindow):
    def __init__(self, parent=None):
        super(ParentClass, self).__init__()

        somevar=1200

        self.ChildItem=ChildClass()
        self.ChildItem.Start(somevar)

Child :

class ChildClass(QtGui.QWidget):
    def Start(self, var):
        super(ChildClass, self).__init__(self)

        self.parent().resize(var,var)

However, even if I have no errors, the last line produces nothing.

I've seen on several examples that the super() could be used to call parent methods, so I assume that it would be a solution for my case. However, I was unable to make it work either.

I have a lot of troubles understanding the super(), it always get into complicated concepts such as multiple inheritance when I just want to do a simple thing.

Answer 1

The problem here has got nothing to do with super, or inheritance. With inheritance, the parent/child relationship is between classes. But in your example, ChildClass doesn't inherit ParentClass, so that is not relevant.

There is another kind of parent/child relationship in Qt, though, and that is between instances. The constructors for widgets have an argument that allows you to pass in a reference to a parent object (which is usually another widget instance). However, the argument is optional, so if you don't explicitly set it, calling parent() afterwards would just return None.

The specific error in your example code, is that the child is never given a parent.

Here is what your example should look like:

class ParentClass(QtGui.QMainWindow):
    def __init__(self, parent=None):
        super(ParentClass, self).__init__()    
        # pass in "self" as the parent
        self.ChildItem = ChildClass(self)
        self.ChildItem.Start(1200)

class ChildClass(QtGui.QWidget):
    def Start(self, var):
        self.parent().resize(var, var)