Python classes: Inheritance vs Instantiation

You should call

super(GUIclass, self).__init__(parent)

This is the proper way to call (all) your inherited __init__() method(s). It has in many cases identical results compared to the mentioned

Frame.__init__(self, parent)

which only lacks the abstraction concerning the inheritance relationships and states the class Frame as the one and only class whose __init__() method you might want to call (more about that later).

The also mentioned

frame = Frame(self.parent)

is wrong in this context. It belongs to another pattern of object relationship, namely contents relationship instead of inheritance relationship (which you aim at). It will create a new object of class Frame instead of initializing the Frame parts of yourself; in inheritance relationships you are a Frame, so you have to initialize yourself as one as well as initializing your specialized parts (which is done in the rest of your __init__() method). In contents relationship models you merely have a Frame.

Now, what about that "slight" difference I mentioned above between calling super(GUIclass, self).__init__(parent) and Frame.__init__(self, parent)?

To understand that you need to dig deeper into inheritance relationships and the various possibilities these offer, especially with multiple inheritance.

Consider a diamond-shaped relationship model which looks like this:

          Frame          /   \    GUIclass  SomeOtherClass          \   /       AnotherClass

In your current scenario you only have the top left two classes, but one never knows what's going to come, and you should always code in a way so that the next user of your code keeps all options.

In this diamond-shaped pattern you have AnotherClass which inherits GUIClass and SomeOtherClass which in turn both inherit Frame.

If you now use the pattern Frame.__init__(self, parent) in both GUIclass and SomeOtherClass, then calling their __init__() methods from the __init__() method of AnotherClass will result in a doubled calling of the Frame's __init__() method. This typically is not intended, and to take care that this does not happen, the super call was invented. It takes care that a decent calling order calls each of the __init__() methods only and exactly once.

You'll have to use Frame.__init__(self, parent) as Tkinter doesn't support super calls