python类调用super的方法，而不是继承的

Question

I have the following problem

class A:
    def __init__(self):
        self.window = "<I am window class>"

class B(A):
    def __init__(self):
        super().__init__()

    def createObject(self):
        print(self.window)  # it works here
        self.c = C()

    def drawRect(self):
        print(self.window)  # does not work when called from class C

class C(B):
    def __init__(self):
        self.draw()

    def draw(self):
        super().drawRect()

app = B()
app.createObject()

I have figured that, when I call the super().drawRect() function in class C it sends the <__main__.C object at 0x0000023D99FA6748> as self.

How can I make class C, in function draw() send <__main__.B object at 0x0000017A2BA95358> in the super().drawRect()

I am asking because in the non-simplified code when I call the function from the B class itself, it prints the self.window, but when called from the C class it prints None

how can this be fixed? what am I doing wrong?

Answer 1

There is no B object when you create an instance of C , there is only a C instance that happens to inherit from B .

So the answer to your question:

How can I make class C, in function draw() send <__main__.B object at 0x0000017A2BA95358> in the super().drawRect()

Is: You can't.

You could create a new B object (but that would defy the purpose of inheritance) alongside your C object.

But it seems more reasonable that you should just call super().__init__ in C.__init__ :

class C(B):
    def __init__(self):
        super().__init__()
        self.draw()

    def draw(self):
        super().drawRect()

It would be weird not to do that anyway. Because inheritance sort of implies that you extend rather than override completely .

But given the fact that you do self.c = C() in your B.createObject method and C inherits from B you should probably figure out another way (eg composition ) instead of inheritance:

class A:
    def __init__(self):
        self.window = "<I am window class>"

class B(A):
    def __init__(self):
        super().__init__()

    def createObject(self):
        print(self.window)  # it works here
        self.c = C(self)

    def drawRect(self):
        print(self.window)  # does not work when called from class C

class C(object):   # don't inherit from B
    def __init__(self, obj):
        self.obj = obj
        self.draw()

    def draw(self):
        self.obj.drawRect()

app = B()
app.createObject()

However this will create a reference cycle because you store the C instance as attribute of your B instance and the B instance as attribute of your C instance. It's also possible that you simplified the code too much, maybe the real code prevents composition in some way. However given the current code it seems like inheriting from B is a bad idea for your class C given that it overrides B s behavior completely and doesn't use any of it's attributes (which it can't anyway because you overrode the methods).

Answer 2

you need to call to

super().__init__() 

in class C constructor

Charlie