Nested classes that inherit from the outer parent's nested classes in Python 3

Question

Suppose I have the following class in Python 3:

class CoolCar:
    @classmethod
    def myWheels(cls):
        cls.Wheels().out()
    class Wheels:
        def __init__(self):
            self.s = "I'm round!"
        def out(self):
            print(self.s)

All well and good. Now I want a derived class:

class TerribleTank(CoolCar):
    class Wheels(CoolCar.Wheels):
        def __init__(self):
            self.s = "I'm square!!"

This works as I would expect:

CoolCar.myWheels()
TerribleTank.myWheels()

But what's bothering me is that I have to write CoolCar twice in the definition of TerribleTank. So I tried this:

class TerribleTank(CoolCar):
    class Wheels(super().Wheels):
        def __init__(self):
            self.s = "I'm square!!"

Which does not work. Now, I know it doesn't work because super() is looking for a first-argument self/cls to begin its search.

So finally my question: Is there something like this that works, so that I don't need to explicitly write that second CoolCar?

@PadraicCunningham: this is a greatly simplified example of what is actually going on. I am using subclasses for various reasons (namespace issues one of them), but also because it allows me to write Wheels.foo in one of my classes, and it simply works, with the right version of Wheels. For more information, please see the Factory Pattern chapter in Marc Summfield's Python in Practice. — Steve D
– Steve D, Commented Jun 17, 2015 at 0:21
Just declare Wheels as a class within the module, rather than encapsulating inside Coolcar. Surely Uncoolcars are entitled to wheels as well? — John Mee
– John Mee, Commented Jun 17, 2015 at 0:27
@JohnMee: TerribleTank will override this Wheel class. Now what does any class that inherits from TerribleTank do? Putting the original Wheels class at the module level only raises the problem one generation, it does not solve it. — Steve D
– Steve D, Commented Jun 17, 2015 at 0:34
I at least cannot see any way to use super how you want, Wheels knows nothing about CoolCar — Padraic Cunningham
– Padraic Cunningham, Commented Jun 17, 2015 at 0:45

zmo · Accepted Answer · 2015-06-17 01:47:41Z

2

What about:

class CoolCar:
    @classmethod
    def myWheels(cls):
        cls.Wheels().out()
    class Wheels:
        def __init__(self):
            self.s = "I'm round!"
        def out(self):
            print(self.s)

class TerribleTank(CoolCar):
    class Wheels(TerribleTank.Wheels):
        def __init__(self):
            self.s = "I'm square!!"

>>> TerribleTank.myWheels()
I'm square!!

basically when you inherit CoolCar in TerribleTank, you set up TerribleTank.Wheels as a reference to CoolCar.Wheels, until you shadow it with your own new definition of it within the TerribleTank definition. So I believe that matches your expectations of not having CoolCar twice in TerribleBank definition ☺

HTH

edited Jun 17, 2015 at 1:47

answered Jun 17, 2015 at 1:22

zmo

24.9k4 gold badges58 silver badges91 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

zmo Over a year ago

N.B.: If you're still unhappy about this because of the use of TerribleBank twice, you might try using metaclasses, but personally I'd rather go to bed than over-engineering something that simple ;)

John Mee Over a year ago

Tweak it to call TerribleTank.myWheels() directly, he doesn't want to create tanks or cars; just their wheels.

Steve D Over a year ago

This is technically correct (the best kind of correct), but I still hold hope there is a cleaner way to do this.

zmo Over a year ago

well, the cleaner way is what you suggested but does not fit you, this is what comes second, and then you might use some metaclass mess. But there's a saying: when you got a problem that needs metaclass to be solved, you end up having a metaproblem ;-)

Collectives™ on Stack Overflow

Nested classes that inherit from the outer parent's nested classes in Python 3

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related