Supercharge Your Classes With Python super()

Multiple Inheritance Overview

There is another use case in which super() really shines, and this one isn’t as common as the single inheritance scenario. In addition to single inheritance, Python supports multiple inheritance, in which a subclass can inherit from multiple superclasses that don’t necessarily inherit from each other (also known as sibling classes).

I’m a very visual person, and I find diagrams are incredibly helpful to understand concepts like this. The image below shows a very simple multiple inheritance scenario, where one class inherits from two unrelated (sibling) superclasses:

To better illustrate multiple inheritance in action, here is some code for you to try out, showing how you can build a right pyramid (a pyramid with a square base) out of a Triangle and a Square:

class Triangle:
    def __init__(self, base, height):
        self.base = base
        self.height = height

    def area(self):
        return 0.5 * self.base * self.height

class RightPyramid(Triangle, Square):
    def __init__(self, base, slant_height):
        self.base = base
        self.slant_height = slant_height

    def area(self):
        base_area = super().area()
        perimeter = super().perimeter()
        return 0.5 * perimeter * self.slant_height + base_area

This example declares a Triangle class and a RightPyramid class that inherits from both Square and Triangle.

You’ll see another .area() method that uses super() just like in single inheritance, with the aim of it reaching the .perimeter() and .area() methods defined all the way up in the Rectangle class.

The problem, though, is that both superclasses (Triangle and Square) define a .area(). Take a second and think about what might happen when you call .area() on RightPyramid, and then try calling it like below:

>> pyramid = RightPyramid(2, 4)
>> pyramid.area()
Traceback (most recent call last):
  File "shapes.py", line 63, in <module>
    print(pyramid.area())
  File "shapes.py", line 47, in area
    base_area = super().area()
  File "shapes.py", line 38, in area
    return 0.5 * self.base * self.height
AttributeError: 'RightPyramid' object has no attribute 'height'

Did you guess that Python will try to call Triangle.area()? This is because of something called the method resolution order.

return 0.5 * self.base * self.height

Method Resolution Order

The method resolution order (or MRO) tells Python how to search for inherited methods. This comes in handy when you’re using super() because the MRO tells you exactly where Python will look for a method you’re calling with super() and in what order.

Every class has an .__mro__ attribute that allows us to inspect the order, so let’s do that:

>>> RightPyramid.__mro__
(<class '__main__.RightPyramid'>, <class '__main__.Triangle'>, 
 <class '__main__.Square'>, <class '__main__.Rectangle'>, 
 <class 'object'>)

This tells us that methods will be searched first in Rightpyramid, then in Triangle, then in Square, then Rectangle, and then, if nothing is found, in object, from which all classes originate.

The problem here is that the interpreter is searching for .area() in Triangle before Square and Rectangle, and upon finding .area() in Triangle, Python calls it instead of the one you want. Because Triangle.area() expects there to be a .height and a .base attribute, Python throws an AttributeError.

Luckily, you have some control over how the MRO is constructed. Just by changing the signature of the RightPyramid class, you can search in the order you want, and the methods will resolve correctly:

class RightPyramid(Square, Triangle):
    def __init__(self, base, slant_height):
        self.base = base
        self.slant_height = slant_height
        super().__init__(self.base)

    def area(self):
        base_area = super().area()
        perimeter = super().perimeter()
        return 0.5 * perimeter * self.slant_height + base_area

Notice that RightPyramid initializes partially with the .__init__() from the Square class. This allows .area() to use the .length on the object, as is designed.

Now, you can build a pyramid, inspect the MRO, and calculate the surface area:

>>> pyramid = RightPyramid(2, 4)
>>> RightPyramid.__mro__
(<class '__main__.RightPyramid'>, <class '__main__.Square'>, 
<class '__main__.Rectangle'>, <class '__main__.Triangle'>, 
<class 'object'>)
>>> pyramid.area()
20.0

You see that the MRO is now what you’d expect, and you can inspect the area of the pyramid as well, thanks to .area() and .perimeter().

There’s still a problem here, though. For the sake of simplicity, I did a few things wrong in this example: the first, and arguably most importantly, was that I had two separate classes with the same method name and signature.

This causes issues with method resolution, because the first instance of .area() that is encountered in the MRO list will be called.

When you’re using super() with multiple inheritance, it’s imperative to design your classes to cooperate. Part of this is ensuring that your methods are unique so that they get resolved in the MRO, by making sure method signatures are unique—whether by using method names or method parameters.

In this case, to avoid a complete overhaul of your code, you can rename the Triangle class’s .area() method to .tri_area(). This way, the area methods can continue using class properties rather than taking external parameters:

class Triangle:
    def __init__(self, base, height):
        self.base = base
        self.height = height
        super().__init__()

    def tri_area(self):
        return 0.5 * self.base * self.height

Let’s also go ahead and use this in the RightPyramid class:

class RightPyramid(Square, Triangle):
    def __init__(self, base, slant_height):
        self.base = base
        self.slant_height = slant_height
        super().__init__(self.base)

    def area(self):
        base_area = super().area()
        perimeter = super().perimeter()
        return 0.5 * perimeter * self.slant_height + base_area

    def area_2(self):
        base_area = super().area()
        triangle_area = super().tri_area()
        return triangle_area * 4 + base_area

The next issue here is that the code doesn’t have a delegated Triangle object like it does for a Square object, so calling .area_2() will give us an AttributeError since .base and .height don’t have any values.

You need to do two things to fix this:

1. All methods that are called with super() need to have a call to their superclass’s version of that method. This means that you will need to add super().__init__() to the .__init__() methods of Triangle and Rectangle.

2. Redesign all the .__init__() calls to take a keyword dictionary. See the complete code below.

class Rectangle:
    def __init__(self, length, width, **kwargs):
        self.length = length
        self.width = width
        super().__init__(**kwargs)

    def area(self):
        return self.length * self.width

    def perimeter(self):
        return 2 * self.length + 2 * self.width

# Here we declare that the Square class inherits from 
# the Rectangle class
class Square(Rectangle):
    def __init__(self, length, **kwargs):
        super().__init__(length=length, width=length, **kwargs)

class Cube(Square):
    def surface_area(self):
        face_area = super().area()
        return face_area * 6

    def volume(self):
        face_area = super().area()
        return face_area * self.length

class Triangle:
    def __init__(self, base, height, **kwargs):
        self.base = base
        self.height = height
        super().__init__(**kwargs)

    def tri_area(self):
        return 0.5 * self.base * self.height

class RightPyramid(Square, Triangle):
    def __init__(self, base, slant_height, **kwargs):
        self.base = base
        self.slant_height = slant_height
        kwargs["height"] = slant_height
        kwargs["length"] = base
        super().__init__(base=base, **kwargs)

    def area(self):
        base_area = super().area()
        perimeter = super().perimeter()
        return 0.5 * perimeter * self.slant_height + base_area

    def area_2(self):
        base_area = super().area()
        triangle_area = super().tri_area()
        return triangle_area * 4 + base_area

There are a number of important differences in this code:

• **kwargs is modified in some places (such as RightPyramid.__init__()):** This will allow users of these objects to instantiate them only with the arguments that make sense for that particular object.

• Setting up named arguments before **kwargs: You can see this in RightPyramid.__init__(). This has the neat effect of popping that key right out of the **kwargs dictionary, so that by the time that it ends up at the end of the MRO in the object class, **kwargs is empty.

Now, when you use these updated classes, you have this:

>>> pyramid = RightPyramid(base=2, slant_height=4)
>>> pyramid.area()
20.0
>>> pyramid.area_2()
20.0

It works! You’ve used super() to successfully navigate a complicated class hierarchy while using both inheritance and composition to create new classes with minimal reimplementation.

Multiple Inheritance Alternatives

As you can see, multiple inheritance can be useful but also lead to very complicated situations and code that is hard to read. It’s also rare to have objects that neatly inherit everything from more than multiple other objects.

If you see yourself beginning to use multiple inheritance and a complicated class hierarchy, it’s worth asking yourself if you can achieve code that is cleaner and easier to understand by using composition instead of inheritance. Since this article is focused on inheritance, I won’t go into too much detail on composition and how to wield it in Python. Luckily, Real Python has published a deep-dive guide to both inheritance and composition in Python that will make you an OOP pro in no time.

There’s another technique that can help you get around the complexity of multiple inheritance while still providing many of the benefits. This technique is in the form of a specialized, simple class called a mixin.

A mixin works as a kind of inheritance, but instead of defining an “is-a” relationship it may be more accurate to say that it defines an “includes-a” relationship. With a mix-in you can write a behavior that can be directly included in any number of other classes.

Below, you will see a short example using VolumeMixin to give specific functionality to our 3D objects—in this case, a volume calculation:

class Rectangle:
    def __init__(self, length, width):
        self.length = length
        self.width = width

    def area(self):
        return self.length * self.width

class Square(Rectangle):
    def __init__(self, length):
        super().__init__(length, length)

class VolumeMixin:
    def volume(self):
        return self.area() * self.height

class Cube(VolumeMixin, Square):
    def __init__(self, length):
        super().__init__(length)
        self.height = length

    def face_area(self):
        return super().area()

    def surface_area(self):
        return super().area() * 6

In this example, the code was reworked to include a mixin called VolumeMixin. The mixin is then used by Cube and gives Cube the ability to calculate its volume, which is shown below:

>>> cube = Cube(2)
>>> cube.surface_area()
24
>>> cube.volume()
8

This mixin can be used the same way in any other class that has an area defined for it and for which the formula area * height returns the correct volume.