Inheritance #

Inheritance is a powerful tool that is very often used to reduce redundant code. If you have more specific versions of a larger class, inheritance can be extremely useful. Using our Animal class from the Objects notes, we can use all its general attributes, but then add more specific classes.

class Animal:
    default_food = []
    def __init__(self, name, energy = 100):
        self.name = name
        self.food = self.default_food[:]
        self.energy = energy
        self.times_fed = 0

    def feed(self, food):
        self.food = self.food + [food]
        self.times_fed += 1
        self.energy += 10

    def play(self):
        if energy <= 20:
            return "Not Enough Energy"
        self.energy -= 20
        self.is_happy = True # Creates new instance variable
        return f"{self.name} has {self.energy} energy."

my_cat = Animal("Sochi")
my_cat.feed("Tuna")

Instead of making my_cat an instance of the Animal class, we could instead create a Cat class that inherits from the Animal class - meaning that it contains the same class, methods, and instance variables as the Animal class (which can then be overridden).

The syntax for creating a class inherited from another class is shown below:

class Cat(Animal):
    pass

Right now, this will create a new Cat class pointing towards the Animal class we’ve already created, meaning that it can also access the class attributes/variables alongside the methods of the Animal class.

In the image above, we can see the Cat class we created pointing towards the Animal class, meaning that in terms of our lookup order, we first look to see if something exists in the instance, then Cat, then Animal.

For example, let’s edit our Cat class a bit, so it has a use, then create an instance:

class Cat(Animal):
    default_food = ["Tuna"]

sochi = Cat("Sochi")

When sochi = Cat("Sochi") is called, it first looks for an __init__ method in its own class (Cat), which doesn’t exist, so it looks up to its parent (Animal) for an __init__, which is found, so that __init__ method is used. If there is no __init__ method in the lookup, that method will simply not be called.

As a result of that, we get the following diagram (in this case, the __init__ method is called from class Animal as that is the highest level where an __init__ is defined.):

When looking for self.default_food[:], default_food from the Cat class is taken, because the created instance first looks to see if there is a default_food already located in the instance (it doesn’t), then looks to where the arrow is pointing, which is the Cat class, then looks for default_food, which is then found.

Our current version of the Cat class doesn’t really do much. We can override our default methods by simply redefining them in our new class. The lookup order will make it such that only our newly defined method will be called.

class Cat(Animal):
    default_food = ["Tuna"]

    def feed(self, food):
        self.food = self.food + [food]
        self.times_fed += 1
        self.energy += 10000

Now our current diagram looks like this. If we call sochi.feed("apple"), it will look at the feed method defined in Cat and then call it because it exists in that class.

The super() Function #

The super() function, at least in the scope of CS 61A, looks at its parent class.

This could be useful if you wanted to use a method of the parent class but add a few extra details. The super() function automatically passes in self.

class Cat(Animal):
    default_food = ["Tuna"]

    def __init__(self, name, energy = 100):
        super().__init__(name, energy) # will automatically define instance variables from the Animal class
        self.is_very_cute = True

    def feed(self, food):
        self.food = self.food + [food]
        self.times_fed += 1
        self.energy += 10000

Now, looking at our diagrams, we have the following:

If we then create an instance of our newly created Cat class, we will do the following:

First, create our instance and find whether an __init__ exists

Then, if it does exist, we execute it.

Use Cases #

Explicit examples will not be given here because they should not be necessary. You can imagine the world of possibilities that you can do with classes and inheritance! Try thinking of how you would make a card game using OOP. Doing that will reduce A LOT of redundant code and make writing this card game easy. In fact, you might see this sort of thing show up in lab questions (alongside the Ants project).