22.3. Overriding Methods¶

If a method is defined for a class, and also defined for its parent class, the subclass’ method is called and not the parent’s. This follows from the rules for looking up attributes that you saw in the previous section.

We can use the same idea to understand overriding methods.

Let’s return to our idea of making Cats, Dogs, and other pets generate a string for their “mood” differently.

Here’s the original Pet class again.

from random import randrange

# Here's the original Pet class
class Pet():
    boredom_decrement = 4
    hunger_decrement = 6
    boredom_threshold = 5
    hunger_threshold = 10
    sounds = ['Mrrp']
    def __init__(self, name = "Kitty"):
        self.name = name
        self.hunger = randrange(self.hunger_threshold)
        self.boredom = randrange(self.boredom_threshold)
        self.sounds = self.sounds[:]  # copy the class attribute, so that when we make changes to it, we won't affect the other Pets in the class

def clock_tick(self):
        self.boredom += 1
        self.hunger += 1

def mood(self):
        if self.hunger <= self.hunger_threshold and self.boredom <= self.boredom_threshold:
            return "happy"
        elif self.hunger > self.hunger_threshold:
            return "hungry"
        else:
            return "bored"

def __str__(self):
        state = "     I'm " + self.name + ". "
        state += " I feel " + self.mood() + ". "
        # state += "Hunger %d Boredom %d Words %s" % (self.hunger, self.boredom, self.sounds)
        return state

def hi(self):
        print(self.sounds[randrange(len(self.sounds))])
        self.reduce_boredom()

def teach(self, word):
        self.sounds.append(word)
        self.reduce_boredom()

def feed(self):
        self.reduce_hunger()

def reduce_hunger(self):
        self.hunger = max(0, self.hunger - self.hunger_decrement)

def reduce_boredom(self):
        self.boredom = max(0, self.boredom - self.boredom_decrement)

Now let’s make two subclasses, Dog and Cat. Dogs are always happy unless they are bored and hungry. Cats, on the other hand, are happy only if they are fed and if their boredom level is in a narrow range and, even then, only with probability 1/2.