Class inheritance

• You can derive new classes from old classes, which is called class inheritance. This is useful if you want to add extra or different functionality but don’t lose what you already implemented. See, for example, a class for an Animal:

class Animal():
  
  def __init__(self, age, color):
    self.age = age
    self.color = color
    
  def crossbreed(self, other):
    return Animal(0, self.color+' and '+other.color)
  
  def make_sound(self):
    print("Animal speaks!")

• We can generate some derived classes by putting the parent class inside the parentheses in the class definition

class Dog(Animal):  # Dog is a subclass of Animal
  
  def __init__(self, breed, age, color):   # reimplementing the constructor method
    self.breed = breed
    self.age = age
    self.color = color
    
  def make_sound(self):   # reimplementing the make_sound method
    print("Woof woof!")
    
    
class Cat(Animal):
  
  def make_sound(self):
    print("Meow!")
    
    
# main chunk
fido = Dog('boxer', 2, 'brown')
whiskers = Cat(2, 'black')
turtle = Animal(1, 'white')
fido.make_sound()
whiskers.make_sound()
turtle.make_sound()

Woof woof!
Meow!
Animal speaks!

• We can treat our Dog and Cat as also members of the Animal class! This is what we call polymorphism. For example, if we call crossbreed in any of the Dog and Cat object
  • But the problem now will be that we can crossbreed animals of different species… which is a mistake! :P