polymorphism

poly.py

# Imagine with have some classes for animals:

class Animal:
    """Base class for all animals."""

    def __init__(self, name):
        self.name = name


class Dog(Animal):
    """Dogs."""

    def bark(self):
        return "woof! barf bark!"


class Cat(Animal):
    """Cats."""

    def meow(self):
        return "meow, purr purr"


class Duck(Animal):
    """Ducks."""
    
    def quack(self):
        return "quack! quack!"


# Let's make some animals
fido = Dog("Fido")
fluffy = Cat("Fluffy")
crumbs = Duck("Crumbs")

# And put them all into a petting zoo

zoo = [fido, fluffy, crumbs]


# Now, we want each animal to make noise, in order. However, each
# class uses a *similar-in-theory* but *differently-named* method
to make noise. So our code would have to be, ugh:


for animal in zoo:
    if isinstance(animal, Dog):
        animal.bark()
    elif isinstance(animal, Cat):
        animal.meow()
    elif isinstance(animal, Duck):
        animal.quack()



# This works, but it's hairy -- especially if we needed to add more
# types of animals (we'd have to change the inside of the for-loop!)


# These methods (bark, meow, quack) all mean basically the same thing:
# "make noise in the right way for this kind of animal".
# Much better, therefore, is to give them the same name:


class Dog(Animal):
    """Dogs."""

    def speak(self):
        return "woof! barf bark!"


class Cat(Animal):
    """Cats."""

    def speak(self):
        return "meow, purr purr"


class Duck(Animal):
    """Ducks."""
    
    def speak(self):
        return "quack! quack!"


# Now our for-loop is much simpler:

for animal in zoo:
    animal.speak()


# That's "polymorphism": different objects from different classes
# can be used interchangeably. When possible, designing your classes
# to be "polymorphic" is good. For example, if you were building a
# special kind of list, similar-but-different from Python's List type,
# you be smart to make as interoperable as possible with a standard
# list by using the same method names ("append", "pop", etc),
# as was sensible