class Animal:

    def speak(self):

        pass  # placeholder

class Dog(Animal):

    def speak(self):

        return "Woof!"

class Cat(Animal):

    def speak(self):

        return "Meow!"

class Parrot(Animal):

    def speak(self):

        return "Squawk!"

# Polymorphism in action

animals = [Dog(), Cat(), Parrot()]

for animal in animals:

    print(animal.speak())