#Python's inheritance and polymorphism

Inheritance is a syntax for creating a new class based on an existing class. The inherited class is called the parent class, and the new class created is called the child class. The child class automatically inherits the same attributes and methods as the parent class.

class ClassName(ParentClassName):
    pass

The parent class is also called the base or superclass, and the child class is also called the derived class.

Example:

class Pet:
    """
    Pet class
    """

    def __init__(self, name: str):
        """
        Constructor

        Args:
            name (str): Name of the pet
        """
        self.__name = name

    def name(self) -> str:
        """
        Get the pet's name

        Returns:
            str: The name of the pet
        """
        return self.__name


# Cat class inherits from Pet
class Cat(Pet):

    # Additional method
    def catch_mouses(self):
        pass


# Dog class inherits from Pet
class Dog(Pet):

    # Additional method
    def guard_gates(self):
        pass


# Create objects using inherited constructor
tom = Cat("Tom")
happy = Cat("Happy")
spike = Dog("Spike")

# Call inherited method
print(tom.name())
print(happy.name())
print(spike.name())

>>> Establishing WebAssembly Runtime.

>>> Standby.

#Polymorphism

Child classes can override methods from the parent class:

class Pet:
    """
    Pet class
    """

    def speak(self):
        """
        Speak method
        """
        print("Some animal is speaking")


class Cat(Pet):
    """
    Cat class, inherits from Pet
    """

    def speak(self):
        """
        Speak method
        """
        print("Meow Meow Meow")


class Dog(Pet):
    """
    Dog class, inherits from Pet
    """

    def speak(self):
        """
        Speak method
        """
        print("Woof Woof Woof")


class Rabbit(Pet):
    """
    Rabbit class, inherits from Pet
    """
    pass


# Create objects
cat = Cat()
dog = Dog()
rabbit = Rabbit()

# Call speak method
cat.speak()       # Calls overridden speak in Cat
dog.speak()       # Calls overridden speak in Dog
rabbit.speak()    # Rabbit does not override speak, calls Pet's speak


# Petting a pet
def fondle(pet: Pet):
    pet.speak()    # Calls speak based on the actual type


# Pet the animals
fondle(cat)
fondle(dog)
fondle(rabbit)

>>> Establishing WebAssembly Runtime.

>>> Standby.

In this example, the function fondle takes a parameter pet of base class type Pet. When calling pet.speak(), it dynamically calls the correct overridden method according to the actual object type.

This behavior — using a unified interface to invoke different behaviors — is called polymorphism.

#super

If a child class overrides the constructor, it needs to call the parent class's constructor within it to initialize inherited attributes. In Python, you use the super() function to access the parent class.

class Pet:
    """
    Pet class
    """

    def __init__(self, name: str):
        """
        Constructor

        Args:
            name (str): Name
        """
        self.name = name


class Cat(Pet):
    """
    Cat class, inherits from Pet
    """

    def __init__(self, name: str, breed: str):
        """
        Constructor

        Args:
            name (str): Name
            breed (str): Breed
        """

        super().__init__(name)  # Call parent constructor

        self.breed = breed      # New attribute


# Create object
cat = Cat("Tom", "Chartreux")
print(cat.name)
print(cat.breed)

>>> Establishing WebAssembly Runtime.

>>> Standby.