Classes can inherit objects (methods) from each other. The class that is being inherited from is called a parent class (also a base class or a superclass). The class that inherits from another class is called a child class (also a derived class or a subclass). It is the third concept of object-oriented programming.

class Animal:
    def voice(self):
        print("Voice!")

class Cat(Animal):
    def breed(self):
        print("I am a Bengal cat!")

cat = Cat()
cat.voice()
cat.breed() 
                                    

The superclass's constructor is automatically called before the subclass's constructor if it isn't overridden. Overriding means providing a new method implementation in the subclass that replaces the method defined in the parent class (e.g., defining a speak() method in a Cat subclass to replace the general speak() method from an Animal parent class).

class Animal:
    def speak(self):
        print("Voice!")

class Cat(Animal):
    def speak(self):
        print("Meow!")

c = Cat()
c.speak()
                                    

In the example below, self.x is assigned in the Example's constructor, making it an instance variable, which printX() (inherited from Functions) can access through self because instance attributes are shared across methods in an object-oriented hierarchy.

class Functions:
    def printX(self):
        print(self.x) 

class Example(Functions):
    def __init__(self):
        self.x = 2

e = Example()
e.printX()
                                    

Using the two methods below, we can check if a class is a subclass of another class or an object is an instance of a given class.

print(issubclass(Cat, Animal))
print(isinstance(cat, Cat))
print(isinstance(cat, Animal))
                                    

Multiple inheritance

A class can inherit features from more than one parent class.

class Animal:
    def voice(self):
        print("Voice!")

class Mammal:
    def appearance(self):
        print("I have fur!")

class Tiger(Animal, Mammal):
    def sound(self):
        print("Raah!")

class Cat(Tiger):
    def breed(self):
        print("I am a Bengal cat!")

cat = Cat()
cat.voice()
cat.appearance()
cat.sound()
cat.breed()
                                    

The super() method

The super() method allows us to call the original method from the parent class when overriding it in a subclass.

class Animal:
    def voice(self):
        print("Voice!")

class Cat(Animal):
    def voice(self):
        super().voice()
        print("Meow!")

cat = Cat()
cat.voice()
                                    

Polymorphism

Polymorphism means operating in the same way on objects of different types. It refers to an object's ability to take on many forms. For example, the + operator behaves polymorphically because it can add numbers and concatenate strings (it works with various data types in a very similar way). It is the fourth concept of object-oriented programming. Example: A man can be a father, a husband, a student, etc. One person can have different roles.

class Person:
    def role(self):
        pass # a placeholder method to be implemented by the subclasses

class Father(Person):
    def role(self):
        print("I am a father!")

class Husband(Person):
    def role(self):
        print("I am a husband!")

class Student(Person):
    def role(self):
        print("I am a student!")

father = Father()
husband = Husband()
student = Student()

for person in [father, husband, student]:
    person.role() # polymorphism: calling the same method "role()" on different objects
                                    

This mechanism can also be used in structural programming.

list1 = ["a", "b", "c"]
string = "abc"
number = 5

def read(parameter): # this function works polymorphically
    print(parameter)
                
read(list1)
read(string)
read(number)