Python Inheritance

Inheritance enables us to characterize a class that takes all the functionality from a parent class and permits us to include more. In this tutorial, you will learn how to use inheritance in Python.

Inheritance in Python

Inheritance is a powerful feature in object-oriented programming.

It refers to defining a new class with little or no modification to an existing class. The new class is called derived (or child) class and the one from which it acquires is known as the base (or parent) class.


Python Inheritance Syntax

class BaseClass:
  Body of base class
class DerivedClass(BaseClass):
  Body of derived class

The derived class inherits features from the base class where new features can be added to it. This results in the re-usability of code.


Example of Inheritance in Python

To demonstrate the use of inheritance, let us take a model.

A polygon is a closed figure with at least 3 sides. State, we have a class called Polygon defined as follows.

class Polygon:
    def __init__(self, no_of_sides):
        self.n = no_of_sides
        self.sides = [0 for i in range(no_of_sides)]

    def inputSides(self):
        self.sides = [float(input("Enter side "+str(i+1)+" : ")) for i in range(self.n)]

    def dispSides(self):
        for i in range(self.n):
            print("Side",i+1,"is",self.sides[i])

This class has data attributes to store the number of sides n and magnitude of each side as a list called sides.

The inputSides() method takes in the magnitude of each side and dispSides() displays these side lengths.

A triangle is a polygon with 3 sides. So, we can create a class called Triangle which inherits from Polygon. This makes all the attributes of the Polygon class available to the Triangle class.

We don’t need to define them again (code reusability). Triangle can be defined as follows.

class Triangle(Polygon):
    def __init__(self):
        Polygon.__init__(self,3)

    def findArea(self):
        a, b, c = self.sides
        # calculate the semi-perimeter
        s = (a + b + c) / 2
        area = (s*(s-a)*(s-b)*(s-c)) ** 0.5
        print('The area of the triangle is %0.2f' %area)

However, the class Triangle has a new method findArea() to find and print the area of the triangle. Here is a sample run.

>>> t = Triangle()

>>> t.inputSides()
Enter side 1 : 3
Enter side 2 : 5
Enter side 3 : 4

>>> t.dispSides()
Side 1 is 3.0
Side 2 is 5.0
Side 3 is 4.0

>>> t.findArea()
The area of the triangle is 6.00

We can see that despite the fact that we didn’t characterize methods like the inputside() or dispSides() for class Triangle independently, we had the option to use them.

On the off chance that a trait isn’t found in the class itself, the pursuit proceeds to the base class. This repeats recursively if the base class is itself derived from other classes.


Method Overriding in Python

In the above example, notice that the __init__() method was defined in both classes, Triangle as well as Polygon. When this happens, the method in the derived class overrides that in the base class. This is to say, __init__() in Triangle gets preference over the __init__ in Polygon.

Generally, when overriding a base method, we tend to extend the definition rather than simply replace it. The same is being done by calling the method in a base class from the one in a derived class (calling Polygon.__init__() from __init__() in Triangle).

A better option would be to use the built-in function super(). So, super().__init__(3) is equivalent to Polygon.__init__(self,3) and is preferred. To learn more about the super() function in Python, visit Python super() function.

Two built-in functions isinstance() and issubclass() are used to check inheritances.

The function instance() returns True if the object is an instance of the class or other classes derived from it. Each and every class in Python inherits from the base class object.

>>> isinstance(t,Triangle)
True

>>> isinstance(t,Polygon)
True

>>> isinstance(t,int)
False

>>> isinstance(t,object)
True

Similarly, issubclass() is used to check for class inheritance.

>>> issubclass(Polygon,Triangle)
False

>>> issubclass(Triangle,Polygon)
True

>>> issubclass(bool,int)
True

Please feel free to give your comment if you face any difficulty here.

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