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 python inheritance

What is Inheritance?

  • Inheritance is used to specify that one class will get most or all of its features from its parent class. It is a feature of Object Oriented Programming.
  • It is a very powerful feature which facilitates users to create a new class with a few or more modification to an existing class.
  • The new class is called child class or derived class and the main class from which it inherits the properties is called base class or parent class.
  • The child class or derived class inherits the features from the parent class, adding new features to it. It facilitates re-usability of code.

Python Inheritance Syntax:

class BaseClass:
  Body of base class
class DerivedClass(BaseClass):
  Body of derived class
  • Derived class inherits features from the base class, adding new features to it. This results into re-usability of code.

Example of Inheritance in Python:

  • To demonstrate the use of inheritance, let us take an example.
  • A polygon is a closed figure with 3 or more sides. Say, we have a class called Polygondefined 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, sides.
  • Method inputSides() takes in magnitude of each side and similarly, dispSides() will display these properly.
  • A triangle is a polygon with 3 sides. So, we can created a class called Triangle which inherits from Polygon.
  • This makes all the attributes available in class Polygon readily available in Triangle. We don't need to define them again (code re-usability). Triangle is 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, 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, even though we did not define methods like inputSides() or dispSides()for class Triangle, we were able to use them.
  • If an attribute is not found in the class, search continues to the base class. This repeats recursively, if the base class is itself derived from other classes.

Image representation:

 python class inheritance
 derived class in python

Syntax 1:

	class DerivedClassName(BaseClassName):  
	    <statement-1>  
	    .  
	    .  
	    .  
	    <statement-N>  

Syntax 2:

	class DerivedClassName(modulename.BaseClassName):  
	    <statement-1>  
	    .  
	    .  
	    .  
    <statement-N>

Parameter explanation:

  • The name BaseClassName must be defined in a scope containing the derived class definition.
  • You can also use other arbitrary expressions in place of a base class name.
  • This is used when the base class is defined in another module.

Python Inheritance Example

  • Let's see a simple python inheritance example where we are using two classes: Animal and Dog. Animal is the parent or base class and Dog is the child class.
  • Here, we are defining eat() method in Animal class and bark() method in Dog class.
  • In this example, we are creating instance of Dog class and calling eat() and bark() methods by the instance of child class only.
  • Since, parent properties and behaviors are inherited to child object automatically, we can call parent and child class methods by the child instance only.
class Animal:
def eat(self):
print 'Eating...'
class Dog(Animal):
def bark(self):
print 'Barking...'
d=Dog()
d.eat()
d.bark()

Python Inheritance Example Output:

Eating...  
Barking...  

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