Inheritance in Python allows us to define a class by using the functionality of the parent class. Let's see an example where we will derive a dog class from an animal class.
Suppose we have an Animal class:
class Animal:
def eat(self):
print('I can eat')
Now, let's derive a Dog class from Animal:
class Animal:
def eat(self):
print('I can eat')
class Dog(Animal):
def bark(self):
print('I can bark')
From the above code, you can see that we have included the parent class inside the bracket along with the child class.
Syntax:
# Dog class inherits Animal class
class Dog(Animal):
# code inside dog class
During inheritance, the object of the child class (Dog) can
access methods and attributes of both the parent class (Animal)
and the child class.
class Animal:
def eat(self):
print('I can eat')
class Dog(Animal):
def bark(self):
print('I can bark')
dog1 = Dog()
dog1.bark()
dog1.eat()
Output
I can bark I can eat
Here, We have created an object of the Dog class named
dog1. We then used this object to access the method of
- parent class,
eat() - child class,
bark()
Inherit Multiple Classes from a Single Class
Similarly, we can also derive another class, named Cat, from the
Animal class:
class Animal:
def eat(self):
print('I can eat')
class Dog(Animal):
def bark(self):
print('I can bark')
class Cat(Animal):
def get_grumpy(self):
print('I am getting grumpy.')
dog1 = Dog()
dog1.bark()
dog1.eat()
cat1 = Cat()
cat1.eat()
Output
I can bark I can eat I can eat
Now the object of the Cat class (cat1) can also
access the method of the parent class (Animal).
Example of Python Inheritance
Let's see one more example of Python Inheritance.
This time we will use inheritance to calculate the perimeter of different polygons like triangles and quadrilaterals using inheritance.
Let's first create a base class called Polygon:
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
perimeter = sum(self.sides)
return perimeter
All polygons, like triangles and quadrilaterals, will derive the features of
Polygon.
Let's create a Triangle class that will inherit from the
Polygon class:
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
perimeter = sum(self.sides)
return perimeter
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
We have redefined the display_info() method to display
information specific to triangles.
Similarly, we can also define a new Quadrilateral class.
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
perimeter = sum(self.sides)
return perimeter
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
class Quadrilateral(Polygon):
def display_info(self):
print('A quadrilateral is a polygon with 4 edges')
Now, let's find the perimeter of a triangle. For this, we will create an
object from Triangle and use its
get_perimeter() method.
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
value = 0
for side in self.sides:
value += side
return value
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
class Quadrilateral(Polygon):
def display_info(self):
print('A quadrilateral is a polygon with 4 edges')
t1 = Triangle([5, 6, 7])
perimeter = t1.get_perimeter()
print('Perimeter:', perimeter)
Output
Perimeter: 18
Here, the Triangle class inherits Polygon, so it is
able to use the get_perimeter() method to find the perimeter of
the triangle.
Method Overriding
If you have noticed in the above example, we have the
display_info() method in both our base class and the derived
classes.
Let's see what will happen if we call the display_info() method
using the t1 object of Triangle.
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
perimeter = sum(self.sides)
return perimeter
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
class Quadrilateral(Polygon):
def display_info(self):
print('A quadrilateral is a polygon with 4 edges')
t1 = Triangle([5, 6, 7])
perimeter = t1.get_perimeter()
print('Perimeter:', perimeter)
t1.display_info()
Output
Perimeter: 18 A triangle is a polygon with 3 edges
We can see that the display_info() method of the
Triangle class is called, and display_info() of its
parent class is not executed.
This is because when the same method is present in both the child and parent classes, the method in the child class will override the same method in the parent class. This concept is called method overriding.
If we need, we can call the display_info() method of our parent
Polygon class from inside its child classes:
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
perimeter = sum(self.sides)
return perimeter
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
# call display_info() of parent class
Polygon.display_info(self)
class Quadrilateral(Polygon):
def display_info(self):
print('A quadrilateral is a polygon with 4 edges')
t1 = Triangle([5, 6, 7])
perimeter = t1.get_perimeter()
print('Perimeter:', perimeter)
t1.display_info()
Output
Parameter: 18 A triangle is a polygon with 3 edges A polygon is a two dimensional shape with straight lines
Note: Polygon is the name of the parent class.
Since we are calling the method using a class rather than an object, we also
need to pass the self object manually.
This code is a bit more unorthodox than what we have been using. There is a
more elegant way to achieve the same task by using the
super() function.
The super() function
The super() function returns a temporary object of the parent
class and allows us to call the methods of the parent class from within the
child class.
class Polygon:
def __init__(self, sides):
self.sides = sides
def display_info(self):
print('A polygon is a two dimensional shape with straight lines')
def get_perimeter(self):
value = 0
for side in self.sides:
value += side
return value
class Triangle(Polygon):
def display_info(self):
print('A triangle is a polygon with 3 edges')
# Polygon.display_info(self)
super().display_info()
class Quadrilateral(Polygon):
def display_info(self):
print('A quadrilateral is a polygon with 4 edges')
t1 = Triangle([5, 6, 7])
perimeter = t1.get_perimeter()
print('Perimeter:', perimeter)
t1.display_info()
Output
Parameter: 18 A triangle is a polygon with 3 edges A polygon is a two dimensional shape with straight lines
In this case, super() is an object of Polygon. We
are using it to call display_info() of the
Polygon class.