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What is inheritance?

Inheritance is the mechanism that allows programmers to create new classes from existing class. By using inhertitance programmers can re-use code they've already written.

Any new class that you create from an existing class is called sub class; existing class is called super class. The sub class gets all of the methods and state variables of the super class by default. The sub class can add new methods and state variables. The sub class can also redefine methods and state variables of its parent.

The following rules you need to know about superclasses and subclasses:

• The subclass inherits all the members of the superclass. So, all data members and methods of the superclass are also the data members and methods of the subclass.
• The private members of the superclass cannot be directly accessed by the members of the subclass.
• The subclass can access the public members of the superclass directly.

Syntax of Inheritance in Python

Any class can be a super class, so the syntax is the same as creating any other class:

class superclass:
    .....

To create a sub class send the super class as a parameter when creating the sub class:

class subclass(superclass):
    .....

Create __init__() function inside subclass and add a call to the parent's __init__() function. We can refer to the methods of super class with super().

class subclass(superclass):
    def __init__(self,param1,param2):
        super().__init__(param1)
        self.param2 = param2
    ....

Alternatively, instead of using super() you can call __init__() using super class name.

class subclass(superclass):
    def __init__(self,param1,param2):
        superclass.__init__(self,param1)
        self.param2 = param2
    ....

Example:

The following program demonstrates the concepts of inheritance :

Program (box.py)

#super class
class Rectangle:

    def __init__(self, length, width):
        self.__length = length
        self.__width = width

    def set_rectangle(self,length, width):
        self.__length = length
        self.__width  = width

    def get_area(self):
        return self.__length * self.__width;

#sub class
class Box(Rectangle):

    def __init__(self, length, width, height):
        Rectangle.__init__(self,length,width)
        self.__height = height

    def set_box(self,length, width, height):
        self.set_rectangle(length,width)
        self.__height = height

    def get_volume(self):
        return self.get_area() * self.__height
    
#creating object
b = Box(12,9,10)
print("Volume is ", b.get_volume())
b.set_box(12.5, 10.5, 9.5)
print("Volume is ", b.get_volume())

Output

Volume is  1080
Volume is  1246.875

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