Inheritance

We can use members of one class inside another class by using the following ways.

By Composition (Has-A Relationship)
By Inheritance (IS-A Relationship)

By Composition (Has-A Relationship)

By using class name or by creating object we can access members of one class inside another class is nothing but composition (Has-A Relationship).
The main advantage of has-a relationship is code reusability.

Example

Python

class Engine: 
a=10 
def __init__(self): 
self.b=20 
def m1(self): 
print('Engine Specific Functionality') 
class Car: 
def __init__(self): 
self.engine=Engine() 
def m2(self): 
print('Car using Engine Class Functionality') 
print(self.engine.a) 
print(self.engine.b) 
self.engine.m1() 
c=Car() 
c.m2()

Output

PowerShell

Car using Engine Class Functionality
10
20
Engine Specific Functionality

Example

Python

class Car: 
def __init__(self,name,model,color): 
self.name=name 
self.model=model 
self.color=color 
def getinfo(self): 
print("Car Name:{} , Model:{} and Color:{}".format(self.name,self.model,self.color)) 
 
class Employee: 
def __init__(self,ename,eno,car): 
self.ename=ename 
self.eno=eno 
self.car=car 
def empinfo(self): 
print("Employee Name:",self.ename) 
print("Employee Number:",self.eno) 
print("Employee Car Info:") 
self.car.getinfo() 
c=Car("Innova","2.5V","Grey") 
e=Employee('Durga',10000,c) 
e.empinfo()

Output

PowerShell

Employee Name: Durga
Employee Number: 10000
Employee Car Info:
Car Name: Innova, Model:2.5V and Color:Grey
In the above program Employee class Has-A Car reference and hence Employee class can access all 
members of Car class.

Example

Python

class X: 
a=10 
def __init__(self): 
self.b=20 
def m1(self): 
print("m1 method of X class") 
class Y: 
c=30 
def __init__(self): 
self.d=40 
def m2(self): 
print("m2 method of Y class") 
def m3(self): 
x1=X() 
print(x1.a) 
print(x1.b) 
x1.m1() 
print(Y.c) 
print(self.d) 
self.m2() 
print("m3 method of Y class") 
y1=Y()
y1.m3()

Output

PowerShell

10
20
m1 method of X class
30
40
m2 method of Y class
m3 method of Y class

By Inheritance(IS-A Relationship)

Whatever variables, methods and constructors available in the parent class by default available to the child classes and we are not required to rewrite. Hence the main advantage of inheritance is code reusability and we can extend existing functionality with some more extra functionality.

Syntax

class childclass(parentclass)

Example

Python

1) class P: 
2) a=10 
3) def __init__(self): 
4) self.b=10 
5) def m1(self): 
6) print('Parent instance method') 
7) @classmethod 
8) def m2(cls): 
9) print('Parent class method') 
10) @staticmethod 
11) def m3(): 
12) print('Parent static method') 
13) 
14) class C(P): 
15) pass 
16) 
17) c=C() 
18) print(c.a) 
19) print(c.b) 
20) c.m1() 
21) c.m2() 
22) c.m3()

Output

PowerShell

10
10
Parent instance method
Parent class method
Parent static method

Example

Python

class P: 
10 methods 
class C(P): 
5 methods

Output

PowerShell

In the above example parent class contains 10 methods and these methods automatically available to the child class and we are not required to rewrite those methods(code reusability) hence child class contains 15 methods.

Note

What ever members present in parent class are by default available to the child class through inheritance.

Example

Python

class P: 
def m1(self): 
print("Parent class method") 
class C(P): 
def m2(self): 
print("Child class method") 
 
c=C(); 
c.m1() 
c.m2()

Output

PowerShell

Parent class method
Child class method

What ever methods present in parent class are automatically available to the child class and hence on the child class reference we can call both parent class methods and child class methods.

Example

Similarly variables also.

Python

class P: 
a=10 
def __init__(self): 
self.b=20 
class C(P): 
c=30 
def __init__(self): 
super().__init__()===>Line-1
self.d=30 
 
c1=C() 
print(c1.a,c1.b,c1.c,c1.d)
If we comment Line-1 then variable b is not available to the child class.

Output

PowerShell

Example

Python

class Person: 
def __init__(self,name,age): 
self.name=name 
self.age=age 
def eatndrink(self): 
print('Eat Biryani and Drink Beer') 
 
class Employee(Person): 
def __init__(self,name,age,eno,esal): 
super().__init__(name,age) 
self.eno=eno 
self.esal=esal 
 
def work(self): 
print("Coding Python is very easy just like drinking Chilled Beer") 
def empinfo(self): 
print("Employee Name:",self.name) 
print("Employee Age:",self.age) 
print("Employee Number:",self.eno) 
print("Employee Salary:",self.esal) 
 
e=Employee('Durga', 48, 100, 10000) 
e.eatndrink() 
e.work() 
e.empinfo()

Output

PowerShell

Eat biryani and drink Beer
Coding python is very easy just like drinking chilled Beer
Employee Name: Durga
Employee Age: 48
Employee Number: 100
Employee Salary: 10000

IS-A vs HAS-A Relationship

If we want to extend existing functionality with some more extra functionality then we should go for is-a relationship
If we dont want to extend and just we have to use existing functionality then we should go for has-a relationship.

Example

Python

Employee class extends person class Functionality.
But Employee class just uses car functionality but not extending.
class Car: 
def __init__(self,name,model,color): 
self.name=name 
self.model=model 
self.color=color 
def getinfo(self): 
print("\tCar Name:{} \n\t Model:{} \n\t Color:{}".format(self.name,self.model,self.color)) 

class Person: 
def __init__(self,name,age): 
self.name=name 
self.age=age 
def eatndrink(self): 
print('Eat Biryani and Drink Beer') 

class Employee(Person): 
def __init__(self,name,age,eno,esal,car): 
super().__init__(name,age) 
self.eno=eno 
self.esal=esal 
self.car=car 
def work(self): 
print("Coding Python is very easy just like drinking Chilled Beer") 
def empinfo(self):
print("Employee Name:",self.name) 
print("Employee Age:",self.age) 
print("Employee Number:",self.eno) 
print("Employee Salary:",self.esal) 
print("Employee Car Info:") 
self.car.getinfo() 
 
c=Car("Innova","2.5V","Grey") 
e=Employee('Durga',48,100,10000,c) 
e.eatndrink() 
e.work() 
e.empinfo()

Output

PowerShell

Eat Biryani and Drink Beer
Coding Python is very easy just like drinking Chilled Beer
Employee Name: Durga
Employee Age: 48
Employee Number: 100
Employee Salary: 10000
Employee Car Info:
 Car Name:Innova
 Model:2.5V
 Color:Grey
In the above example Employee class extends Person class functionality but just uses Car class 
functionality.

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