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Inheritance in Python

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  • Difficulty Level : Easy
  • Last Updated : 07 Jul, 2022
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Inheritance is the capability of one class to derive or inherit the properties from another class. 

Benefits of inheritance are: 

  • It represents real-world relationships well.
  • It provides the reusability of a code. We don’t have to write the same code again and again. Also, it allows us to add more features to a class without modifying it.
  • It is transitive in nature, which means that if class B inherits from another class A, then all the subclasses of B would automatically inherit from class A.

Python Inheritance Syntax

Class BaseClass:
    {Body}
Class DerivedClass(BaseClass):
    {Body}

Creating a Parent Class

Creating a Person class with Display methods.

Python3




# A Python program to demonstrate inheritance
 
class Person(object):
   
  # Constructor
  def __init__(self, name, id):
    self.name = name
    self.id = id
 
  # To check if this person is an employee
  def Display(self):
    print(self.name, self.id)
 
 
# Driver code
emp = Person("Satyam", 102) # An Object of Person
emp.Display()


Output:

Satyam 102

Creating a Child Class

Here Emp is another class which is going to inherit the properties of the Person class(base class).

Python3




class Emp(Person):
   
  def Print(self):
    print("Emp class called")
     
Emp_details = Emp("Mayank", 103)
 
# calling parent class function
Emp_details.Display()
 
# Calling child class function
Emp_details.Print()


Output:

Mayank 103
Emp class called

Example of Inheritance in Python 

Python3




# A Python program to demonstrate inheritance
 
# Base or Super class. Note object in bracket.
# (Generally, object is made ancestor of all classes)
# In Python 3.x "class Person" is
# equivalent to "class Person(object)"
 
 
class Person(object):
 
    # Constructor
    def __init__(self, name):
        self.name = name
 
    # To get name
    def getName(self):
        return self.name
 
    # To check if this person is an employee
    def isEmployee(self):
        return False
 
 
# Inherited or Subclass (Note Person in bracket)
class Employee(Person):
 
    # Here we return true
    def isEmployee(self):
        return True
 
 
# Driver code
emp = Person("Geek1"# An Object of Person
print(emp.getName(), emp.isEmployee())
 
emp = Employee("Geek2"# An Object of Employee
print(emp.getName(), emp.isEmployee())


Output: 

Geek1 False
Geek2 True

What is object class?

Like the Java Object class, in Python (from version 3. x), the object is the root of all classes. 

  • In Python 3.x, “class Test(object)” and “class Test” are same. 
  • In Python 2. x, “class Test(object)” creates a class with the object as a parent (called a new-style class), and “class Test” creates an old-style class (without an objecting parent). 

Subclassing (Calling constructor of parent class)

A child class needs to identify which class is its parent class. This can be done by mentioning the parent class name in the definition of the child class. 

Eg: class subclass_name (superclass_name)

Python3




# Python code to demonstrate how parent constructors
# are called.
 
# parent class
class Person(object):
 
    # __init__ is known as the constructor
    def __init__(self, name, idnumber):
        self.name = name
        self.idnumber = idnumber
 
    def display(self):
        print(self.name)
        print(self.idnumber)
 
# child class
 
 
class Employee(Person):
    def __init__(self, name, idnumber, salary, post):
        self.salary = salary
        self.post = post
 
        # invoking the __init__ of the parent class
        Person.__init__(self, name, idnumber)
 
 
# creation of an object variable or an instance
a = Employee('Rahul', 886012, 200000, "Intern")
 
# calling a function of the class Person using its instance
a.display()


Output: 

Rahul
886012

‘a’ is the instance created for the class Person. It invokes the __init__() of the referred class. You can see ‘object’ written in the declaration of the class Person. In Python, every class inherits from a built-in basic class called ‘object’. The constructor i.e. the ‘__init__’ function of a class is invoked when we create an object variable or an instance of the class.
The variables defined within __init__() are called the instance variables or objects. Hence, ‘name’ and ‘idnumber’ are the objects of the class Person. Similarly, ‘salary’ and ‘post’ are the objects of the class Employee. Since the class Employee inherits from class Person, ‘name’ and ‘idnumber’ are also the objects of class Employee.

Python program to demonstrate error if we forget to invoke __init__() of the parent

If you forget to invoke the __init__() of the parent class then its instance variables would not be available to the child class. 

The following code produces an error for the same reason. 

Python3




class A:
    def __init__(self, n='Rahul'):
        self.name = n
 
 
class B(A):
    def __init__(self, roll):
        self.roll = roll
 
 
object = B(23)
print(object.name)


Output : 

Traceback (most recent call last):
  File "/home/de4570cca20263ac2c4149f435dba22c.py", line 12, in 
    print (object.name)
AttributeError: 'B' object has no attribute 'name'

Different types of Inheritance:

  • Single inheritance: When a child class inherits from only one parent class, it is called single inheritance. We saw an example above.
  • Multiple inheritances: When a child class inherits from multiple parent classes, it is called multiple inheritances. 

Unlike java, python shows multiple inheritances.

Python3




# Python example to show the working of multiple
# inheritance
 
 
class Base1(object):
    def __init__(self):
        self.str1 = "Geek1"
        print("Base1")
 
 
class Base2(object):
    def __init__(self):
        self.str2 = "Geek2"
        print("Base2")
 
 
class Derived(Base1, Base2):
    def __init__(self):
 
        # Calling constructors of Base1
        # and Base2 classes
        Base1.__init__(self)
        Base2.__init__(self)
        print("Derived")
 
    def printStrs(self):
        print(self.str1, self.str2)
 
 
ob = Derived()
ob.printStrs()


Output: 

Base1
Base2
Derived
Geek1 Geek2
  • Multilevel inheritance: When we have a child and grandchild relationship. 

Python3




# A Python program to demonstrate inheritance
 
# Base or Super class. Note object in bracket.
# (Generally, object is made ancestor of all classes)
# In Python 3.x "class Person" is
# equivalent to "class Person(object)"
 
 
class Base(object):
 
    # Constructor
    def __init__(self, name):
        self.name = name
 
    # To get name
    def getName(self):
        return self.name
 
 
# Inherited or Sub class (Note Person in bracket)
class Child(Base):
 
    # Constructor
    def __init__(self, name, age):
        Base.__init__(self, name)
        self.age = age
 
    # To get name
    def getAge(self):
        return self.age
 
# Inherited or Sub class (Note Person in bracket)
 
 
class GrandChild(Child):
 
    # Constructor
    def __init__(self, name, age, address):
        Child.__init__(self, name, age)
        self.address = address
 
    # To get address
    def getAddress(self):
        return self.address
 
 
# Driver code
g = GrandChild("Geek1", 23, "Noida")
print(g.getName(), g.getAge(), g.getAddress())


Output: 

Geek1 23 Noida
  • Hierarchical inheritance More than one derived class are created from a single base.
  • Hybrid inheritance: This form combines more than one form of inheritance. Basically, it is a blend of more than one type of inheritance.

For more details please read this article: Types of inheritance in Python

Private members of the parent class 

We don’t always want the instance variables of the parent class to be inherited by the child class i.e. we can make some of the instance variables of the parent class private, which won’t be available to the child class. 
We can make an instance variable private by adding double underscores before its name. For example,

Python3




# Python program to demonstrate private members
# of the parent class
 
 
class C(object):
    def __init__(self):
        self.c = 21
 
        # d is private instance variable
        self.__d = 42
 
 
class D(C):
    def __init__(self):
        self.e = 84
        C.__init__(self)
 
 
object1 = D()
 
# produces an error as d is private instance variable
print(object1.d)


Output : 

  File "/home/993bb61c3e76cda5bb67bd9ea05956a1.py", line 16, in 
    print (object1.d)                     
AttributeError: type object 'D' has no attribute 'd'

Since ‘d’ is made private by those underscores, it is not available to the child class ‘D’ and hence the error.


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