OOP in Python | Set 3 (Inheritance, examples of object, issubclass and super)

We have discussed following topics on Object Oriented Programming in Python

• Object Oriented Programming in Python | set-1
• Object Oriented Programming in Python | Set 2 (Data Hiding and Object Printing)

In this article, Inheritance is introduced.

One of the major advantages of Object Oriented Programming is re-use. Inheritance is one of the mechanisms to achieve the same. In inheritance, a class (usually called superclass) is inherited by another class (usually called subclass). The subclass adds some attributes to superclass.

Below is a sample Python program to show how inheritance is implemented in Python.

# 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 employee
    def isEmployee(self):
        return False


# Inherited or Sub class (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)

How to check if a class is subclass of another?
Python provides a function issubclass() that directly tells us if a class is subclass of another class.

# Python example to check if a class is
# subclass of another

class Base(object):
    pass   # Empty Class

class Derived(Base):
    pass   # Empty Class

# Driver Code
print(issubclass(Derived, Base))
print(issubclass(Base, Derived))

d = Derived()
b = Base()

# b is not an instance of Derived
print(isinstance(b, Derived))

# But d is an instance of Base
print(isinstance(d, Base))

Output:

True
False
False
True

What is object class?
Like Java Object class, in Python (from version 3.x), object is 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 object as parent (called new style class) and “class Test” creates old style class (without object parent). Refer this for more details.

Does Python support Multiple Inheritance?
Unlike Java and like C++, Python supports multiple inheritance. We specify all parent classes as comma separated list in bracket.

# Python example to show 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')

How to access parent members in a subclass?

1. Using Parent class name

   # Python example to show that base
   # class members can be accessed in
   # derived class using base class name
   class Base(object):
   
       # Constructor
       def __init__(self, x):
           self.x = x
   
   class Derived(Base):
   
       # Constructor
       def __init__(self, x, y):
           Base.x = x
           self.y = y
   
       def printXY(self):
   
          # print(self.x, self.y) will also work
          print(Base.x, self.y)
   
   
   # Driver Code
   d = Derived(10, 20)
   d.printXY()
   Output:

   (10, 20)
   

2. Using super()
   We can also access parent class members using super.

   # Python example to show that base
   # class members can be accessed in
   # derived class using super()
   class Base(object):
   
       # Constructor
       def __init__(self, x):
           self.x = x
   
   class Derived(Base):
   
       # Constructor
       def __init__(self, x, y):
   
           ''' In Python 3.x, "super().__init__(name)"
               also works'''
           super(Derived, self).__init__(x)
           self.y = y
   
       def printXY(self):
   
          # Note that Base.x won't work here
          # because super() is used in constructor
          print(self.x, self.y)
   
   
   # Driver Code
   d = Derived(10, 20)
   d.printXY()
   Output:

   (10, 20)
   

Note that the above two methods are not exactly the same. In the next article on inheritance, we will covering following topics.
1) How super works? How accessing a member through super and parent class name are different?
2) How Diamond problem is handled in Python?

Exercise:
Predict the output of following Python programs

1. 
   class X(object):
       def __init__(self, a):
           self.num = a
       def doubleup(self):
           self.num *= 2
   
   class Y(X):
       def __init__(self, a):
           X.__init__(self, a)
       def tripleup(self):
           self.num *= 3
   
   obj = Y(4)
   print(obj.num)
   
   obj.doubleup()
   print(obj.num)
   
   obj.tripleup()
   print(obj.num)
   Output:

   4
   8
   24
   

2. # Base or Super class
   class Person(object):
       def __init__(self, name):
           self.name = name
   
       def getName(self):
           return self.name
   
       def isEmployee(self):
           return False
   
   # Inherited or Subclass (Note Person in bracket)
   class Employee(Person):
       def __init__(self, name, eid):
   
           ''' In Python 3.0+, "super().__init__(name)"
               also works'''
           super(Employee, self).__init__(name)
           self.empID = eid
   
       def isEmployee(self):
           return True
   
       def getID(self):
           return self.empID
   
   # Driver code
   emp = Employee("Geek1", "E101")
   print(emp.getName(), emp.isEmployee(), emp.getID())
   Output:

   ('Geek1', True, 'E101')