Classes and Objects _

BCS-DS-427B: PYTHON
Ms. Swati Hans
Assistant Professor, CSE
Manav Rachna International Institute of Research and Studies, Faridabad
1

Classes & Objects
Python Lecture

Classes
❖ A class is a user-defined blueprint or prototype from which objects are created.
❖ Classes provide a means of bundling data and functionality together.
❖ Creating a new class creates a new type of object, allowing new instances of that type to be
made.
❖ Like function definitions begin with the def keyword in Python, class definitions begin with a
class keyword.
Class Definition Syntax:
class ClassName:
# variables
………..........
# functions

Example
class Rectangle:
def area(self): #function1
self.length=0 #variable1
self.breadth=0 #variable2
print("area is",self.length*self.breadth)
def perimeter(self): #function2
print("perimeter is ",2*(self.length+self.breadth))
r1=Rectangle()
r1.area()
r1.perimeter()
In this class Rectangle, we have two
Member variables, length & breadth
and two member functions area &
perimeter

Objects
❖ An object is an instance of
class
❖ When an object of a class is
created, class is said to be
instantiated.
❖ All the instances share the
attributes and the behavior
of the class.
❖ But the values of those
attributes, i.e. the state are
unique for each object.
❖ A single class may have any
number of instances.

Objects
Rectangle class
Variables included are:
length
breadth
Functions included are:
area( )
perimeter( )
Object1
length=31
breadth=43
Object2
length=35
breadth=56
Each object has its own instance variables
Considering rectangle class told earlier with its member variables length & breadth and
its functions area() & perimeter()

Accessing Attributes
Attributes of class can be accessed using Dot operator
rect1=Rectangle( ) #create objects
rect2=Rectangle( )
print(rect1.length, rect1.breadth) # access variables
rect1.area( ) # access functions
rect2.area( )

Self Parameter in Methods
In object-oriented programming, whenever we define methods for a class, we use self as the first
parameter in each case.
class car:
def setdata(self, model, color):
self.model = model
self.color = color
def show(self):
print("Model is", self.model)
print("color is", self.color)
audi = car()
volvo=car()
audi.setdata("audi a4", "blue")
Volvo.setdata("volvo 488", "silver")
audi.show() # same output as car.show(audi)
ferrari.show() # same output as car.show(ferrari)

__init__ method(Constructor )
Constructors are generally used for instantiating an object. The task of constructors is to initialize(assign values) to the data
members of the class when an object of class is created. In python constructors are defined by including init method in class
Syntax of constructor declaration :
Class A:
def __init__(self):
# body of the constructor
Types of constructors :
 default constructor
 parameterized constructor

Default Constructor
The default constructor is simple constructor which doesn’t accept any arguments. Constructor in
which no other argument included except self which is a reference to the instance being
constructed.
class Rectangle:
def __init__(self):
self.l=0 # in this default constructor, by default values are initialized with zero
self.b=0
def area(self): Output
print(self.l*self.b)
rec1=Rectangle( )
rec1.area( ) #area is displayed using default values
rect1.l=45
rect1.b=50
rec1.area( ) #area is displayed as per updated values

Parameterized constructor
Constructor with parameters is known as parameterized constructor. Constructor in
which other arguments are also included along with self which are provided by the
programmer
class Rectangle:
def __init__(self,len,bre):
self.l=len # in this constructor, member variables l,b are initialized with arguments
len,bre
self.b=bre
def area(self): Output
rect1=Rectangle(3,4) #arguments are passes while creating object

Copy Constructor
This constructor copies values of one object to other object.
class Rectangle:
l=0
b=0
def __init__(self,obj1=None): #by default it will assign None to obj1,if object is not passed
if(obj1!=None):
self.l=obj1.l
self.b=obj1.b
def area(self):
rec1=Rectangle() #here No object is passed in constructor
rec1.l=4
rec1.b=5
rec2=Rectangle(rec1) #rec1 object is passed, so copies the value in rec1 to rec2
rec1.area( )
rec2.area( )
Output

Classes and Objects _

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