Python's Object Oriented Programming (OOPs) | Blog 1

About class, object, reference variables, self variable, constructors and Instance Varaiable

Python Sep 20, 2022 0 285 Add to Reading List

Python's Object Oriented Programming (OOPs) | Blog 1

What is Class:

In Python everything's an object. To create objects we required some Model or Plan or Blueprint, which is nothing but class.
We can write a class to represent properties (attributes) and actions (behavior) of object.
Properties can be represented by variables
Actions can be represented by Methods.
Hence class contains both variables and methods.

How to Define a class?

We can define a class by using the class keyword.

Syntax:

class className:

''' documentation string '''

variables:instance variables,static and local variables

methods: instance methods,static methods,class methods

Documentation string represents the description of the class. Within the class doc string is always optional. We can get doc strings by using the following 2 ways.

print(classname.__doc__)
help(classname)

Example:

class Student:
''''' This is student class with required data'''
print(Student.__doc__)
help(Student)

Within the Python class we can represent data by using variables.

There are 3 types of variables allowed.

Instance Variables (Object Level Variables)
Static Variables (Class Level Variables)
Local variables (Method Level Variables)

Within the Python class, we can represent operations by using methods. The following are various types of allowed methods

Instance Methods
Class Methods
Static Methods

Example for class:

class Student:
'''''Developed by durga for python demo'''
Def __init__(self):
self.name='durga'
self.age=40
self.marks=80

def talk(self):
print("Hello I am :",self.name)
print("My Age is:",self.age)
print("My Marks are:",self.marks)

What is Object:

Physical existence of a class is nothing but an object. We can create any number of objects for a class. Syntax to create object: referencevariable = classname()

Example: s = Student()

What is Reference Variable:

The variable which can be used to refer to an object is called a reference variable. By using reference variables, we can access properties and methods of objects.

Program: Write a Python program to create a Student class and create an object to it. Call the method talk() to display student details

class Student:
def __init__(self,name,rollno,marks):
self.name=name
self.rollno=rollno
self.marks=marks
def talk(self):
print("Hello My Name is:",self.name)
print("My Rollno is:",self.rollno)
print("My Marks are:",self.marks)
s1=Student("Durga",101,80)
s1.talk()

Output:

D:\thecobox>py test.py

Hello My Name is: Durga

My Rollno is: 101

My Marks are: 80

Self variable:

self is the default variable which is always pointing to the current object (like this keyword in Java) . By using self we can access instance variables and instance methods of objects.

Note:

self should be first parameter inside constructor

def __init__(self):

self should be first parameter inside instance methods

def talk(self):

Constructor Concept:

Constructor is a special method in python.
The name of the constructor should be __init__(self)
Constructor will be executed automatically at the time of object creation.
The main purpose of the constructor is to declare and initialize instance variables.
Per object constructor will be executed only once.
Constructor can take at least one argument(at least self)
Constructor is optional and if we are not providing any constructor then python will provide default constructor.

Example:

def __init__(self,name,rollno,marks):

self.name=name

self.rollno=rollno

self.marks=marks

Program to demonstrate constructor will execute only once per object:

class Test:
def __init__(self):
print("Constructor execution...")
def m1(self):
print("Method execution...")
t1=Test()
t2=Test()
t3=Test()
t1.m1()

Output

Constructor execution...

Method execution...

Program:

class Student:
''''' This is student class with required data'''
def __init__(self,x,y,z):
self.name=x
self.rollno=y
self.marks=z

def display(self):
print("Student Name:{}\nRollno:{} \nMarks:{}".format(self.name,self.rollno,self.marks) )

s1=Student("Durga",101,80)
s1.display()
s2=Student("Sunny",102,100)
s2.display()

Output

Student Name:Durga

Rollno:101

Marks:80

Student Name:Sunny

Rollno:102

Marks:100

Differences between Methods and Constructors:

Method	Constructor
1. Name of the method can be any name	1. Constructor name should be always __init__
2. Method will be executed if we call that method	2. Constructor will be executed automatically at the time of object creation.
3. Per object, method can be called any number of times.	3. Per object, Constructor will be executed only once
4. Inside the method we can write business logic	4. Inside Constructor we have to declare and initialize instance variables

Types of Variables:

Inside Python class 3 types of variables are allowed.

Instance Variables (Object Level Variables)
Static Variables (Class Level Variables)
Local variables (Method Level Variables)
Instance Variables:

If the value of a variable is varied from object to object, then such type of variables are called instance variables.

For every object a separate copy of instance variables will be created.

Where we can declare Instance variables:

Inside Constructor by using self variable
Inside Instance Method by using self variable
Outside of the class by using object reference variable
Inside Constructor by using self variable:

We can declare instance variables inside a constructor by using self keyword. Once we create an object, automatically these variables will be added to the object.

Example:

class Employee:
def __init__(self):
self.eno=100
self.ename='Durga'
self.esal=10000
e=Employee()
print(e.__dict__)

Output: {'eno': 100, 'ename': 'Durga', 'esal': 10000}

Inside Instance Method by using self variable:

We can also declare instance variables inside the instance method by using self variables. If any instance variable is declared inside the instance method, that instance variable will be added once we call that method.

Example:

class Test:
def __init__(self):
self.a=10
self.b=20
def m1(self):
self.c=30
t=Test()
t.m1()
print(t.__dict__)

Output

{'a': 10, 'b': 20, 'c': 30}

Outside of the class by using object reference variables:

We can also add instance variables outside of a class to a particular object.

class Test:
def __init__(self):
self.a=10
self.b=20
def m1(self):
self.c=30
t=Test()
t.m1()
t.d=40
print(t.__dict__)

Output {'a': 10, 'b': 20, 'c': 30, 'd': 40}

How to access Instance variables:

We can access instance variables within the class by using self variables and outside of the class by using object reference.

class Test:
def __init__(self):
self.a=10
self.b=20

def display(self):
print(self.a)
print(self.b)
t=Test()
t.display()
print(t.a,t.b)

Output

10 20

How to delete instance variables from the object: 1. Within a class we can delete instance variable as follows

del self.variableName

From outside of class we can delete instance variables as follows

del objectreference.variableName

Example:

class Test:
def __init__(self):
self.a=10
self.b=20
self.c=30
self.d=40
def m1(self):
del self.d
t=Test()
print(t.__dict__)
t.m1()
print(t.__dict__)
del t.c
print(t.__dict__)

Output

{'a': 10, 'b': 20, 'c': 30, 'd': 40}

{'a': 10, 'b': 20, 'c': 30}

{'a': 10, 'b': 20}

Note: The instance variables which are deleted from one object,will not be deleted from other objects.

Example:

class Test:
def __init__(self):
self.a=10
self.b=20
self.c=30
self.d=40
t1=Test()
t2=Test()
del t1.a
print(t1.__dict__)
print(t2.__dict__)

Output

{'b': 20, 'c': 30, 'd': 40}

{'a': 10, 'b': 20, 'c': 30, 'd': 40}

If we change the values of instance variables of one object then those changes won't be reflected in the remaining objects, because for every object we separate copies of instance variables are available.

Example:

class Test:
def __init__(self):
self.a=10
self.b=20
t1=Test()
t1.a=888
t1.b=999
t2=Test()
print('t1:',t1.a,t1.b)
print('t2:',t2.a,t2.b)