(i) The variables which are defined inside the class is public by default. These variables can be accessed anywhere in the program using dot operator.
(ii) A variable prefixed with double underscore becomes private in nature. These variables can be accessed only within the class

In the program, class variable num is shared by all three objects of the class Sample. It is initialized to zero and each time an object is created, the num is incremented by 1. Since, the variable shared by all objects, change made to num by one object is reflected in other objects as well

Program to illustrate Constructor:
class Sample:
def _init_(self, num):
print("Constructor of class Sample...")
self.num=num
print("The value is :", num)
S = Sample(10)
Output:
constructor of class Sample..
The value is:10

Constructor -( - - init - - ( ))
Destructor -(- - del - - ( ))

Python class function or method is very similar to ordinary function with a small difference. The class method must have the first argument  named as self.

(i) Object - name = class_name ()
(ll) Object - name = class_member

(i) "Init" is a special function begin an end with double underscore in Python act as a Constructor .
(ii) Constructor function will automatically executed when an object is created.
General format of __init__ method
                                    (Constructor function)
def __init__ (self, [args........])
<statements>

Output:
>>>
10
line 7, in <module>
print(S._num)
AttributeError: 'Sample' object has no attribute '_num'
>>>

Once a class is created, next to create an object or instance of that class. This process of creating object is called as "Class Instantiation".

(i) Class is the main building block in Python.
(ii) Class is a template for the object.
(iii) Object is a collection of data and function that act on those data.
(iv) Objects are also called as instances of a class.

1. It denotes class name
2. x, y is a class variables of the class
3. S is an object created to access the members of the class

Program to illustrate about the _del_( ) method
class Sample:
num=()
def _init_(self, var):
Sample.num+= 1
self.var=var
print("The object value is = ", var)
print("The value of class variable is = ", Sample.num)
def _del_(self):
Sample.num-=1
print("Object with value %d is exit from the scope"%self.var)
S1=Sample(15)
S2=Sample(35)
S3=Sample(45)

class Sample:
def _init_(self, n1, n2):
self.n1=n1
self._n2=n2
def display(self):
print("Class variable 1 = ", self.n1)
print("Class variable 2 = ", self._n2)
S=Sample(12,14)
S.display()
print("Value 1 = ", S.n1)
print("Value 2 = ". S._n2)
Reason: The print statements defined within class will successfully display the values of n1 and n2, even though the class variable n2 is private because, in this case, n2 is called by a method defined inside the class. But, when we try to access the value of n2 from outside the class Python throws an error because, private variable cannot be accessed from outside the class.

(i) The above class "Sample': has only a constructor with one argument named as num. When the constructor gets executed, first the print statement, prints the "Constructor of class Sample ... " then, the passing value to the constructor is assigned to self.num and finally it prints the value passed along with the given string.
(ii) The above constructor gets executed automatically, when an object S is created with actual parameter 10. Thus, the Python displays the following output.
(iii) Constructor of class Sample ...
The value is: 10
Class variable defined within constructor keep count of number of objects created with the class.

class Student:
mark1, mark2, mark3 = 45, 91, 71
#class variable
​​​​​#class method
def process(self):
sum = Student.mark1 + Student.mark2 + Student.mark3
avg = sum/3
print("Total Marks = ", sum)
print("Average Marks = ", avg)
return
S=Student()
S.process()
Output:
Total marks = 207
Average Marks = 69.0

(i) Python class function or Method is very similar to ordinary function with a small difference that, the class method must have the first argument named as self.
(ii) No need to pass a value for this argument when we call the method. Python provides its value automatically.
(iii) Even if a method takes no arguments, it should be defined with the first argument called self.
(iv) If a method is defined to accept only one argument it will take it as two arguments ie. self and the defined argument.

(i) Object is a collection of data and function that act on those data. Class is a template for the object
(ii) According to the concept of Object Oriented Programming, objects are also called as instances of a class or class variable.
(iii) In Python, everything is an object. For example, all integer variables that we use in our program is an object of class into Similarly all string variables are also object of class string.

Good Morning Bindu Madhavan.

In line No.8, del F.display will not come.

Variables defined inside a class are called as "Class Variable" and functions are called as "Methods. Class variable and methods are together known as members of the class. The class members should be accessed through objects or instance of class. A class can be defined anywhere in a Python program.
Defining classes: In Python, a class is defined by using the keyword class. Every class has a unique name followed by a colon (:).
Syntax for Defining a Class:
class class_name:
statement_1
statement_2
....................
....................
statement_n

class MyStore:
_prod_code= []
_prod_name= []
_ cost_price= []
_prod_quant= []
def getdata(self):
self.p = int(input("Enter no. of products you need to store: "))
for x in range(self.p):
self._prod_code.
append(int(input("Enter Product Code: ")))
self._prod_name.append(str(input("Enter Product Name: ")))
self._ cost_price.append(int(input("Enter Cost price: ")))
def display(self):
print("Stock in Stores")
print(" ---------------------------------- ")
print("Product Code \t Product Name \t Cost Price")
print(" ------------------------------------- ")
for x in range(self.p):
print(self._prod_code[x], "\t\t", self._prod_name[x], "\t\t", self._cos_ price [x])
print(" ---------------------------------")
def print_bill(self):
total_price = 0
for x in range( self.p):
q=int(input("Enter the quantify for the product code %d : "%self._ prod_code [x]))
self._prod_quant.append(q)
total_price = total_price +self._cost_
price[x]*self._prod_quant[x]
print(" Invoice Receipt ")
print(" --------------------------------------")
print("Product Code\t Product Name\t
Cost Price\t Quantity \t Total Amount")
print(" ------------------------------------ ")
for x in range(self.p):
print(self._prod_code[x], "\t\t", self._prod_name[x], "\t\t",
self._cost_price[x], "\t\t", self._prod_quant[x], "\t\t",
self._pro d_ quan t [x] *self._cost_price [x])
print(" -----------------------------------")
print(" Total Amount = ", total_price)

Class String:
def _init_(self):
self.uppercase=()
self.lowercase=()
self.vowels=()
self.consonants=()
self.spaces=()
self.string= " "
def getstr (self):
self.string=str(input("Enter a String: "))
def count_upper(self):
for ch in self.string:
if (ch.isupper()):
self.uppercase+= 1
def count_Iowert(self):
for ch in self.string:
if (ch.islower()):
self.lowercase+= 1
def count_vowels(self):
for ch in self.string:
if (ch in ('A', 'a', 'e', 'E', 'i', 'I', 'o', 'O', 'u', 'U')):
self.vowels+=1
def count_consonants(self):
for ch in self.string:
if (ch not in ('A', 'a', 'e', 'E', 'i', 'I', 'o', 'O', 'u', 'U')):
self.consonants+= 1
def count_space(self):
for ch in self.string:
if (ch==" ''):
self.spaces+= 1
def execute(self):
self.count_upper()
self.count_lower ()
self.count_vowels ()
self.count_consonants()
self.count_space()
def display(self):
print("The given string contains ...")
print("%d Uppercase letters"%self. uppercase)
print("%d Lowercase letters"%self. lowercase)
print("%d Vowels"%self.vowels)
print("%d Consonants"%self.consonants)
print("%d Spaces"%self.spaces)
S = String()
S.getstr()
S.execute()
S.display()
Output:
Enter a string: Welcome To Learn Computer Science
The given string contains...
5 Uppercase letters
24 Lowercase letters
13 vowels
20 consonants
4 Spaces

class Library:
def _init_(self):
self.bookname= ""
self.author='"
def getdata(self):
self.bookname = input("Enter Name of the Book: ")
self.author = input("Enter Author of the Book: ")
def display(self):
print("Name of the Book: ",self.bookname)
print("Author of the Book: ",self.author)
print("\n")
book= [] #empty list
ch = 'y'
while( ch=='y'):
print("1. Add New Book \n 2.Display Books")
resp = int(input("Enter your choice: "))
if(resp==1):
L=Library()
L.getdata()
book.append(L)
elif(resp==2):
for x in book:
x.display()
else:
print("Invalid input....")
ch = input("Do you want continue ....")
Output
1. Add New Book
2. Display Books
Enter your choice: 1
Enter Name of the Book: Programming in C++
Enter Author of the Book: K. Kannan
Do you want to continue...y
1. Add New Book
2. Display Books
Enter your choice: 1
Enter Name of the Book: Advanced Python
Enter Author of the Book: Dr. vidhya
Do you want continue.... n

class Circle:
pi=3.14
def _init_(self.radius):
self.radius=radius
def area(self):
return Circle. pi*(self.radius**2)
def circumference(self):
return 2*Circle.pi*self.radius
r=int(input("Enter Radius: "))
Ce=Circle(r)
print("The Area =",C.area())
print("The Circumference =", Cxircumferencet())

class test;
def check (self, num)
if num> 0:
print (num, "is positive number")
else:
print (num, "is negative number")
n = test ()
x = int (input("Enter the number"))
n. check (x)

class Odd_Even:
even = 0 #class varaiable
def check(self, num):
if num%2==0:
print(num," is Even number")
else:
print(num," is Odd number")
n=Odd Even()
x = int(input("Enter a value:"))
n.check(x)
when we execute this program, Python accepts the value entered by the user and passes it to the method check through object.
Output:1 
Enter a value: 4
4 is Even number
Output: 2
Enter a value: 5 
5 is Odd number

class Mystore:
_prod_code = [ ]
_prod_name = [ ]
_cost price = [ ]
_prod quant = [ ]
def getdata(self):
selt.p= int (input ("Enter no. of products you need to store:"))
for x in range (self.p):
self._prod_code. append (int (input ("Enter Product Code:")))
self._prod_name. append (str (input ("Enter Product Name:"))
self._ cost_price. append (int (input ("Enter cost price:"))
def display (self):
print ("stock in stores")
print ("---------------------")
print ("Product Code\t Product name\t cost price")
print ("------------------")
for x in range (self.p):
print (self._prod_code [x], "\t\t", self._prod_name [x], "It \t", self._cost price [x])
print ("------------------------------")
def print_bill (self):
total_price =0
for x in range (self.p):
q= int(input("Enter the quantity for the product code %d:" %self._prod_code [x]))
self. Prod_quant.append (q)
total_price = total_price + self. _cost_price|x]* self._prod_quant [x]
print ("Invoice Receipt")
print ("---------------")
print ("Product Code\t Product Name\t Cost Price\t Quantity\t Total Amount")
print ("----------------")
for x in range (self.p):
print (self._Prod_code [x), "\t \t", self_prod-name[x], "\t \t",
self._cost_price[x], "\t \t", self._prod-quant [x], "\t \t",
self._prod_quant[x]*self._cost price [x])
print ("-------------------")
print ("Total Amount =", total_price")
S=Mystore ()
S.getdata ()
S.display ()
S.print_bill ()
Output:
Enter no. of products you need to store: 5
Enter Product code: 101
Enter Product Name: Notebook
Enter cost price: 25
Enter Product code: 201
Enter Product Name: pencil
Enter cost price: 35
Enter Product code: 301
Enter Product Name: Pen
Enter cost price: 35
Enter Product code: 401
Enter Product Name: Colour Pencils
Enter cost price: 50
Enter Product code: 501
Enter Product Name: Sketches
Enter cost price: 120
Stock in stores

Enter the quantity for the product code 101:10
Enter the quantity for the product code 201:15
Enter the quantity for the product code 301:10
Enter the quantity for the product code 401:20
Enter the quantity for the product code 501:10
Invoice Receipt