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 - - ( ))

(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>

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

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.

Output:
The object value is = 15
The count of object created = 1
The object value is = 35
The count of object created = 2
The object value is = 45
The count of object created = 3

Constructor :
(i) Constructor is the special function that is created. In Python, there is a special function called "init", which act as a constructor.
(ii) It must begin and end with double underscore.
General format of Constructor :
General format of _init_ method (Constructor function)
def _init_(self, [args .... ]):
<statement>
Destructor :
(i) Destructor is also a special method to destory the objects.
(ii) In Python,_del_() method is used as destructor.It is just opposite to constructor.

Code:
class Sample:
def_init_(self, n1, n2):
            self._n1=nt
            self_n2-n2
def sum(self):
           print ("Class Variable 1:" self_n1)
           print ("Class Variable 2:", self_n2)
S=Sample (5,10)
S.sum()
Output:
Class Variable 1:5
Class Variable 2: 10
Sum: 15

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 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