(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

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

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.

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".

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 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

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