Python Constructor and Destructor

 

 

    Constructor And Destructor In Python classes

                              Constructors in python

 

Syntax

def   __init__(self,*arg):

   <data members initialization n>

# A sample class with init method

 

class Person:

    #init method or constructor

    def __init__(self, name):

        self.name=name

    # Sample method

    def welcome(self):

      print("Hi sir, I am", self.name)

per=Person('Ramesh Kumar')

per.welcome()

Output

Hi, sir I am Ramesh Kumar

 

Constructor Overloading

For example:

class jknum:

   def __init__(self,x=1,y=0):

         self.cx=x

         self.cy=y

   def callme(self):

                 print("x value=%d, y value=%d"%(self.cx,self.cy))

b1=jknum(10)

b2=jknum(20,30)

b1.callme()

b2.callme()

Output

x value=10, Y value=0

y value=20, y value=30

 

Two kinds of attributes

1.Data Attributes

2.Class Attributes

1.Data Attributes

class Store:

    '''A class store with a dataattributes self.Book_name.'''

    def  __init__(self,n=0):

                  self.book_name=n

    def print_name(self):

                  print("bookname",self.book_name)

2.Class Attributes

Example:

 

class classattrib:

   num=10

   def count_fun(self):

           self.__class__.num+=1

N1=classattrib()

N2=classattrib()

print("\n num value...",N1.num)

print("\n num value...",N2.num)

print("\n num value...",N1.__class__.num)

print("\n num value...",N2.__class__.num)

print("\n num value...", classattrib.num)

N1.count_fun()

N2.count_fun()

print("\n num value...",N1.num)

print("\n num value..",N2.num)

print("\n num value...",N1.__class__.num)

print("\n num value...",N2.__class__.num)

print("\n num value...",classattrib.num)

Output

num value... 10

num value... 10

num value... 10

num value... 10

num value... 10

num value... 12

num value... 12

num value... 12

num value... 12

num value... 12

PRIVATE METHODS AND PRIVATE VARIABLES

In python, there is no existence of "private" instance variables which cannot be accessed expect inside an object. However, a convention is being followed by most python code and coders i.e.., a name prefixed with an underscore, for e.g. _Pmember should be treated as a non-public part of the API or any python code, whether it is a function ,a method or a data member. It cannot be execute through class object directly. It can be execute only through member function.

 

EXAMPLE :

class number: 

""" class consist _a and _b are two private variables and _callme() a private function. It cannot ne execute through class object directly. It can be execute only through public function c is public variables it can be access via class object directly"""

    _a=0

_b=0

c=0

def _callme(self,x,y):

                 self._a=x

                 self._a=y

def cal(self):

               s=self._a+self._b+self.c

                return(s)

def maincall(self,x,y):

                 self._callme(x,y)

obj=number()

obj.c=40

obj.maincall(10,20)

print("sum value = %d " %obj.cal())

 

OUTPUT:

Sum value = 60

 

DATA HIDING AND OBJECT PRINTING

In python, we use double underscore (_) before the attributes name and those attributes will not be directly visible outside.

FOR EXAMPLE:

class MyClass:

        #Hidden member of My Class

         _Hidvar = 0

        #A member method that changes

        #_Hidvar

         def add(self,N):

               self._Hidvar =self._Hidvar + N

               print(self._Hidvar)

#Driver code

myObject = MyClass()

myObject.add(2)

myObject.add(5)

print(myObject._Hidvar)

OUTPUT

2

7

7

PRINTING OBJECTS

EXAMPLE #1

class Test:

        def __init__(self,a,b):

                    self.a=a

                    self.b=b

        def __repr__(self):

               return("test a:%s b:%s" %(self.a,self.b))

        def  __str__(self):

               return("from str method of test:a is %s, b is %s" %(self.a,self.b))

#driver code

t=Test(1234,5678)

print(t)                      #This calls_str_()

print([t])                    # This calls_repr_()

OUTPUT:

from str method of test:a is 1234, b is 5678

[test a:1234 b:5678]

DESTRUCTOR

FOR EXAMPLE

class sample:

         def __init__(self,x,y):

                     self.x=x

                     self.y=y

         def __del__(self):

                print("An object removed from memory")

         def disp_data(self):

                print("x value-{},y value-{}".format(self.x,self.y))

 

s1=sample(2,4)

s2=sample(4,5)

s3=sample(10,12)

s1.disp_data()

s2.disp_data()

s3.disp_data()

#del command remove one reference and invoke destructor method _ del _ () automatically

del s1

del s2

del s3

#program end and the output for the following

 

OUTPUT

x value-2,y value-4

x value-4,y value-5

x value-10,y value-12

An object removed from memory

An object removed from memory

An object removed from memory