Constructor and Destructor in Python

Constructor and Destructor in Python

Constructor is the special function that is automatically executed when an object of a class is created. In Python, there is a special function called “init” which act as a Constructor. It must begin and end with double underscore. This function will act as an ordinary function; but only difference is, it is executed automatically when the object is created. This constructor function can be defined with or without arguments. This method is used to initialize the class variables.

General format of __init__ method (Constructor function)

def __init__(self, [args ……..]):

<statements>

Example : 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)

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 itprints the value passed along with the given string.

The above constructor gets executed automatically, when an object S is created with actual parameter 10. Thus, the Python display the following output.

Constructor of class Sample...

The value is : 10

Class variable defined within constructor keep count of number of objects created with the class.

Example : Program to illustrate class variable to keep count of number of objects created.

class Sample:

num=0

def __init__(self, var):

Sample.num+=1

self.var=var

print("The object value is = ", var)

print("The count of object created = ", Sample.num)

S1=Sample(15)

S2=Sample(35)

S3=Sample(45)

In the above 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

Since, the variable shared by all objects, change made to num by one object is reflected in other objects as well. Thus the above program produces the output given below.

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

Note: class variable is similar to static type in C++

Destructor is also a special method gets executed automatically when an object exit from the scope. It is just opposite to constructor. In Python, __del__( ) method is used as destructor.

Example : Program to illustrate about the __del__( ) method

class Sample:

num=0

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)