constructors in python

_ _new_ _ 

• __new__ is for creating objects.

• Use __new__ when you need to control the creation of a new instance.

• __new__ is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class.

• __new__ is static, class method .

• __new__ takes cls as parameter.

• __new__ is good for immutable object as they cannot be changed once they are assigned. So we can return new instance which has new state.

class A:

    

    def __new__(cls):

        print "A.__new__ is called"  # -> this is never called

A( )

_ _init_ _

• __init__ is for initializing objects.

•  Use __init__ when you need to control initialization of a new instance.

• In contrast, __init__ doesn't return anything,it's only responsible for initializing the instance after it's been created.

• __init__ is instance method. 

• __init__ takes self as parameter.

• __init__ called on it automatically.

class A:

    def __init__(self):
        print "A.__init__ called"


A( )



class A:

    def __init__(self):
        return 29


A( )


class A(object):  # -> don't forget the object specified as base

    def __new__(cls):
        print "A.__new__ called"
        return super(A, cls).__new__(cls)

    def __init__(self):
        print "A.__init__ called"


A( )

Example:

class ExampleClass(object):

     def __new__(cls, value):

         print"Creating new instance..."

         #Call the superclass constructor to create the instance.

         instance = super(ExampleClass, cls).__new__(cls)

         return instance

     def __init__(self, value):

        print"Initialising instance..."

        self.x= value

exampleInstance = ExampleClass(42)

print(exampleInstance.x)