Continuing with the series of back to basics OOP articles, this article covers the differences between __init__ and __new__.

This article is based on an article in English, understanding new and init (broken link) .

Many people who have done object-oriented programming in Python have never used the __new__ method; the one they use as a constructor is __init__.

The reality is that the __init__ method creates the object and then initializes it; it is not the constructor as such, whereas the __new__ method only constructs the object.

This article will work with Python 3.

We have the following class:

class A(object):

    pass

So Class A inherits from object in Python 3.

Now a class that uses __new__ and __init__ is created:

class A(object):


    def __new__(cls):


        print "A.__new__ es llamado"


        return super(A, cls).__new__(cls)





    def __init__(self):


        print "A.__init__ es llamado"





A()

The output is the following:

A.__new__ es llamado
A.__init__ es llamado

Now let's look at an example where __init__ will not be called:

class A(object):
    def __new__(cls):
        print "A.__new__ es llamado"

    def __init__(self):
        print "A.__init__ es llamado"

print ("Solo llamando el objeto A() o instanciandolo")

A()
a = A()
print ("*"*40)
print ("Ejecutando un print con el objeto A()")
print (A())

As you can see, a print is executed in the __init__ method and also in the __new__ method.

The output is the following:

Solo llamando el objeto A() o instanciandolo
A.__new__ es llamado
A.__new__ es llamado
****************************************
Ejecutando un print con el objeto A()
A.__new__ es llamado
None

As you can see, when calling A() or instantiating it, only the output of the __new__ method is shown, and not that of __init__; only when print(A()) is executed does it return the output of the __new__ method, and the __init__ method returns None.

Now the class with only the __new__ method will be shown, where a print is executed and a number is returned:

class A(object):

    def __new__(cls):


        print "A.__new__ es llamado"


        return 29


print ("Solo llamando al objeto")


A()


print ("*"*30)


print ("Llamando al objeto desde un print")


print A()

The output is the following:

Solo llamando al objeto A.new es llamado

---

Llamando al objeto desde un print A.new es llamado 29

When only calling the object it just shows the print, but when running the print it shows the method's print and the integer it returns.

Now the same will be done but with the init method:

class A(object):

def __init__(self):

    print "A.__init__ es llamado"

    return 33

try:

print ("Solo llamando al objeto")

A()

except (TypeError):

print ("Error de tipo")

The output is the following:

Solo llamando al objeto
A.__init__ es llamado
Error de tipo

In this case the return gives error de tipo. The only way for the __init__ method to return something is for it to be of type None.

Now the use of an alternative class called Ejemplo is shown:

class Ejemplo(object):


    def __str__(self):


        return "Ejemplo"


class A(object):


    def __new__(cls):


        return Ejemplo()


class B(object):


    def __new__(cls):


        return super(B, cls).__new__(Ejemplo)


print ("Prueba con A")


print A()


print ("Prueba con B")


print B()

The output returns the following:

Prueba con A
Ejemplo
Prueba con B
Ejemplo

As can be seen, there are cases where using __new__ is useful and others where __init__ can still be used.

The __new__ method is used in design patterns with python, which will be the next series of articles in this blog.

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