What is the __new__ in python method used for?

According to the definition below:

  

Use __ new __ when you need to control the creation of a new   instance of the class. Use __ init __ when you need to control   the initialization of a new instance.

     

__ new __ is the first step in creating an instance. He is   called first, and is responsible for returning a new instance of the   your class. In contrast, __ init __ does not return anything, it is only   responsible for initializing the instance after the class is   created.

     

In general, you should not override __ new __ unless it is   a subclass, an immutable type such as str , int , unicode or    tuple .

That is, you can use __new__ to have a control of the creation of the class instance and after it uses __init__ to pass arguments, see an example:

class Fabrica(object):
    """ pra quê serve o metodo __new__ in python? """

    def __new__(cls[, ...]):
        # seu código aqui
        # definir uma rotina no momento da criação da instancia.
    def __init__(self, nome):
        # aqui ocorre a inicialização da instancia, 
        # pode iniciar os atributos da classe aqui.
        self.nome = nome

class inch(float):
    "Converte de polegadas para metros"
    def __new__(cls, arg=0.0):
        return (float.__new__(cls, arg*0.0254))

print(inch(12))

Output: 0.3048

But, this usage I found very interesting in the article, can really come in handy if you use the standard singleton , follow the example:

class Singleton(object):
    def __new__(cls, *args, **kwds):
        it = cls.__dict__.get("__it__")
        if it is not None:
            return it
        cls.__it__ = it = object.__new__(cls)
        it.init(*args, **kwds)
        return it
    def init(self, *args, **kwds):
        pass

Sources:
Documentation.
Python's use of __new__ and __init__ ? UnifyingTypesClasses , I highly advise you to read if you want to know more about it.