Need to declare an attribute in __init__

Question

I was doing a Dice in python3, so far so good. but I don’t understand why I need to put the attributes in init, being that having them or not will work in the "same way" and I can’t see a "sense".

I learned Object Orientation with Java and there really things make more sense or I’m not understanding. Python’s Object Orientation is a bit confusing for me.

I’m sorry for the way I’m expressing/writing, maybe it sounds gross.

class Dado(object):
    def __init__(self, cor="vermelho", lado=6):
      self._cor = cor

    def setCor(self, cor):
      self._cor = cor


     def getCor(self):
      return self._cor

or

class Dado(object):
    def __init__(self):
      pass

    def setCor(self, cor):
      self._cor = cor

    def getCor(self):
      return self._cor

Why do I need to declare an attribute in init? or why this is necessary?

Like, it works both ways.

Answer 1

There is no written rule that you should declare the rules in your init() It’s just a matter of convention. When you declare the variables in init it is the equivalent of declaring their variables in the constructor. About your question:

Por que eu preciso declarar um atributo no init? ou por que isso é necessário?

It is not necessary to declare an attribute in init. It is not necessary. Everything works normal. This only serves as a shortcut to create an instance of your class in a more declarative way, or even to set default values at class startup.

The equivalent of this statement you made in java python would be:

//classe
class Dado {
     private String cor;
     private int lado;
     //construtor padrão do java não precisa ser declarado sempre vai existir 
     //public Dado(){
     //}
     //construtor 2
     public Dado(String cor, int lado) {
         this.cor = cor == null ? "vermelho" : cor;
         this.lado = lado == 0 ? 6 : lado
     }
     //getters e setters
}

But I would like to point out that it is not necessary to have the second constructor, because we will always have the default constructor of java, without parameters. Declaring a variable in the init method is equivalent to declaring a variable in the java constructor. Only this but the constructor is not required, everything works normal without declaring variables in the constructor.