Classes and objects

The class is the mold, the object is the thing

A class describes what objects of a type are like and what they do. Each object created from it is an instance, with its own data:

class Cachorro:
    def __init__(self, nome, idade):
        self.nome = nome
        self.idade = idade

    def latir(self):
        return f"{self.nome} diz: au au"


rex = Cachorro("Rex", 3)
mel = Cachorro("Mel", 5)
print(rex.nome, mel.nome)
print(rex.latir())

Rex Mel
Rex diz: au au

The __init__ is the initializer: it runs right after the object is created and prepares the initial data. (The one who actually creates the object is the __new__, which you rarely write.) The word self is the object itself, and it is through it that each method knows whose data it is.

What self really is

A call like rex.latir() is just sugar for Cachorro.latir(rex). Python passes the object as the first argument. Seeing this once unravels the mystery of self:

print(Cachorro.latir(mel))
print(type(rex).__name__, isinstance(rex, Cachorro))

Mel diz: au au
Cachorro True

A representation that helps with debugging

Printing an object without __repr__ shows something like <Ponto object at 0x...>, which helps no one. Define the __repr__ in almost every class: it should show the data and preferably look like the code that recreates the object:

class Ponto:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f"Ponto(x={self.x}, y={self.y})"


p = Ponto(1, 2)
print(p)
print(vars(p))

Ponto(x=1, y=2)
{'x': 1, 'y': 2}

The vars() shows the object's attributes like a dictionary. This is how Python stores the data of each instance.

When to create a class

I create a class when I have state and behavior that go together, like a bank account, which has a balance and knows how to deposit and withdraw. If the class only has methods and no data, it probably should be a function (or a module). A Calculadora with somar(a, b) and subtrair(a, b) without any attributes is the way to write Java inside Python.