Chapter 27, Intermediate

Attributes and methods

Instance attribute, class attribute, instance method, class method, and static method. Each has a role, and confusing them generates subtle bugs.

Code for this chapter: intermediario/cap27_atributos_metodos.py

Instance and class attributes

An attribute of instance belongs to an object. A class attribute is defined in the body of the class and is shared by all instances:

intermediario/cap27_atributos_metodos.pylinhas 10 a 20

class Contador:
    total_criados = 0

    def __init__(self, nome):
        self.nome = nome
        Contador.total_criados += 1


a = Contador("a")
b = Contador("b")
print(Contador.total_criados, a.total_criados)

Output

2 2

When reading a.total_criados, Python first looks in the object, then in the class, then in the parent classes. When assigning a.nivel = ..., it always creates the attribute in the object, without touching the class:

intermediario/cap27_atributos_metodos.pylinhas 22 a 29

class Config:
    nivel = "padrão"


c = Config()
print(c.nivel)
c.nivel = "personalizado"
print(c.nivel, Config.nivel)

Output

padrão
personalizado padrão

The trap of mutable class attributes

It's the same problem of the mutable default value from chapter 18. A list in the class body is the same one for all instances:

intermediario/cap27_atributos_metodos.pylinhas 34 a 43

class Turma:
    alunos = []

    def adicionar(self, nome):
        self.alunos.append(nome)


t1, t2 = Turma(), Turma()
t1.adicionar("Ana")
print(t2.alunos)

Output

['Ana']

The second group "gained" Ana without anyone asking. The fix is to create the list inside the __init__, so each instance has its own:

intermediario/cap27_atributos_metodos.pylinhas 45 a 55

class TurmaCorreta:
    def __init__(self):
        self.alunos = []

    def adicionar(self, nome):
        self.alunos.append(nome)


t1, t2 = TurmaCorreta(), TurmaCorreta()
t1.adicionar("Ana")
print(t2.alunos)

Output

[]

Instance, class, and static methods

Type	Decorator	First parameter	When to use
Instance	none	`self`	Uses or alters the object's data
Class	`@classmethod`	`cls`	Alternative constructors and class logic
Static	`@staticmethod`	none	Utility function that belongs to the class theme

The most valuable use of the @classmethod is the alternative constructor: to create an object from another representation, like text:

intermediario/cap27_atributos_metodos.pylinhas 60 a 79

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

    @classmethod
    def de_texto(cls, texto):
        nome, idade = texto.split(",")
        return cls(nome.strip(), int(idade))

    @staticmethod
    def eh_maior_de_idade(idade):
        return idade >= 18

    def __repr__(self):
        return f"Pessoa({self.nome!r}, {self.idade})"


p = Pessoa.de_texto("Ana, 30")
print(p, Pessoa.eh_maior_de_idade(p.idade))

Output

Pessoa('Ana', 30) True

Notice in the cls(...) instead of Pessoa(...): thus, a subclass that uses de_texto receives an object of the subclass, not of the base class.

Exercise 1

Product from a dictionary

Create Produto(nome, preco) with the alternative constructor de_dict(dados), which receives {"nome": ..., "preco": ...}.

Instance and class attributes

The trap of mutable class attributes

Instance, class, and static methods

View solution