Encapsulation and @property

Conventions, not locks

In Python, access control is a convention among adults:

• titular: public, anyone can use it.
• _saldo: with a single underscore, means "internal use, do not touch unless you are from the class".
• __pin: with double underscores, Python does the name mangling: renames the attribute to _Classe__pin, to avoid name collisions in subclasses.

class Conta:
    def __init__(self):
        self.titular = "Ana"
        self._saldo = 100
        self.__pin = 1234


conta = Conta()
print(conta.titular, conta._saldo)
try:
    conta.__pin
except AttributeError as erro:
    print(erro)
print(conta._Conta__pin)

Ana 100
'Conta' object has no attribute '__pin'
1234

The __pin is not protected: it just has another name. The goal of the double underscore is to avoid collisions in inheritance, not to prevent access. Whoever wants the value, finds it.

property: validation without changing the interface

In Java, you write getSaldo() and setSaldo() from day one. In Python, you start with a common attribute. If you later need to validate, you replace it with a @property without changing anything for those who use the class, because access remains conta.saldo:

class ContaBancaria:
    def __init__(self, saldo=0):
        self._saldo = saldo

    @property
    def saldo(self):
        return self._saldo

    @saldo.setter
    def saldo(self, valor):
        if valor < 0:
            raise ValueError("saldo não pode ser negativo")
        self._saldo = valor


c = ContaBancaria(50)
c.saldo = 80
print(c.saldo)
try:
    c.saldo = -1
except ValueError as erro:
    print(erro)

80
saldo não pode ser negativo

Calculated and read-only properties

A @property without a setter becomes a calculated and read-only attribute. It's the right way to expose a derived value, which is never outdated, because it is recalculated with each read:

class Retangulo:
    def __init__(self, largura, altura):
        self.largura = largura
        self.altura = altura

    @property
    def area(self):
        return self.largura * self.altura


r = Retangulo(3, 4)
print(r.area)
try:
    r.area = 10
except AttributeError as erro:
    print(erro)

12
property 'area' of 'Retangulo' object has no setter