8.1 Classes, Objects, and Encapsulation

Object-oriented programming, or OOP, models things as objects. An object stores data and provides behavior. A class is a template, and an object is an instance of that class.

Classes and Objects

The BankAccount class below represents a bank account. owner, account, and balance are attributes. deposit() and withdraw() are methods.

class BankAccount:
    def deposit(self, amount):
        self.balance += amount

    def withdraw(self, amount):
        self.balance -= amount

account = BankAccount()
account.owner = "Alice"
account.account = "6250941006528599"
account.balance = 50

account.deposit(100)
print("Balance:", account.balance)

Output:

Balance: 150

Here, self means the current object. When account.deposit(100) runs, self refers to account.

Encapsulation

Encapsulation hides internal implementation details and lets outside code access or change state through clear methods.

If account balance can be changed freely from outside, it is hard to keep data valid. A better design makes balance private and checks changes through methods.

class BankAccount:
    def __init__(self, owner, account, balance):
        self.__owner = owner
        self.__account = account
        self.__balance = balance

    def get_balance(self):
        return self.__balance

    def deposit(self, amount):
        if amount <= 0:
            return False
        self.__balance += amount
        return True

    def withdraw(self, amount):
        if amount <= 0 or amount > self.__balance:
            return False
        self.__balance -= amount
        return True

Private attributes are not meant to make code mysterious. They keep state changes inside trusted methods.

Constructor

__init__() is the constructor. It runs automatically when an object is created.

account = BankAccount("Alice", "6250941006528599", 50)

Constructors are commonly used to set initial object state. If an object should always have certain attributes, initialize them in __init__().