Classes and Encapsulation

Object-oriented programming (OOP) models program concepts as objects. An object keeps data together with behavior that operates on that data. A class describes shared structure and behavior; each object is an instance created from that template.

🏠 Blueprint → housesOne blueprint creates houses with different addresses and colors.

🍪 Cutter → cookiesThe cutter defines a shape, while every cookie is independent.

📝 Account form → accountsFields and rules are shared; every account has its own owner and balance.

Classes, objects, attributes, and methods

Attributes describe what an object has; methods describe what it can do. Objects share method code, while self directs that code to the current object.

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(account.balance)

Encapsulation and valid state

If arbitrary outside code can make a balance negative, the account enters an invalid state. Encapsulation is not secrecy; it funnels changes through operations such as deposit() and withdraw(), where rules can be checked. Double-underscore names trigger name mangling to reduce accidental outside access. This is a design mechanism, not security encryption.

The constructor

__init__() runs automatically for a new object. It gathers required values at construction time so every instance begins with complete, valid state.

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