12. Object-Oriented Design

OOD Design Template

// Step 1: Identify key entities (nouns)
// Step 2: Define relationships (inheritance, composition)
// Step 3: List behaviors (verbs → methods)
// Step 4: Apply design patterns (Singleton, Factory, Strategy, Observer)
// Step 5: Discuss trade-offs (coupling vs cohesion, extensibility)

ATM Machine

# Key classes
class Card:
    def __init__(self, number, pin, account):
        self.number = number; self.pin = pin
        self.account = account

class Account:
    def __init__(self, acct_no, balance=0):
        self.acct_no = acct_no; self.balance = balance

    def withdraw(self, amount):
        if amount > self.balance: raise InsufficientFunds()
        self.balance -= amount
        return amount

class ATM:
    def __init__(self, cash_available):
        self.cash_available = cash_available

    def authenticate(self, card, pin):
        return card.pin == pin

    def withdraw(self, card, amount):
        if amount > self.cash_available: raise ATMOutOfCash()
        cash = card.account.withdraw(amount)
        self.cash_available -= amount
        return cash

Key OOD Problems