Low Level Design (LLD)

# ENCAPSULATION - Hide internal state
class BankAccount:
    def __init__(self):
        self.__balance = 0  # Private
    
    def deposit(self, amount):  # Controlled access
        if amount > 0:
            self.__balance += amount

# ABSTRACTION - Hide complexity
class PaymentProcessor(ABC):
    @abstractmethod
    def process(self, amount): pass  # What, not how

# INHERITANCE - Share behavior
class Car(Vehicle):
    def __init__(self):
        super().__init__()  # Reuse parent

# POLYMORPHISM - Same interface, different behavior
def make_sound(animal: Animal):
    animal.speak()  # Works for Dog, Cat, Bird...

# SINGLETON - One instance
class Logger:
    _instance = None
    def __new__(cls):
        if not cls._instance:
            cls._instance = super().__new__(cls)
        return cls._instance

# FACTORY - Create without specifying class
def create_vehicle(type: str) -> Vehicle:
    return {"car": Car, "bike": Bike}[type]()

# STRATEGY - Swappable algorithms
class PaymentContext:
    def __init__(self, strategy: PaymentStrategy):
        self.strategy = strategy
    def pay(self, amount):
        return self.strategy.process(amount)

# OBSERVER - Notify on changes
class Stock:
    def __init__(self):
        self.observers = []
    def notify(self):
        for obs in self.observers:
            obs.update(self)