class Subject:
    def __init__(self):
        self._observers = []

    def attach(self, observer):
        self._observers.append(observer)

    def notify(self, message):
        for observer in self._observers:
            observer.update(message)

class WeatherStation(Subject):
    @property
    def temperature(self):
        return self._temperature

    @temperature.setter
    def temperature(self, temp):
        self._temperature = temp
        self.notify(f"Temperature changed to {temp}")

class CreditCardPayment(PaymentStrategy):
    def pay(self, amount):
        return f"Paid ${amount} by Credit Card"

class PayPalPayment(PaymentStrategy):
    def pay(self, amount):
        return f"Paid ${amount} by PayPal"

cart.set_payment_strategy(CreditCardPayment())
print(cart.checkout())

class LightOnCommand(Command):
    def __init__(self, light):
        self.light = light

    def execute(self):
        return self.light.turn_on()

    def undo(self):
        return self.light.turn_off()

remote.set_command(1, LightOnCommand(living_room_light))
remote.press_button(1)   # Living Room light is ON
remote.press_undo()      # Living Room light is OFF