Composition

Questions 1 to 5 of 6

1. 1. In the Car example from this lesson, what is the relationship between Car and Engine?

   Car subclasses Engine, so a car is a kind of engineCar and Engine share one static method that both callCar has an Engine, held in a private attribute such as self._engineCar must be a subclass of Engine before it can call start()
2. 2. A hybrid car needs petrol and electric behaviour, so a developer writes this. What does it print? ``python class PetrolCar: def start(self): return "petrol engine: fuel and spark" class ElectricCar: def start(self): return "electric engine: battery to motor" class HybridCar(PetrolCar, ElectricCar): pass print(HybridCar().start()) ``

   electric engine: battery to motorpetrol engine: fuel and sparkTypeError: HybridCar cannot have two parentsAttributeError: 'HybridCar' object has no attribute 'start'
3. 3. A class marks a name as fixed with typing.Final, then a line reassigns it. What happens when this runs? ``python from typing import Final class Car: ENGINE: Final = "petrol" Car.ENGINE = "electric" print(Car.ENGINE) ``

   TypeError: ENGINE is final and cannot be reassignedAttributeError: 'Car' object has no attribute 'ENGINE'petrol, because the first value stays fixedelectric, because Python runs the assignment and Final is only a hint for a type checker
4. 4. True or false: a Garage that holds many Car objects and forwards start_car to the right car is an example of delegation.

   False, because delegation needs a parent class, and the garage has noneTrue, because the garage passes the request to the car it holds instead of doing the work itself
5. 5. What does this program print? ``python class PetrolEngine: def start(self): return "petrol engine: fuel and spark" class ElectricEngine: def start(self): return "electric engine: battery to motor" class Car: def __init__(self, engine): self._engine = engine def swap_engine(self, new_engine): self._engine = new_engine def drive(self): return self._engine.start() car = Car(PetrolEngine()) before = car.drive() car.swap_engine(ElectricEngine()) print(before + " | " + car.drive()) ``

   petrol engine: fuel and spark | electric engine: battery to motorelectric engine: battery to motor | petrol engine: fuel and sparkelectric engine: battery to motor | electric engine: battery to motorpetrol engine: fuel and spark | petrol engine: fuel and spark