为什么我不用用自我。在从另一个类创建的属性前面？

Question

对于那些需要上下文的人，我正在阅读Python速成课程手册，第176页。 对于最后一行代码，为什么我能够在battery之前键入my_tesla.battery.describe_battery()而不使用self.

class Car():

  def __init__(self,make,model,year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer = 20000

  def get_description(self):
    """This function gives a full name of the cars make, model and year and returns it to the function."""
    long_name = str(self.year) + " " + self.make + " " + self.model
    return long_name.title()

  def odometer_reading(self):
    """This function prints the odometer reading of the vehicle."""
    return self.odometer

  def update_mileage(self, mileage):
    if mileage >= self.odometer:
      self.odometer = mileage
    else:
      print("You can not roll back the odometer!")  


  def increase_mileage(self, mile):
    self.odometer += mile
    return self.odometer



class Battery():

  """This creates the classes __init__ method and sets a default battery_size to 70"""
  def __init__(self,battery_size=70):
    self.battery_size = battery_size

  def describe_battery(self):
      print("This car has a " + str(self.battery_size) + "-kWh battery.")



class ElectricCar(Car):
  """Initializes the parents class - Car() into the child class - ElectricCar()."""
  def __init__(self,make,model,year):
    super().__init__(make,model,year)
    self.battery = Battery()




my_tesla=ElectricCar('tesla','model s', 2016)
print(my_tesla.get_description())
my_tesla.battery.describe_battery()`

我希望必须键入：

my_tesla.self.battery.describe_battery()以允许使用属性-battery