Part.3.B.2.classes-2这里为什么类要封装呢，为什么不是在外部直接改变，这样不是更方便吗？

[Achuan-2]

from IPython.core.interactiveshell import InteractiveShell
InteractiveShell.ast_node_interactivity = "all"
import datetime

class Golem:
    __population = 0
    __life_span = 10
  
    def __init__(self, name=None):
        self.name = name
        self.built_year = datetime.date.today().year
        self.__active = True
        Golem.__population += 1
  
    def say_hi(self):
        print('Hi!')
  
    def cease(self):
        self.__active = False
        Golem.__population -= 1
  
    def is_active(self):
        if datetime.date.today().year - self.built_year >= Golem.__life_span:
            self.cease
        return self.__active
  
    @property
    def population(self):
        return Golem.__population
  
    @population.setter
    def population(self, value):
        Golem.__population = value

g = Golem('Clay')
g.population
g.population = 100
ga = Golem('New')
g.population
ga.population
help(Golem)
Golem.__dict__
g.__dict__
hasattr(Golem, 'population')
getattr(Golem, 'population')
setattr(Golem, 'population', 10000)
g.population    # 所以，在很多的情况下，不把数据封装在 Class 内部的话，后面会有很多麻烦。