限制动态添加属性，__slots__

python允许在定义class时，定义一个特殊的__slots__变量，来限制class实例梦添加的属性。

>>> class Person(object):
    __slots__ = ("name", "age")

>>> P = Person()
>>> P.name = "老王"
>>> P.age = 20
>>> P.score = 100
Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
AttributeError: Person instance has no attribute 'score'
>>>

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使用需要注意定义的属性只对当前类的实例起作用，对继承的子类不起作用。

In [67]: class Test(Person):
    ...:     pass
    ...:

In [68]: t = Test()

In [69]: t.score = 100