實(shí)現(xiàn)了__set__(), __get__()或__delete__()的對(duì)象，且描述符屬性只能定義在類級(jí)別。
@property可利用描述符實(shí)現(xiàn)。(@classmethod, @staticmethod等也可以用描述符實(shí)現(xiàn))

利用描述符來進(jìn)行類型檢查。
<b>Python中屬性引用解析的執(zhí)行方式，優(yōu)先級(jí)鏈搜索屬性。</b>數(shù)據(jù)描述符（實(shí)現(xiàn)了__set__(), __get__()）優(yōu)先級(jí)>實(shí)例變量>非數(shù)據(jù)描述符(只實(shí)現(xiàn)__get__())。在下例中實(shí)際給屬性賦值時(shí), u_0.name='test'等同于type(u_0).__dict__['name'].__set__(u_0, 'test')，取值，訪問屬性u_.name等同于type(u_0).__dict__['name'].__get__(u_0, type(u_0))

class Field(object):
    _expected_type = type(None)
    
    def __init__(self, name):
        self.name = name
    
    def __set__(self, instance, value):
        if not isinstance(value, self._expected_type):
            raise TypeError('excepted %s' % self._expected_type)
        # setattr(instance, self.name, value)
        instance.__dict__[self.name] = value

    def __get__(self, instance, owner):
        if not instance:
            return self
        # getattr(instance, self.name)
        return instance.__dict__[self.name]


class StringField(Field):
    _expected_type = str


class IntField(Field):
    _expected_type = int


class User_0(object):
    name = StringField('name')
    age = IntField('age')

    def __init__(self, name, age):
        self.name = name
        self.age = age

u_0 = User_0(name='JiangW', age=22)
u_0.name = 'test'
print(u_.age)

# TypeError: excepted <class 'int'>
# User_0(name='JiangW', age='22')

其他方式實(shí)現(xiàn)類型檢查。
利用函數(shù)和@property。property實(shí)際是一個(gè)描述符類，CheckType()方法返回的實(shí)際是一個(gè)<class 'property'>的對(duì)象， 該對(duì)象也是實(shí)現(xiàn)了__set__(), __get__()。

def CheckType(name, expected_type):
    _name = name
    
    @property
    def res(self):
        return getattr(self, _name)
    
    @res.setter
    def res(self, value):
        if not isinstance(value, expected_type):
            raise TypeError('excepted type %s' % excepted_type)
       setattr(self, _name, value)

    return res