p = Person()
    p.name = 'Bob'	# 名字屬性必須是str
    p.age = 18	        # 年齡必須是int
    p.height = 1.83	# 身高必須是float

class Descriptor(object):
    def __get__(self, instance, cls):
        # instance用于區(qū)分使用類訪問(wèn)x,還是使用實(shí)例訪問(wèn)x
        print('in __get__', instance, cls)
        return instance.__dict__['x']
    def __set__(self, instance, value):
        # 在set中對(duì)于類型進(jìn)行檢查
        print('in __set__')
        instance.__dict__['x'] = value
    def __delete__(self, instance):
        print('in __del__')
class A(object):
    # 在類中定義一個(gè)類屬性x
    x = Descriptor()
a = A()
# 會(huì)被Descriptor的__get__方法所截獲
print(a.x)
# 直接使用類A訪問(wèn)類屬性,instance會(huì)被傳入None
print(A.x)
# 會(huì)被Descriptor的__set__方法所截獲
a.x = 5
# 會(huì)被Descriptor的__del__方法所截獲
del a.x
'''
通常來(lái)說(shuō)在描述符這些方法當(dāng)中訪問(wèn)的是instance.__dict__這個(gè)字典，
也就是對(duì)于它的真正屬性進(jìn)行操作。
'''
a = A()
a.x = 5
print(a.__dict__)

class Attr(object):
    def __init__(self, name, type_):
        self.name = name
        self.type_ = type_
    def __get__(self, instance, cls):
        return instance.__dict__[self.name]
    def __set__(self, instance, value):
        # 對(duì)字段類型做檢測(cè)
        if not isinstance(value, self.type_):
            raise TypeError('expected an %s' % self.type_)
        instance.__dict__[self.name] = value
    def __delete__(self, instance):
        del instance.__dict__[self.name]
class Person(object):
    # 定義一個(gè)name字段，申請(qǐng)描述符實(shí)例
    name = Attr('name', str)
    age = Attr('age', int)
    height = Attr('height', float)
p = Person()
p.name = 'Bob'
print(p.name)
# age賦值字符串類型拋出異常錯(cuò)誤
# p.age = '17'