class DemoDescriptor:
    def __get__(self, instance, owner):
        print('執(zhí)行了__get__')
        return 42

class MyClass:
    x = DemoDescriptor()

obj = MyClass()
print(obj.x)  # 輸出42，中間會(huì)打印“執(zhí)行了__get__”

class Typed:
    def __init__(self, name, typ):
        self.name = name
        self.typ = typ

    def __get__(self, instance, owner):
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        if not isinstance(value, self.typ):
            raise TypeError(f'屬性{self.name}必須是{self.typ}')
        instance.__dict__[self.name] = value

class Person:
    age = Typed('age', int)    # 用描述器控制
    name = Typed('name', str)

p = Person()
p.age = 18            # 自動(dòng)類型檢查
p.name = '小明'
# p.age = 'not int'   # 拋出TypeError

class Demo:
    @property
    def val(self):
        return 666

def typed_field(name, typ):
    class _TypedDescriptor:
        def __get__(self, instance, owner):
            return instance.__dict__[name]
        def __set__(self, instance, value):
            if not isinstance(value, typ):
                raise TypeError(f"屬性{name}必須是{typ}")
            instance.__dict__[name] = value
    return _TypedDescriptor()

class Student:
    age = typed_field('age', int)
    name = typed_field('name', str)

stu = Student()
stu.age = 18    # √
# stu.age = 'oops' # TypeError

class ReadOnly:
    def __init__(self, value):
        self._value = value
    def __get__(self, instance, owner):
        return self._value

class System:
    version = ReadOnly('1.0.0')

print(System.version)   # '1.0.0'