浅谈Python中的描述符(Descriptor)

简单点

class Descriptor:
    def __get__(self, instance, owner):
        print('__get__ called. '）
class foo:
    t = Descriptor()
if __name__ == '__main__':
    c = foo()
    c.t    #print  __get__ called.

详细点

import types
class Descriptor:
    def __call__(self, *args, **kwargs):
        print('Descriptor called')
    def __get__(self, instance, owner):
        if instance is not None:
            print('__get__ called. ' + 'instance is ' + str(instance) + '; owner is ' + str(owner))
            return types.MethodType(self, instance)
        else:
            print('__get__ called. ' + 'instance is ' + str(instance) + '; owner is ' + str(owner))
            return self
    def __set__(self, instance, value):
        print('set ' + str(instance) + ';value: ' + str(value))
class foo:
    t = Descriptor()
class bar:
    def __init__(self):
        self.t=Descriptor()
if __name__ == '__main__':
    c = foo()
    print('test 1')
    # 相当于foo.__dict__['t'].__get__(None, foo)
    foo.t
    print('test 2')
    # 相当于foo.__dict__['t'].__get__(t, foo)
    c.t
    print('test 3')
    c.t()
    print('test 4')
    c.t = 1
    print('test 5')
    d=bar()
    d.t
'''
test 1
__get__ called. instance is None; owner is <class '__main__.foo'>
test 2
__get__ called. instance is <__main__.foo object at 0x00000273839F4320>; owner is <class '__main__.foo'>
test 3
__get__ called. instance is <__main__.foo object at 0x00000273839F4320>; owner is <class '__main__.foo'>
Descriptor called
test 4
set <__main__.foo object at 0x00000273839F4320>;value: 1
test 5
'''

有什么用

如果想创建一个新形式的实例属性，可以以描述符类的形式定义其功能

class Integer:
    def __init__(self, name):
        self.name = name
    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.name]
    def __set__(self, instance, value):
        if not isinstance(value, int):
            raise TypeError('Expected an int')
        instance.__dict__[self.name] = value
    def __delete__(self, instance):
        del instance.__dict__[self.name]
class Point:
    x = Integer('x')
    y = Integer('y')
    def __init__(self, x, y):
        self.x = x
        self.y = y
if __name__ == '__main__':
    p = Point(2, 3)
    print(p.x) # calls Point.x.__get__(p,Point)
    p.y = 5 
    try:
        p.x = 2.3
    except TypeError as e:
        print(e)

如果仅仅是像访问某个特定类中的一个属性，并对此做定制化处理，直接用property更加简单。
在需要大量重用代码的情况下，描述符更加有用。