Python 描述符 data 和 non-data 两种类型

仅包含__get__的,是non-data descriptor, 如果实例__dict__包含同名变量, 则实例优先; 如果还包含__set__, 则是data descriptor, 优先于实例__dict__同名变量.

class c:
    pass

class a:
    def __init__(self, name):
        self.name = name
    def __get__(self, ins, cls):
        print('call %s get: '%type(self).__name__, end='')
        ins.__dict__[self.name] = c()
        return ins.__dict__[self.name]

class a2(a):
    def __set__(self, ins, val):
        ins.__dict__[self.name] = val
    
class t:
    x=a('x')
    y=a2('y')

b = t()
print(b.x)
print(b.x)
print(b.y)
print(b.y)

结果:

>>> 
call a get: <__main__.c object at 0x0000000002A47B00>
<__main__.c object at 0x0000000002A47B00>
call a2 get: <__main__.c object at 0x0000000002B27208>
call a2 get: <__main__.c object at 0x0000000002B27278>