Question

These two codes will give two different outputs, why ?

class Test:
    def __get__(self, instance, owner):
        return 42
    def __set__(self, instance, value):
        pass

class A:
    a = Test()

a = A()
print(a.a) // print 42
a.a = 0
print(a.a) // print 42

And

class Test:
    def __get__(self, instance, owner):
        return 42
    def __set__(self, instance, value):
        pass

class A:
    pass

a = A()
a.a = Test()
print(a.a) // print <__main__.Test object at 0xb700d6cc>
a.a = 0
print(a.a) // print 0

How are stored attributes in the Python engine ?

Answer 1

Your Test class isn’t called “attribute”, it’s a descriptor. Descriptors work, by definition, only when stored on a (new style, for the poor Python 2 users) class. Descriptor objects on non-class objects have no special meaning, they are treated like any other object. Their __get__ method is ignored when they are retrieved, and their __set__ is ignored when they are replaced.