对属性装饰器及其相关的getter / setter方法感到困惑

Question

I've been doing some Python, and I realised I Haven't actually know a lot about the property decorator, so I tried making a simple example. This is the code I used:

class foo():
    def __init__(self):
        self.__test = 0

    @property
    def test(self):
        return self.__test

    @test.setter
    def test(self, value):
        self.__test = value

    @test.getter
    def test(self):
        self.__test += 1
        return self.__test

Then I started playing with it in the interactive shell:

>>> bar = foo()
>>> bar.test
1
>>> bar.test
2

So far the object behaved as I expected it to.

Then I tried checking out the setter method

>>> bar.test = 5
>>> bar.test
5
>>> bar.test
5

Weird. For some reason the value of __test wasn't incremented.

>>> bar._foo__test
2

I thought I had set __test to be equal to 5.

What's going on?

Answer 1

The problem is that your foo class is an old style class, descriptors (and as such properties) are only intended to work with new style classes.

From the doc :

Note that descriptors are only invoked for new style objects or classes (a class is new style if it inherits from object or type)

In this case, with an old style class setting bar.test = 5 creates a test attribute in the instance dict, which shadows the property from the class dict:

>>> bar = foo()
>>> foo.__dict__
{'test': <property object at 0x7f302e64c628>, '__module__': '__main__', '__doc__': None, '__init__': <function __init__ at 0x7f302e658b18>}
>>> bar.test   # test property from class dict is used
1
>>> bar.__dict__
{'_foo__test': 1}
>>> bar.test = 5   # sets test on instance
>>> bar.__dict__
{'test': 5, '_foo__test': 1}

So the solution is simple: make foo a new style class by inheriting from object