python - 了解 Python 描述符

Question

我正在尝试更好地理解描述符。

我不明白为什么在 foo 方法中描述符 __get__ 方法未被调用。

据我了解描述符 __get__ 当我通过点运算符访问对象属性或使用 __getattribute__() 时，方法总是被调用.

根据Python documentation :

class RevealAccess(object):
    def __init__(self, initval=None, name='var'):
        self.val = initval
        self.name = name

    def __get__(self, obj, objtype):
        print('Retrieving', self.name)
        return self.val

    def __set__(self, obj, val):
        print('Updating', self.name)
        self.val = val

class MyClass(object):
    x = RevealAccess(10, 'var "x"')
    y = 5

    def foo(self):
        self.z = RevealAccess(13, 'var "z"')
        self.__getattribute__('z')
        print(self.z)

m = MyClass()
m.foo()
m.z # no print
m.x # prints var x

Answer 1

z 是实例 上的属性，而不是类上的属性。描述符协议(protocol)仅适用于从类中检索的属性。

来自Descriptor HOWTO :

For objects, the machinery is in object.__getattribute__() which transforms b.x into type(b).__dict__['x'].__get__(b, type(b)).

并且在 Implementing Descriptors section Python 数据模型:

The following methods only apply when an instance of the class containing the method (a so-called descriptor class) appears in an owner class (the descriptor must be in either the owner’s class dictionary or in the class dictionary for one of its parents).

类字典中找不到你的m.z； type(m).__dict__['z'] 不存在；它位于 m.__dict__['z'] 中。这里m是实例，owner类是MyClass，z没有出现在中所有者类字典。