python - 覆盖的属性访问不起作用(如预期)

Question

以下模块的主要目标是为某些名称提供一种“常量”语义。

class ConstantError(Exception):

    def __init__(self, msg):
            self._msg = msg


class Constant(object):

    def __init__(self, name):
            self._name = name

    def __get__(self, instance, owner):
            return instance._content[self._name]

    def __set__(self, instance, value):
            raise ConstantError, 'Illegal use of constant'


class Constants(object):

    def __init__(self, content):
            self._content = content
            for k in self._content:
                    setattr(self, k, Constant(k))

num_const = Constants({
    'one': 1,
    'two': 2
})

使用时:

>>> from const import *
>>> dir(num_const)
['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', '__weakref__', '_content', 'one', 'two']

所以 one 和 two 存在，但属性访问是diappointing:

>>> num_const.one
<const.Constant object at 0x7faef4871710>
>>> 

在这种情况下我希望 1 。我哪里错了？

Answer 1

描述符协议(protocol)仅适用于类的属性，而不适用于类实例的属性。请参阅How-To Guide for Descriptors