对内置对象的属性分配

问题描述

这行得通:

class MyClass(object):
    pass

someinstance = MyClass()
someinstance.myattribute = 42
print someinstance.myattribute
>>> 42

但这不是:

someinstance = object()
someinstance.myattribute = 42
>>> AttributeError: 'object' object has no attribute 'myattribute'

为什么?我有一种感觉，这与 object 作为内置类有关，但我觉得这并不令人满意，因为我在 MyClass 的声明中没有更改任何内容.

Why? I've got a feeling, that this is related to object being a built-in class, but I find this unsatisfactory, since I changed nothing in the declaration of MyClass.

推荐答案

Python 将属性存储在 dict 中.你可以给MyClass添加属性，看它有一个__dict__:

Python stores attributes in a dict. You can add attributes to MyClass, see it has a __dict__:

>>> class MyClass(object):
>>>   pass
>>> dir(MyClass)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']

重要的区别是 object 没有 __dict__ 属性.

The important difference is that object has no __dict__ attribute.

>>> dir(object)
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

更详细的解释:

• 无法设置对象类的属性
• 为什么不能添加属性在python中反对?