向 python 对象添加属性
这是一件困扰我一段时间的事情。为什么我不能做：
>>> a = ""
>>> a.foo = 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'foo'
...而我可以做以下事情？
>>> class Bar():
... pass
...
>>> a = Bar()
>>> a.foo = 10 #ok!
这里有什么规则？您能给我一些描述吗？
It's a thing that bugged me for a while. Why can't I do:
>>> a = ""
>>> a.foo = 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'foo'
...while I can do the following?
>>> class Bar():
... pass
...
>>> a = Bar()
>>> a.foo = 10 #ok!
What's the rule here? Could you please point me to some description?
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您可以向任何具有 __dict__ 的对象添加属性。
x = object()就没有它。class定义的类都具有它。如果一个对象使用 __slots__ / 没有 __dict__ ，通常是为了节省空间。例如，在
str中，拥有一个字典就太过分了——想象一下非常短的字符串的膨胀量。如果要测试给定对象是否具有
__dict__，可以使用hasattr(obj, '__dict__')。这读起来可能也很有趣：
另一篇关于Python数据模型（包括
__dict__、__slots__等）的有趣文章是this 来自 python 参考。You can add attributes to any object that has a
__dict__.x = object()doesn't have it, for example.__slots__also do not have it.classhave it unless the previous statement applies.If an object is using
__slots__/ doesn't have a__dict__, it's usually to save space. For example, in astrit would be overkill to have a dict - imagine the amount of bloat for a very short string.If you want to test if a given object has a
__dict__, you can usehasattr(obj, '__dict__').This might also be interesting to read:
Another interesting article about Python's data model including
__dict__,__slots__, etc. is this from the python reference.