为python对象添加属性 [英] Adding attributes to python objects
问题描述
这件事困扰了我一段时间.为什么我做不到:
<预><代码>>>>一个 = "">>>a.foo = 2回溯(最近一次调用最后一次):文件<stdin>",第 1 行,在 <module> 中AttributeError: 'str' 对象没有属性 'foo'...虽然我可以执行以下操作?
<预><代码>>>>类酒吧():... 经过...>>>a = 酒吧()>>>a.foo = 10 #ok!这里的规则是什么?你能指点我一些描述吗?
您可以向任何具有 __dict__
的对象添加属性.
x = object()
没有它,例如.- 字符串和其他简单的内置对象也没有.
- 使用
__slots__
的类也没有. - 用
class
定义的类都有它,除非前面的语句适用.
如果一个对象正在使用 __slots__
/没有 __dict__
,通常是为了节省空间.例如,在 str
中,如果有一个 dict 就有点矫枉过正了——想象一下一个非常短的字符串的膨胀量.
如果你想测试给定的对象是否有__dict__
,你可以使用hasattr(obj, '__dict__')
.
这个阅读起来可能也很有趣:
<块引用>某些对象,例如内置类型及其实例(列表、元组等)没有 __dict__
.因此,无法对它们设置用户定义的属性.
另一篇关于 Python 数据模型的有趣文章,包括 __dict__
、__slots__
等,是 this 来自 python 参考.
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?
You can add attributes to any object that has a __dict__
.
x = object()
doesn't have it, for example.- Strings and other simple builtin objects also don't have it.
- Classes using
__slots__
also do not have it. - Classes defined with
class
have 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 a str
it 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 use hasattr(obj, '__dict__')
.
This might also be interesting to read:
Some objects, such as built-in types and their instances (lists, tuples, etc.) do not have a
__dict__
. Consequently user-defined attributes cannot be set on them.
Another interesting article about Python's data model including __dict__
, __slots__
, etc. is this from the python reference.
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