这件事困扰了我一段时间.为什么我做不到:
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'
...虽然我可以做到以下几点?
...while I can do the following?
>>> class Bar():
... pass
...
>>> a = Bar()
>>> a.foo = 10 #ok!
这里的规则是什么?你能指点我一些描述吗?
What's the rule here? Could you please point me to some description?
您可以为任何具有 __dict__
的对象添加属性.
You can add attributes to any object that has a __dict__
.
x = object()
没有它,例如.__slots__
的类也没有.class
定义的类有它,除非前面的语句适用.x = object()
doesn't have it, for example.__slots__
also do not have it.class
have it unless the previous statement applies.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,使用 dict 就太过分了——想象一下非常短的字符串会产生多大的膨胀.
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.
如果要测试给定对象是否有 __dict__
,可以使用 hasattr(obj, '__dict__')
.
If you want to test if a given object has a __dict__
, you can use hasattr(obj, '__dict__')
.
这也可能很有趣:
某些对象,例如内置类型及其实例(列表、元组等)没有 __dict__
.因此无法在它们上设置用户定义的属性.
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.
另一篇关于Python的数据模型包括__dict__
、__slots__
等有趣的文章是this 来自 python 参考.
Another interesting article about Python's data model including __dict__
, __slots__
, etc. is this from the python reference.
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