添加属性Python对象 [英] Adding attributes to python objects

问题描述

这是窃听我一会儿的事。为什么我不能做的：

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'

......虽然我可以做以下？

...while I can do the following?

>>> class Bar():
...     pass
... 
>>> a = Bar()
>>> a.foo = 10 #ok!

什么是规则吗？能否请你点我的一些介绍？

What's the rule here? Could you please point me to some description?

推荐答案

您可以添加属性有任何对象的 __ __字典。

You can add attributes to any object that has a __dict__.

• X =对象（）没有它，例如。


• 字符串等简单的内置对象也没有了。


• 使用类 __ __插槽也没有了。


• 与类定义的类有它，除非previous声明适用。

• 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.

如果一个对象是使用 __插槽__ /不具有 __字典__ ，它通常是为了节省空间。例如，在 STR 这将是矫枉过正有字典 - 想象膨胀量在很短的字符串

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.

如果你想测试一个给定的对象有一个 __字典__ ，您可以用 hasattr（OBJ，'__dict __'）。

If you want to test if a given object has a __dict__, you can use hasattr(obj, '__dict__').

<一个href=\"http://www.cafepy.com/article/python_attributes_and_methods/python_attributes_and_methods.html#the-dynamic-dict\">This也可能是有趣的阅读：

This might also be interesting to read:

一些对象，如内建类型及其实例（列表，元组等）不具有 __ __字典。因此用户定义的属性不能对它们进行设置。

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.

有关Python的包括数据模型的另一个有趣的文章 __字典__ ， __插槽__ 等是的这个从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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