为什么不能在python中向对象添加属性？

Why can't you add attributes to object in python?

本问题已经有最佳答案，请猛点这里访问。

(用python shell编写)

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>>> o = object()
>>> o.test = 1

Traceback (most recent call last):
File"<pyshell#45>", line 1, in <module>
o.test = 1
AttributeError: 'object' object has no attribute 'test'
>>> class test1:
pass

>>> t = test1()
>>> t.test

Traceback (most recent call last):
File"<pyshell#50>", line 1, in <module>
t.test
AttributeError: test1 instance has no attribute 'test'
>>> t.test = 1
>>> t.test
1
>>> class test2(object):
pass

>>> t = test2()
>>> t.test = 1
>>> t.test
1
>>>

为什么对象不允许您向其添加属性？

注意，object实例没有__dict__属性：

1 2	>>> dir(object()) ['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__']

在派生类中演示此行为的示例：

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>>> class Foo(object):
... __slots__ = {}
...
>>> f = Foo()
>>> f.bar = 42
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute 'bar'

号

引用slots上的文件：

[...] The __slots__ declaration takes a sequence of instance variables and reserves just enough space in each instance to hold a value for each variable. Space is saved because __dict__ is not created for each instance.

号

编辑：为了回答托马斯的评论，OP的测试班是一个"老式"的班。尝试：

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>>> class test: pass
...
>>> getattr(test(), '__dict__')
{}
>>> getattr(object(), '__dict__')
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute '__dict__'

你会注意到有一个__dict__实例。对象类可能没有定义__slots__，但结果是相同的：缺少__dict__，这是防止动态分配属性的原因。我已经重新整理了我的答案，使它更清晰(把第二段移到顶部)。