python - 为什么不能在 python 中为对象添加属性？

Question

这个问题在这里已经有了答案:

Can't set attributes on instance of "object" class (7 个回答)

关闭 8年前。

(用 Python shell 编写)

>>> 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
>>> 

为什么对象不允许你给它添加属性？

Answer 1

注意 object 实例没有 __dict__ 属性:

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

在派生类中说明此行为的示例:

>>> 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'

引用 slots 上的文档:

[...] 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.

编辑:从评论中回答 ThomasH，OP 的测试类是一个“旧式”类。试试:

>>> 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__'

您会注意到有一个 __dict__ 实例。对象类可能没有定义 __slots__，但结果是一样的:缺少 __dict__，这是阻止动态分配属性的原因。我已经重新组织了我的答案以使其更清晰(将第二段移到顶部)。