为什么不能在 python 中为对象添加属性?

2022-01-13 00:00:00 python attributes instances

问题描述

(用 Python shell 编写)

(Written in Python shell)

>>> 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?

Why doesn't object allow you to add attributes to it?

解决方案

注意 object 实例没有 __dict__ 属性:

Notice that an object instance has no __dict__ attribute:

>>> 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:

An example to illustrate this behavior in a derived class:

>>> 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:

Quoting from the docs on slots:

[...] __slots__ 声明采用一系列实例变量，并在每个实例中保留足够的空间来保存每个变量的值.节省空间是因为 __dict__ 不是为每个实例创建的.

[...] 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.

从评论中回答 ThomasH，OP 的测试类是一个旧式"类.试试:

To answer ThomasH from the comments, OP's test class is an "old-style" class. Try:

>>> 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).

and you'll notice there is a __dict__ instance. The object class may not have a __slots__ defined, but the result is the same: lack of a __dict__, which is what prevents dynamic assignment of an attribute. I've reorganized my answer to make this clearer (move the second paragraph to the top).

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