无法设置对象类的属性 [英] Can't set attributes of object class

问题描述

所以，我与Python玩弄一边回答<一个href=\"http://stackoverflow.com/questions/1528932/how-to-create-inline-objects-with-properties-in-python/\">this问题，我发现这是无效的：

So, I was playing around with Python while answering this question, and I discovered that this is not valid:

o = object()
o.attr = 'hello'

由于一个 AttributeError的：'对象'对象有没有属性'ATTR。然而，从对象继承的任何类，它是有效的：

due to an AttributeError: 'object' object has no attribute 'attr'. However, with any class inherited from object, it is valid:

class Sub(object):
    pass

s = Sub()
s.attr = 'hello'

印刷 s.attr 显示'你好'预期。为什么会出现这样的情况？在Python语言规范什么规定，你不能分配属性香草对象？

Printing s.attr displays 'hello' as expected. Why is this the case? What in the Python language specification specifies that you can't assign attributes to vanilla objects?

推荐答案

的实例对象做的不可以随身携带 __字典__ - 如果它这样做了，可怕的循环依赖问题（因为字典之前，像其他大多数的一切，从<$ C $继承C>对象 ;-)，这将鞍的每个的对象在Python有字典，这将意味着的许多的每个对象的字节的开销，目前没有或需要一个字典（基本上，没有随意分配的属性不具备或需要的字典中的所有对象）。

An instance of object does not carry around a __dict__ -- if it did, before the horrible circular dependence problem (since dict, like most everything else, inherits from object;-), this would saddle every object in Python with a dict, which would mean an overhead of many bytes per object that currently doesn't have or need a dict (essentially, all objects that don't have arbitrarily assignable attributes don't have or need a dict).

例如，采用优 pympler 项目（您可以通过从的这里），我们可以做一些测量...

For example, using the excellent pympler project (you can get it via svn from here), we can do some measurements...:

>>> from pympler import asizeof
>>> asizeof.asizeof({})
144
>>> asizeof.asizeof(23)
16

您不希望每 INT 占用144个字节，而不是仅仅16，右 - ？）

You wouldn't want every int to take up 144 bytes instead of just 16, right?-)

现在，当你犯了一个类（无论从那个继承），事物的变化...

Now, when you make a class (inheriting from whatever), things change...:

>>> class dint(int): pass
... 
>>> asizeof.asizeof(dint(23))
184

...的 __ __字典的是的现在又增加了（加，多一点开销） - 因此力实例可以拥有任意的属性，但是你付出相当的空间成本灵活性。

...the __dict__ is now added (plus, a little more overhead) -- so a dint instance can have arbitrary attributes, but you pay quite a space cost for that flexibility.

所以，如果你想 INT s的只是有一个的额外属性 foobar的。 ..？这是一个难得的需要，但是Python的确为目的的特殊机制...

So what if you wanted ints with just one extra attribute foobar...? It's a rare need, but Python does offer a special mechanism for the purpose...

>>> class fint(int):
...   __slots__ = 'foobar',
...   def __init__(self, x): self.foobar=x+100
... 
>>> asizeof.asizeof(fint(23))
80

......不是的非常的形成微小作为 INT ，你要知道！ （甚至两个 INT S，一个自和一个 self.foobar - 第二个可以被重新分配），但肯定远远好力

...not quite as tiny as an int, mind you! (or even the two ints, one the self and one the self.foobar -- the second one can be reassigned), but surely much better than a dint.

在这个类有 __插槽__ 特殊的属性（字符串序列），那么类语句（更precisely，默认元类，键入）做的不可以装备该类的每一个实例与 __字典__ （因此有任意属性的能力），只是一个有限的，刚性集插槽（可各持一个参考一些对象主要地方）用给定的名字。

When the class has the __slots__ special attribute (a sequence of strings), then the class statement (more precisely, the default metaclass, type) does not equip every instance of that class with a __dict__ (and therefore the ability to have arbitrary attributes), just a finite, rigid set of "slots" (basically places which can each hold one reference to some object) with the given names.

在换回失去的灵活性，你获得了很多每个实例的字节（前提是你有闲逛身边实例不计其数可能有意义的，但是，有是的用例的）。

In exchange for the lost flexibility, you gain a lot of bytes per instance (probably meaningful only if you have zillions of instances gallivanting around, but, there are use cases for that).

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