为什么setattr在绑定方法上失败 [英] Why does setattr fail on a bound method

问题描述

在以下情况中，setattr在第一次调用中成功，但在第二次调用中失败:

In the following, setattr succeeds in the first invocation, but fails in the second, with:

AttributeError: 'method' object has no attribute 'i'

这是为什么，是否有一种方法可以在方法上设置属性，使其仅存在于一个实例上，而不是针对类的每个实例?

Why is this, and is there a way of setting an attribute on a method such that it will only exist on one instance, not for each instance of the class?

class c:

    def m(self):

        print(type(c.m))
        setattr(c.m, 'i', 0)

        print(type(self.m))
        setattr(self.m, 'i', 0)

Python 3.2.2

推荐答案

简短的答案:无法将自定义属性添加到绑定方法中.

The short answer: There is no way of adding custom attributes to bound methods.

长答案如下.

在Python中，有个功能对象和一种方法对象.定义类时，def语句创建一个 function对象，该对象位于该类的名称空间中:

In Python, there are function objects and method objects. When you define a class, the def statement creates a function object that lives within the class' namespace:

>>> class c:
...     def m(self):
...         pass
...
>>> c.m
<function m at 0x025FAE88>

函数对象具有特殊的__dict__属性，该属性可以保存用户定义的属性:

Function objects have a special __dict__ attribute that can hold user-defined attributes:

>>> c.m.i = 0
>>> c.m.__dict__
{'i': 0}

方法对象是不同的野兽.它们是微小的对象，仅持有对相应功能对象(__func__)的引用，并对其宿主对象(__self__)的引用:

Method objects are different beasts. They are tiny objects just holding a reference to the corresponding function object (__func__) and one to its host object (__self__):

>>> c().m
<bound method c.m of <__main__.c object at 0x025206D0>>
>>> c().m.__self__
<__main__.c object at 0x02625070>
>>> c().m.__func__
<function m at 0x025FAE88>
>>> c().m.__func__ is c.m
True

方法对象提供了一种特殊的__getattr__，该属性将属性访问转发给功能对象:

Method objects provide a special __getattr__ that forwards attribute access to the function object:

>>> c().m.i
0

对于__dict__属性也是如此:

>>> c().m.__dict__['a'] = 42
>>> c.m.a
42
>>> c().m.__dict__ is c.m.__dict__
True

但是，设置属性遵循默认规则，并且由于它们没有自己的__dict__，因此无法设置任意属性.

Setting attributes follows the default rules, though, and since they don't have their own __dict__, there is no way to set arbitrary attributes.

这类似于定义__slots__并且没有__dict__插槽的用户定义类，当尝试设置不存在的插槽时会引发AttributeError(请参见

This is similar to user-defined classes defining __slots__ and no __dict__ slot, when trying to set a non-existing slot raises an AttributeError (see the docs on __slots__ for more information):

>>> class c:
...     __slots__ = ('a', 'b')
...
>>> x = c()
>>> x.a = 1
>>> x.b = 2
>>> x.c = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'c' object has no attribute 'c'

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