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
在下面,setattr
在第一次调用中成功,但在第二次调用中失败:
AttributeError: 'method' 对象没有属性 'i'
为什么会这样,有没有办法在方法上设置属性,使其只存在于一个实例上,而不存在于类的每个实例上?
c 类:def m(自我):打印(类型(厘米))setattr(cm, 'i', 0)打印(类型(self.m))setattr(self.m, 'i', 0)
Python 3.2.2
简短的回答:无法向绑定方法添加自定义属性.
长答案如下.
在Python中,有函数对象和方法对象.当您定义一个类时,def
语句会创建一个位于类的命名空间中的函数对象:
函数对象有一个特殊的 __dict__
属性,可以保存用户定义的属性:
方法对象是不同的野兽.它们是微小的对象,只持有对相应函数对象(__func__
)的引用和对其宿主对象(__self__
)的引用:
方法对象提供了一个特殊的__getattr__
,它将属性访问转发到函数对象:
对于 __dict__
属性也是如此:
虽然设置属性遵循默认规则,但由于它们没有自己的__dict__
,因此无法设置任意属性.
这类似于定义 __slots__
并且没有 __dict__
槽的用户定义类,当尝试设置不存在的槽时会引发 AttributeError
(有关详细信息,请参阅 __slots__
上的文档):
In the following, setattr
succeeds in the first invocation, but fails in the second, with:
AttributeError: 'method' object has no attribute 'i'
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
The short answer: There is no way of adding custom attributes to bound methods.
The long answer follows.
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>
Function objects have a special __dict__
attribute that can hold user-defined attributes:
>>> c.m.i = 0
>>> c.m.__dict__
{'i': 0}
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
Method objects provide a special __getattr__
that forwards attribute access to the function object:
>>> c().m.i
0
This is also true for the __dict__
property:
>>> c().m.__dict__['a'] = 42
>>> c.m.a
42
>>> c().m.__dict__ is c.m.__dict__
True
Setting attributes follows the default rules, though, and since they don't have their own __dict__
, there is no way to set arbitrary attributes.
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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