static, class and instance methods (Reposting On Python-List Prohibited)

[Peter Otten]

ast wrote:

> 
> "Lawrence D’Oliveiro" <lawrencedo99 at gmail.com> a écrit dans le message de
> news:f5314bdd-a98f-4a16-b546-bd8efe4dd568 at googlegroups.com...
>> On Thursday, October 6, 2016 at 7:54:08 PM UTC+13, ast wrote:
>>> But there is no decorator, why ? Is python doing the conversion
>>> of funct2 to a descriptor itself, behind the scene ?
>>
>> Every function is already a descriptor. So you get the instance-method
>> behaviour automatically.
>>
>>> * static methods are decorated too
>>
>> This is to turn off the instance-method behaviour.
> 
> 
> I do not understand your answer.
> Consider this function:
> 
> def add(a, b):
>     return a+b
> 
> You say that a function is always stored as
> a descriptor object, so when I execute
> 
> sum = f(4, 6)
> 
> from which class it is supposed to come from ?

It doesn't matter. You can invoke it in the usual way

>>> def add(a, b): return a + b
... 
>>> add(2, 3)
5

and even put it into an instance:

>>> class A: pass
... 
>>> a = A()
>>> a.foo = add
>>> a.foo(3, 4)
7

Only when you put it into a class the first argument will be the instance

>>> A.bar = add
>>> a.bar(5, 6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: add() takes 2 positional arguments but 3 were given

because under the hood add.__get__() is invoked. Doing it manually:

>>> add.__get__(a)(5, 6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: add() takes 2 positional arguments but 3 were given

Unlike Python functions those implemented in C don't support the descriptor 
protocol out of the box, so

>>> A.baz = sum
>>> a.baz([1,2,3])
6

does not pass the instance a as an argument.