[Python-ideas] Add a way to test for a descriptor in Python Code

Steven D'Aprano steve at pearwood.info
Tue Oct 30 06:30:31 EDT 2018

On Tue, Oct 30, 2018 at 04:40:40AM -0400, Joy Diamond wrote:

> """
> For classes, the machinery is in type.__getattribute__() which transforms
> B.x into B.__dict__['x'].__get__(None, B). In pure Python, it looks like:
> def __getattribute__(self, key):
>     "Emulate type_getattro() in Objects/typeobject.c"
>     v = object.__getattribute__(self, key)
>     if hasattr(v, '__get__'):
>         return v.__get__(None, self)
>     return v
> """
> However, the call to `hasattr(v, '__get__')` appears to me to be incorrect.

I agree, but only because it fails to take into account that dunder 
methods like __get__ are only looked up on the class, not the instance. 
I believe a more accurate eumulation would be:

    if hasattr(type(v), '__get__'):
        return type(v).__get__(None, self)

Actually, on further investigation, I think it ought to be:

if inspect.hasattr_static(type(v), '__get__')

except that there is no hasattr_static, there's only a getattr_static. 
So perhaps there ought to be a hasattr_static as well.

> The question is *NOT* whether 'v' has an attribute '__get__'; *BUT* whether
> `v` has a symbol `__get__` in any of the classes in it's method resolution
> order.

What's the difference as you see it?


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