[Python-Dev] 'hasattr' is broken by design

[Guido van Rossum]

On Mon, Aug 23, 2010 at 3:45 PM, P.J. Eby <pje at telecommunity.com> wrote:
> At 06:12 PM 8/23/2010 -0400, Yury Selivanov wrote:
>>
>> BTW, is it possible to add new magic method __hasattr__?  Maybe not
>> in Python 3.2, but in general.
>
> In order to do this properly, you'd need to also add __has__ or __exists__
> (or some such) to the descriptor protocol; otherwise you break descriptors'
> ability to operate independently of the class they're used in.  You would
> probably also need a __hasattribute__, in order to be able to properly
> synchronize with __getattr__/__getattribute__.
>
> Seems like overkill to me, though, as I'm not sure how such a protocol
> actually helps ORM or persistence schemes (and I've written a few).  Pretty
> much, if you're trying to check for the existence of an attribute, you're
> probably about to be getting that attribute anyway.  (i.e. why query the
> existence of an attribute you *don't* intend to use?)

Right. This sounds like way too big a gun to kill this particular
mosquito. If Yury wants to write a PEP I won't stop him, but I expect
that it will be rejected for want of important use cases compared to
the complexity of the solution. There just are too many places that
would be affected, for too little value.

So just be warned.

OTOH I still think that fixing hasattr() to be mroe like getattr(obj,
key, None) has a high value and a relative low risk.

-- 
--Guido van Rossum (python.org/~guido)