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

[Benjamin Peterson]

2010/8/23 Steven D'Aprano <steve at pearwood.info>:
> On Tue, 24 Aug 2010 06:50:19 am Guido van Rossum wrote:
>
>> > * Is there anything that hasattr(obj, key) can or should do that
>> >   can't already be done with getattr(obj, key, None)?
>> >   If not, do we really need to change anything?
>>
>> getattr(obj, 'key', None) returns None when obj.key exists and has
>> the value None. The workaround is ugly.
>
> Why do you say it's ugly? It's a short, sweet, simple two-liner:
>
> mark = object()
> getattr(obj, 'key', mark) is not mark
>
> Nothing ugly about it at all. But if somebody really objected to using a
> two lines, they could put it in a utility function.

That's not all, though. You still have to test it with an "if" and
that gets cumbersome after a while.

>
> I have always thought that hasattr() does what it says on the box: it
> tests for the *existence* of an attribute, that is, one that statically
> exists rather than being dynamically generated. In other words, it is a
> key in the instance __dict__ or is inherited from the class __dict__ or
> that of a superclass, or a __slot__.
>
> Now that I know that hasattr doesn't do what I thought it does or what
> the name implies it does, it has little or no utility for me. In the
> future, I'll just write a try...except block and catch errors if the
> attribute doesn't exist.

Well, that's exactly what hasattr does...



-- 
Regards,
Benjamin