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

[Brett Cannon]

On Mon, Aug 23, 2010 at 17:04, Benjamin Peterson <benjamin at python.org> wrote:
> 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.

It is also non-obvious to any beginner. Are we really going to want to
propagate the knowledge of this trick as a fundamental idiom? I would
rather leave hasattr in that instance. But I'm +1 on only swallowing
AttributeError.

-Brett

>
>>
>> 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
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