[Python-Dev] 'hasattr' is broken by design (original) (raw)

Brett Cannon brett at python.org
Tue Aug 24 02:21:50 CEST 2010

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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