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

Steven D'Aprano steve at pearwood.info
Tue Aug 24 01:56:27 CEST 2010

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.

It still doesn't cope with dynamically-generated attributes that are either expensive or have side-effects (both of which are probably poor design, but nevertheless I'm sure they're out there), but neither does the existing hasattr.

* Why do people typically use hasattr() instead getattr()?

> Aren't they are really trying to just determine > whether a key exists somewhere in the MRO? > If so, then doing anything more than that is probably a > surprise. Most users who call hasattr() probably don't even know what MRO means.

Well, yes, but most users never write getattr or getattribute methods either.

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.

-- Steven D'Aprano

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