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 _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
