[Python-Dev] Re: Confusing "hasattr" behaviour

Terry Reedy tjreedy at udel.edu
Thu Feb 24 22:25:07 CET 2005

"J. David Ibanez" <jdavid at itaapy.com> wrote in message 
news:421B8C4B.5050003 at itaapy.com...

Given that the behavior of hasattr is clearly defined in Lib Manual 2.1 as 
equivalent to

def hasattr(obj, name):
    getattr(obj, name)
    return True
   return False

I am not sure what could be confusing about it.  It is a simple getattr 
wrapper converting 'got something' to True and 'did not get anything' 
(raised an exception instead) to False.  Users should know this so they 
don't wastefully write 'if hasattr(o,n): x = getattr(o,n)'

>Today I have spent a while to hunt down a couple of bugs in my 
>because "hasattr" was catching the exceptions.

If you want a different behavior, you can write your own version now, 
without waiting for a future that may never come, that only converts 
AttributeError to False and that possibly collapses all others to one type 
of your choosing.  To make sure that even this does not mask a bug, one 
needs to make sure that .__getattr__ methods do not pass on unintended 
bug-indicating AttributeErrors.

Terry J. Reedy

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