How best to check for existance of an attribute?

[Roy Smith]

I've got a function which augments an object passed to it by adding a new 
attribute.  Something like this:

def addStuff (obj):
   obj.newAttr = "stuff"

I want it to be an error to call addStuff() more than once for a given 
object.  The simpliest way to do this would be with something like perl's 
defined() function:

def addStuff (obj):
   if defined (obj.newAttr):
      raise MultipleCallError
   else:
      obj.newAttr = "stuff"

What's the best way to simulate defined()?  I could think of a few 
possibilities.  For example, either of the following seem like they would 
work:

def addStuff (obj):
   if 'newAttr' in dir (obj):
      raise MultipleCallError
   else:
      obj.newAttr = "stuff"

def addStuff (obj):
   try:
      obj.newAttr
      raise MultipleCallError
   except AttributeError:
      obj.newAttr = "stuff"

Any reason to pick one over the other, or perhaps a third way?