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Date: Fri, 28 Oct 2011 11:20:20 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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To: dhyams <dhyams@gmail.com>,  python-list@python.org
Subject: Re: How to mix-in __getattr__ after the fact?
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:15112

dhyams wrote:
> Python 2.7.2
> 
> I'm having trouble in a situation where I need to mix-in the
> functionality of __getattr__ after the object has already been
> created.  Here is a small sample script of the situation:
> 
> =============snip
> 
> import types
> 
> class Cow(object):
>   pass
>   # this __getattr__ works as advertised.
>   #def __getattr__(self,a):
>   #   print "CAUGHT INCLASS: Attempting to get attribute ",a
> 
> 
> def attrgetter(self,a):
>    print "CAUGHT: Attempting to get attribute ",a
> 
> bessie = Cow()
> 
> bessie.__getattr__ = types.MethodType(attrgetter,bessie,Cow)
> 
> # here, I should see my printout "attempting to get attribute"
> # but I get an AttributeException
> print bessie.milk
> ======================snip
> 
> If I call __getattr__ directly, as in bessie.__getattr__('foo'), it
> works as it should obviously; so the method is bound and ready to be
> called.  But Python does not seem to want to call __getattr__
> appropriately if I mix it in after the object is already created.  Is
> there a workaround, or am I doing something wrongly?
> 
> Thanks,

Python only looks up __xxx__ methods in new-style classes on the class 
itself, not on the instances.

So this works:

8<----------------------------------------------------------------
class Cow(object):
   pass

def attrgetter(self, a):
    print "CAUGHT: Attempting to get attribute", a

bessie = Cow()

Cow.__getattr__ = attrgetter

print bessie.milk
8<----------------------------------------------------------------

~Ethan~