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Re: How to mix-in __getattr__ after the fact?

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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~

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How to mix-in __getattr__ after the fact? dhyams <dhyams@gmail.com> - 2011-10-28 10:34 -0700
  Re: How to mix-in __getattr__ after the fact? Jerry Hill <malaclypse2@gmail.com> - 2011-10-28 14:14 -0400
  Re: How to mix-in __getattr__ after the fact? Ethan Furman <ethan@stoneleaf.us> - 2011-10-28 11:20 -0700
  Re: How to mix-in __getattr__ after the fact? Lie Ryan <lie.1296@gmail.com> - 2011-10-29 13:26 +1100
    Re: How to mix-in __getattr__ after the fact? dhyams <dhyams@gmail.com> - 2011-10-31 05:01 -0700
      Re: How to mix-in __getattr__ after the fact? DevPlayer <devplayer@gmail.com> - 2011-11-01 22:50 -0700
      Re: How to mix-in __getattr__ after the fact? Lie Ryan <lie.1296@gmail.com> - 2011-11-08 15:17 +1100
        Re: How to mix-in __getattr__ after the fact? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-11-08 05:58 +0000
      Re: How to mix-in __getattr__ after the fact? Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-11-08 11:21 +0100

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