How to mix-in getattr after the fact?

Fri Oct 28 14:20:20 EDT 2011

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~

How to mix-in __getattr__ after the fact?

How to mix-in getattr after the fact?