Question on overriding implicit lookups

Bruno Desthuilliers bdesth.quelquechose at
Tue Oct 2 10:27:05 CEST 2007

David Ells a écrit :
> In Python we have a wonderful facility for customizing attribute
> access by defining __getattr__ or __getattribute__ in our classes.
> Unfortunately (or perhaps fortunately, for reasons I don't know), this
> facility only works for explicit attribute access, i.e. accessing
> What I am in need of, for reasons I'll keep out of the
> discussion for now, is a similar facility for implicit attribute
> access, such as the call to foo.__len__ when something like len(foo)
> is executed. I would like some way to customize access during these
> implicit lookups without having to define each one in the class
> myself. Essentially what I am looking for is a mechanism like
> __getattribute__, only one that will work for these implicit lookups
> to functions like __len__, __str__, __getitem__, and so on. Hopefully
> my question makes sense.

An obvious solution would be to define "each one" of these magic methods 
in a mixin class. Else, here's a Q&D but working solution :

class Test(object):
     def _get_magic_attribute(self, name):
         return "YADDA YADDA"

     for _name in ('__len__', '__str__', '__repr__'):
         exec "%s = lambda self, _name='%s' : " \
               "self._get_magic_attribute(_name)" % (_name, _name)
     del _name

For a cleaner version, you may want to use a custom metaclass, but that 
seems a bit overkill to me.

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