delegation pattern via descriptor

[kedra marbun]

On Jul 8, 4:02 pm, Bruno Desthuilliers <bruno.
42.desthuilli... at websiteburo.invalid> wrote:
> kedra marbun a écrit :
>
>
>
> > On Jul 7, 2:46 am, Bruno Desthuilliers
> > <bdesth.quelquech... at free.quelquepart.fr> wrote:
> >> Gregory Ewing a écrit :
>
> >>> Bruno Desthuilliers wrote:
> >>>> kedra marbun a écrit :
> >>>>> if we limit our discussion to py:
> >>>>> why __{get|set|delete}__ don't receive the 'name' & 'class' from
> >>>>> __{getattribute|{set|del}attr}__
> >>>>> 'name' is the name that is searched
> >>>> While it would have been technically possible, I fail to imagine any use
> >>>> case for this.
> >>> I think he wants to have generic descriptors that are
> >>> shared between multiple attributes, but have them do
> >>> different things based on the attribute name.
> >> I already understood this, but thanks !-)
>
> >> What I dont understand is what problem it could solve that couldn't be
> >> solved more simply using the either _getattr__ hook or hand-coded
> >> delegation, since such a descriptor would be so tightly coupled to the
> >> host class that it just doesn't make sense writing a descriptor for this.
>
> > yeah, i finally can agree descriptor isn't supposed to be used as
> > delegation in general, it should be the job of __getattr__
>
> > however i still think passing name would open up some other
> > possibilities of use
>
> Nothing prevents you to pass a "name" to the descriptor instance when
> instanciating it, ie:
>
> class Desc(object):
>      def __init__(self, name):
>          self.name = name
>     def __get__(self, inst, cls):
>         # ...
>     def __set__(self, inst, value):
>         # ...
>
> class Foo(object):
>      bar = Desc("bar")
>      baaz = Desc("baaz")
>
> Ok, this is not necessarily what you were looking for, but it's IMHO
> less brittle than relying on which attribute name was looked up (which
> is something the descriptor shouldn't have to care about).
>
>
>
> > btw, is there a common approach to let the interface of a class that
> > uses __getattr__, to include names that are delegated?
>
> In Python 2.x, not that I know (but it may have passed under my radar).
> If what you want it to automate delegation of a set of methods without
> too much manual coding, you can use a custom metaclass that will add the
> relevant methods to the class, based on (ie) a list (or mapping) of
> methods names. But that might be a bit overkill.
>
> > class A:
> >    def do_this(self): ...
>
> > class B:
> >    a = A()
>
> I don't see the point of using delegation on a class attribute. That's
> typically what inheritance is for.

ah, our friend mixin, it's just a sample though
>
> >    def do_that(self): ...
>
> >    def __getattr__(self, name):
> >            try:
> >                    return types.MethodType(getattr(self.a, name), self)
>
> Err... Did you try the simple way ?
>                          return getattr(self.a, name)

argh, that should be
class A:
   def do_this(self, ins): ...