bruno modulix wrote:
.....
>
> Could you elaborate ? Or at least give an exemple ?
.....
in answer to Bengt & Bruno here is what I'm sort of playing with. Alex suggests
class change as an answer, but that looks really clunky to me. I'm not sure
what
Alex means by
> A better design might be to use, instead of the builtin
> type 'property', a different custom descriptor type that is specifically
> designed for your purpose -- e.g., one with a method that instances can
> call to add or remove themselves from the set of "instances overriding
> this ``property''" and a weak-key dictionary (from the weakref module)
> mapping such instances to get/set (or get/set/del, if you need to
> specialize "attribute deletion" too) tuples of callables.
I see it's clear how to modify the behaviour of the descriptor instance, but is
he saying I need to mess with the descriptor magic methods so they know what
applies to each instance?
## my silly example
class ObserverProperty(property):
def __init__(self,name,observers=None,validator=None):
self._name = name
self._observers = observers or []
self._validator = validator or (lambda x: x)
self._pName = '_' + name
property.__init__(self,
fset=lambda inst, value: self.__notify_fset(inst,value),
)
def __notify_fset(self,inst,value):
value = self._validator(value)
for obs in self._observers:
obs(inst,self._pName,value)
inst.__dict__[self._pName] = value
def add(self,obs):
self._observers.append(obs)
def obs0(inst,pName,value):
print 'obs0', inst, pName, value
def obs1(inst,pName,value):
print 'obs1', inst, pName, value
class A(object):
x = ObserverProperty('x')
a=A()
A.x.add(obs0)
a.x = 3
b = A()
b.x = 4
#I wish I could get b to use obs1 instead of obs0
#without doing the following
class B(A):
x = ObserverProperty('x',observers=[obs1])
b.__class__ = B
b.x = 7
--
Robin Becker
--
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