On May 27, 8:56 pm, Francesco Bochicchio <bieff...@gmail.com> wrote: > On 27 Mag, 14:37, eb303 <eric.brunel.pragma...@gmail.com> wrote: > > > > > Hello all, > > > I've been using Python properties quite a lot lately and I've found a > > few things that are a bit annoying about them in some cases. I > > wondered if I missed something or if anybody else has this kind of > > problems too, and if there are better solutions than the ones I'm > > using ATM. > > > The first annoyance is when I want to specialize a property in a > > subclass. This happens quite often actually, and it is even sometimes > > the reason why a plain attribute is turned into a property: a subclass > > needs to do more things than the superclass when the property is > > updated for example. So, of course, my first try was: > > > class A(object): > > def __init__(self): > > self._p = None > > def _get_p(self): > > return self._p > > def _set_p(self, p): > > self._p = p > > p = property(_get_p, _set_p) > > class B(A): > > def _set_p(self, p): > > ## Additional things here… > > super(B, self)._set_p(p) > > > And of course, it doesn't work: the property has been bound to > > A._set_p in A, so any new definition of _set_p in any subclass does > > not replace the set method for the property. So I always have to add a > > line: > > p = property(A._get_p, _set_p) > > in the subclass too. This is a bit awkward to me, since I have to > > specify the superclass's name (super(…) can't be used, since it should > > take B as an argument, and B isn't defined yet…). Do I miss something? > > Is this the way to do it, or is there a better one? > > Don't know if is better, but you could add a level of indirection to > solve it > > class A(object): > def __init__(self): > self._p = None > def _get_p(self): > return self._p > def _set_p(self, p): > self._p = p > def _virtual_get_p (self): _get_p(self) > def _virtual_set_p (self,v): _set_p(self, v) > p = property(_virtual_get_p, _virtual_set_p) > > At this point, the subclasses of A can reimplement _get_p and _set_p > as they like (I think) > > Ciao > ----- > FB
Well, I've thought about that too and it should work, but that makes 2 function calls instead of one for every property access… I'd really like to avoid that. By the way, I think your 'virtual' methods should be written as: def _virtual_get_p (self): return self._get_p() def _virtual_set_p (self,v): self._set_p(v) Thanks anyway. - Eric - -- http://mail.python.org/mailman/listinfo/python-list
