Minor annoyances with properties

[Francesco Bochicchio]

On 27 Mag, 14:37, eb303 <eric.brunel.pragma... at 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