[Python-ideas] of properties and metaclasses

Tal Einat taleinat at gmail.com
Tue Jul 24 12:30:21 CEST 2007

On 7/24/07, Ron Adam <rrr at ronadam.com> wrote:
> >> The same object in a class might look like...
> >>
> >> property foo(self):
> >>     self.__value__ = None
> >>     def __get__(self):
> >>         return self.__value__
> >>     def __set__(self, value):
> >>         self.__value__ = value
> >
> > In the first version, how is anything supposed to know that
> > the __value__ inside the function is an implicit attribute
> > of self rather than a local variable of the function
> In the first example self isn't implicit, it's non-existent.  It would
> work
> more like a generator that yields it value on gets, and receives its value
> on sets.
> In the second version self is passed to the functions explicitly so it can
> work in an instance.  But other than that it's the same.  It could also
> keep state between calls, in which case all instances of that class would
> share those values.  (just like instances share class attributes)
>    class bar(object):
>       property foo(self):
>           countgets = 0
>           self.__value__ = None
>           def __get__()
>               countgets += 1
>               return countgets, self.__value__
>           def __set__(value)
>               self.__value__ = value
> In this case countgets will be the total gets of all subclasses of bar.
> This could be done with a class attribute as well, but that breaks the
> idea
> of having a single self contained component.  Having self contained
> components is more conducive to a modular design where you can more easily
> reuse parts.

Pardon me for interrupting... Don't the existing descriptors do all of this,
with almost exactly the same syntax? You just have to set a class variable
to an instance of the descriptor class, which is one extra line of code.
Additionally, this allows reuse of a descriptor class for multiple
properties, which as-far-as-I-can-tell your models don't.

Perhaps I'm missing something, but I can't see the actual benefit of your
suggestions (besides inline-ing of properties' names).

- Tal
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