[Python-ideas] of properties and metaclasses
rrr at ronadam.com
Tue Jul 24 18:25:44 CEST 2007
Tal Einat wrote:
> On 7/24/07, *Ron Adam* <rrr at ronadam.com <mailto: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
> 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
> 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
> reuse parts.
> Pardon me for interrupting...
You aren't interrupting at all, but adding to it. ;-)
> 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.
Yes you are correct and it is why I said at the beginning one of the
problems is you can't make a lot of these without resorting to meta
> Perhaps I'm missing something, but I can't see the actual benefit of
> your suggestions (besides inline-ing of properties' names).
Inline-ing property names isn't really the point at all. A much bigger
change is having a way to use them outside of classes for indirect references.
foo = ['a', 'b', 'c', 'd']
foo[n] = value
print thirdfoo ---> 'c'
thirdfoo = 'z' # Doesn't rebind thirdfoo to 'z'!
print thirdfoo ---> 'z'
print foo ---> ['a', 'b', 'z', 'd']
Now as far as re-use goes, it could be more like a class with an __init__
method. Then it becomes almost exactly like the current descriptor objects
too. (Maybe a good thing.)
The reason 'class' is replaced by 'property' is that it needs some way to
tell the interpreter that this has different access characteristics than a
normal name/object binding so it can be used outside of a class.
That change (potentially) makes descriptors/properties independent from the
classes they are in.
Another way to do it might be to have a indirect name space. Then it
becomes a matter of moving a class (or suitable function) to "indirect" to
activate it as a local descriptor. Or "__indirect__" in the case of class
And yes this is still a rather abstract idea. The question is would it be
beneficial to generalize and formalize descriptors and properties? (but not
necessarily in the exact manner I'm describing.)
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