On 14 Mar 2005 01:19:23 -0800, "Martin Miller" <[EMAIL PROTECTED]> wrote:
>In answer to my question about instance properties not working, Bengt >Richter suggest using: >> > >>> class InstProp(object): >> > ... def __getattribute__(self, attr): >> > ... p = object.__getattribute__(self, attr) >> > ... if isinstance(p, property): return p.__get__(self) >> > ... return p > >and more generally for any descriptor object: > >> >>> class InstProp(object): >> ... def __getattribute__(self, attr): >> ... p = object.__getattribute__(self, attr) >> ... if hasattr(p, '__get__'): return p.__get__(self, >type(self)) >> ... return p > >Both the above makes the '__get__' method of any property attributes of >the instance to be used. However, it does not cause attempts to be made >to access their "__set__' methods when assigning to them (which should >fail in my case because my properties don't have such a method because >they are read-only). > >Just overriding '__getattribute__' alone is insufficent to make >instance property/descriptor attributes fully function. To do so also >requires overriding the __setattr__ method so it checks for a '__set__' >method and then uses it if one is found (or delegates it to >object.__setattr__ if not). > >Similarily, an override for '__delattr__' would also be need for >complete property functionality, I believe. > As I said in my first post, """ If you want robust writeable and/or deleteable properties, you will have to do a little more work, but it can be done. """ I'm still leaving it as an exercise ;-) >For just making instance attributes read-only, it seems to be that the >simplist solution would be to override __setattr__ and make it check to >see if the attribute is write protected or not, which is really all I >need for the specific task I'm trying to accomplish (which is >esstentially what Jeremy Bowers said in his reply). > >What still puzzles me, though, is why all the above to make properties >work on instances is necessary in the first place. It's certainly not >clear (to me) from what is said in the How-to at: >> >http://users.rcn.com/python/download/Descriptor.htm#invoking-descriptors >I suspect that it may be simply a performance issue, in other words, it >was considered too slow to check for instance property/discriptors -- >although *why* is not clear to me. I suspect the desired semantics re precedence have evolved to make normal programs easily implementable, and that probably drove the implementation: """ For objects, the machinery is in object.__getattribute__ which transforms b.x into type(b).__dict__['x'].__get__(b, type(b)). The implementation works through a precedence chain that gives (my added [1,2,3]) [1] data descriptors priority over instance variables, [2] instance variables priority over non-data descriptors, [3] and assigns lowest priority to __getattr__ if provided. The full C implementation can be found in PyObject_GenericGetAttr() in Objects/object.c. """ Regards, Bengt Richter -- http://mail.python.org/mailman/listinfo/python-list
