On Aug 2, 7:05 am, Bruno Desthuilliers <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] a écrit : > (snip) > > > Last post -- I swear. > > > I failed to realize that it's all part of an extremely well defined > > attribute resolution protocol, and handled via the descriptor > > specification. Discussing descriptors was on the TODO list for the > > type/class unification document, but there's a very fulfilling > > explanation by Raymond > > Hettinger:http://users.rcn.com/python/download/Descriptor.htm > > Also, the official doc is here:http://docs.python.org/ref/descriptors.html > > > Maybe the documentation for the property builtin should make reference > > to the descriptor specification? If nobody thinks this is silly, I'll > > submit a documentation patch in a few days. > > What to say ? As an old-time pyton user (well... 7+ years now) and > meta-programming addict, it's nothing new to me, but the fact that you > didn't realize it is certainly a clear indication that documentation is > not really up to date, so yes, submitting a patch is certainly a pretty > good idea. > > > Sorry for the spam -- hope someone besides me learns from it! > > What's to be learned is mostly that we (the whole community) should take > more care of the doc (which I never contributed FWIW, so I'm the first > one that should feel in guilt here).
Actually I was wrong about this! Of course, the problem was insufficient test coverage :) The key addition: assert spam.useful_attr == 12 # FAILS!!! I failed to realize it in the last posts... When you don't declare a descriptor as a class attribute the descriptor protocol isn't invoked! The descriptor protocol looks in the /class/ attributes -- it transforms spam.useful_attr to type(spam).__dict__['useful_attr'].__get__(spam, type(spam)), the caveat being /type(spam)/. >From the doc: (http://users.rcn.com/python/download/Descriptor.htm) The implementation works through a precedence chain that gives data descriptors priority over instance variables, instance variables priority over non-data descriptors, and assigns lowest priority to __getattr__ if provided. The data descriptors are only looked up in the class attributes -- the instance attributes are returned as-is, so the above assertion fails! The addition of 'print spam.useful_attr' produces <property object at 0x[something]>, because the portion of the descriptor protocol that invokes __get__ is does not cover instance attributes (as declared in __init__). So, we're back to the original problem, with a bit more insight. What if we declare the useful_attr property as a class attribute in the constructor after the LibraryClass initialization function? It only works once! Once you set a class attribute on the first instantiation it's obviously there the next time you try to instantiate. Now our test looks like this: def test(): class _Fake(object): pass external_obj = _Fake() external_obj.proxy_useful_attr = 12 spam = MyInheritedClass(external_obj) assert spam.useful_attr == 12 eggs = MyInheritedClass(external_obj) assert eggs.useful_attr == 12 Which provides coverage for the INCORRECT class-attribute-on- instantiation solution, since eggs won't be able to instantiate. The solution which fails this test looks like this: class MyInheritedClass(LibraryClass): # WRONG! Don't use this. def __init__(self, external_obj): LibraryClass.__init__(self) def get_useful_attr(would_be_self): # has to be unbound return would_be_self._external_obj.proxy_useful_attr MyInheritedClass.useful_attr = property(get_useful_attr) self._external_obj = external_obj At this point it seems like the only viable option is to hack getattribute to add special cases. To do this, we continue to use the unbound method getter as our argument to property, but add the getattribute hack which allows for specified instance attributes that have the descriptor protocol invoked on them: class MyInheritedClass(LibraryClass): # ... def __init__(self, external_obj): LibraryClass.__init__(self) def get_useful_attr(would_be_self): return would_be_self._external_obj.proxy_useful_attr self.useful_attr = property(fget=get_useful_attr) self._external_obj = external_obj def __getattribute__(self, attr_name): """ Hacks getattribute to allow us to return desired instance attributes with the descriptor protocol. """ instance_descrs = ['useful_attr'] if attr_name in instance_descrs and attr_name in self.__dict__: attr = self.__dict__[attr_name] return attr.__get__(self, type(self)) return object.__getattribute__(self, attr_name) This passes the above test, but obviously requires a hack -- is there a more elegant way to do it? - Chris
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