Duncan Booth wrote: > danielx wrote: > > >>>> Foo.func = dan # <-- Appearantly, something magical happens here, > because... > >>>> Foo.func > ><unbound method Foo.dan> > >>>> f = Foo.func > >>>> f is dan # <-- things begins to look suprising here. > > False > >>>> ismethod(f) > > True > > > > Imagine my surprise. Why would Python do this? > > > > Nothing magical happens at the point you said. The assignment is just an > assignment and you saw that the function was stored unchanged when you > looked in the class's dictionary. > > The magic happens when you access a member of a class or an instance. If > the value is a descriptor then its __get__ method is called. Roughly > speaking, Foo.func is equivalent to: > > >>> Foo.__dict__['func'].__get__(None, Foo) > <unbound method Foo.dan> > > Python does this so that it can support other descriptor types such as > property, classmethod, staticmethod. > > You can see this happening if you define your own descriptor: > > >>> class MyDesc(object): > def __get__(self, *args): > print "__get__ called", args > return None > > > >>> d = MyDesc() > >>> Foo.oops = d > >>> Foo.oops > __get__ called (None, <class __main__.Foo at 0x00B3F930>) > > http://docs.python.org/ref/descriptor-invocation.html has a description of > this. Although it claims "Note that descriptors are only invoked for new > style objects or classes (ones that subclass object() or type())." the > descriptor mechanism is partially implemented for old style classes. > Several aspects of descriptors don't work properly though in old-style > classes which is one reason why you should always use new-style classes. > > > Privates don't have to be entirely absent from Klass.__dict__ > > (which would make Python not introspective); they can just be invisible > > when using the dot-syntax. > > You could implement that using a data descriptor, but if you are going to > prevent access to your private variables using the dot operator, then your > code is going to look pretty silly with a lot of references to > self.__dict__['theprivate'] which doesn't gain anything in readability over > self.__theprivate.
I believe you are talking about the same thing as Leif, but I can't quite tell. I'll need to read your paper as well :P. -- http://mail.python.org/mailman/listinfo/python-list
