On Sunday, February 3, 2013 10:35:30 PM UTC-6, Steven D'Aprano wrote: > On Sun, 03 Feb 2013 17:08:47 -0800, Saul Spatz wrote: > > > > > I don't understand what's going on at all. Can't I dynamically define > > > __getattr__? How should I go about it? > > > > Special "dunder" methods (DoubleUNDERscore) are looked up only on the > > class, not on instances. That means that if you try to dynamically > > provide a dunder method by assigning it to an instance, say with: > > > > self.__nonzero__ = lambda self: True > > > > it will *not* be automatically used by Python. The only way to > > dynamically add dunder methods is to add them to the class, but of course > > that means that all instances see the same method. > > > > > > In your case, you try doing this inside the __init__: > > > > self.__getattr__ = lambda x, name: getattr(self.canvas, name) > > > > > > Why not just define a __getattr__ method the normal way? In your class, > > define a method: > > > > def __getattr__(self, name): > > return getattr(self.canvas, name) > > > > This technique is called automatic delegation. > > > > Even if this does not quite do what you are trying to do, you will > > eliminate one major stumbling block and be that much closer to a working > > solution. > > > > > > > By the way, another thing that > > > didn't work was calling the method delegate instead of __getattr__. > > > Then after the constructor call, I wrote self.__getattr__ = > > > self.delegate. This crashed as before on self.create_text. > > > > It is pointless to tell us that Python "crashed" if you don't show us > > *exactly* what you did, by copying and pasting the *actual* code, > > complete with the full traceback. Otherwise we are just guessing what you > > did and what error you saw. > > > > I'm pretty confident that Python didn't "crash", in the commonly accepted > > meaning of the word meaning a core dump or equivalent. I'm guessing you > > meant that Python raised a perfectly normal exception, like > > > > > > Traceback (most recent call last): > > ... > > NameError: name 'self' is not defined > > > > > > If you pay attention to the exception messages that Python prints for > > you, you will not only find it easier to debug your code, but you can > > also ask more sensible questions using accepted terminology. > > > > > > > > -- > > Steven
Thanks. The class versus instance lookup explains it. I didn't mean that python crashed, but that my app did. Now I have another question. If dunder methods are looked up only in the class, not the instance, why did defining __nonzero__ the way I did work? Shouldn't I have had to define it with a def? Is __nonzero__ a special case? -- http://mail.python.org/mailman/listinfo/python-list
