On Fri, 21 Dec 2007 10:30:31 -0800 (PST), Mangabasi <[EMAIL PROTECTED]> wrote: >On Dec 21, 1:11 pm, Jean-Paul Calderone <[EMAIL PROTECTED]> wrote: >> On Fri, 21 Dec 2007 09:49:51 -0800 (PST), Mangabasi <[EMAIL PROTECTED]> >> wrote: >> > [snip] >> >> >Hi Jean-Paul, >> >> >Sorry, I should have spelled this out in my post but I did not. For >> >several reasons I do not wish to couple the pos object with the Body >> >instances. In my case, I did not want pos objects (in my example they >> >were lists but they can be other objects as well) to care about their >> >parents. So the question is: How do we detect this in the Body? >> >> You can't. If you have this: >> >> b = Body([1, 2]) >> b.pos[0] = 3 >> >> Then Body gets asked for the value of the attribute; then, that >> object (a list in the case of this example) gets told to change >> its 0th element to 3. There is no way for Body to get in the >> way of the second message, except by doing what I said - adding >> a wrapper around the list so that it _can_ get in the way. >> >> This doesn't require the list to know anything about Body, though. >> _Body_ should apply the wrapper. >> >> For example, new-style: >> >> class Body(object): >> def get_pos(self): >> return ChangeNoticingListWrapper(self._pos) >> def set_pos(self, value): >> self._pos = value >> pos = property(get_pos, set_pos) >> def __init__(self, pos): >> self.pos = pos >> >> class ChangeNoticingListWrapper(object): >> def __init__(self, wrapped): >> self.wrapped = wrapped >> def __setitem__(self, index, value): >> print 'Changing', index, 'to', value >> self.wrapped[index] = value >> >> b = Body([1, 2]) >> b.pos[0] = 3 >> >> Jean-Paul > > >Hi Jean-Paul, > >I see what you are saying but my contrived example is not doing a good >job explaining the real issue I am having here. > >When you say "The Body gets asked for the value of the attribute" that >means that Body's __dict__ is being asked to provide a value >corresponding to its 'pos' key, right? Now I may want to ask a more >specific question here. How do I modify Body's __dict__ so that I can >perform a task when __getitem__ method of the __dict__ is called?
You can't do that. Fortunately, there's no reason to. Changing the behavior of __getitem__ on the __dict__ of a Body instance would let you address the exact same use-case as a property or a __getattr__ implementation lets you address. Maybe you are asking whether there is a special method called __getitem__ which is invoked when attributes are looked up? If so, yes, there is. You can also use a property, as in the example I gave. None of that has anything to do with looking up values in the result of the attribute lookup, though. For example, it doesn't help you change the behavior of "pos[0]". Jean-Paul -- http://mail.python.org/mailman/listinfo/python-list
