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? -- http://mail.python.org/mailman/listinfo/python-list
