On 19/05/2007 3.34, Raymond Hettinger wrote:
* Make sets listenable for changes (proposed by Jason Wells):
s = set(mydata) def callback(s): print 'Set %d now has %d items' % (id(s), len(s)) s.listeners.append(callback) s.add(existing_element) # no callback s.add(new_element) # callback
-1 because I can't see why sets are so specials (compared to other containers or objects) to provide a builtin implementation of the observer pattern. In fact, in my experience, real-world use cases of this pattern often require more attention to details (eg: does the set keep a strong or weak reference to the callback? What if I need to do several *transactional* modifications in a row, and thus would like my callback to be called only once at the end?). -- Giovanni Bajo