[Python-Dev] Py2.6 buildouts to the set API

Giovanni Bajo rasky at develer.com
Sat May 19 13:33:11 CEST 2007

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

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