On Mon, Oct 26, 2009 at 1:40 PM, Raymond Hettinger <python@rcn.com> wrote: ..

FWIW, here is a recipe that can retrieve canonical objects from any container, letting look-up a preferred representative of an equivalence class: http://code.activestate.com/recipes/499299/

This is very clever, but I am not sure whether existence of this recipe is an argument for or against adding functionality to sets. On one hand, it does provide a solution for O(1) retrieval of an equivalent item, but it is far from obvious. Overloading == with a mutating method is borderline black magic! Furthermore, it is not clear how to generalize this recipe to efficiently insert the value that is not found in the set. Finally, overloading both __eq__ and __getattr__ with python methods is likely to negate any performance gains of an O(1) over O(n) algorithm for typical sets of built-in types. The use case in this recipe is not addressed by the "return value from set.add(..)" proposal. One would need a different "get-like" method for that.