There was a somewhat ancient discussion on OrderedDict and OrderedSet before: http://mail.python.org/pipermail/python-dev/2005-March/051915.html The resolution seemed to be that neither of them should be in stdlib. Now that OrderedDict is in and Raymond Hettinger has created a solid OrderedSet implementation: http://code.activestate.com/recipes/576694/ , could the latter also be included in collections? Here's a very generic use-case: def _update_key(dct, key, val): """ Update a key in dict *dct*. If they key already exists in *dct* but the value doesn't, a set of previous values is created and the value added to it. """ if key in dct: if dct[key] == val: return s = set(dct[key]) s.update(val) dct[key] = s else: dct[key] = val The problem is that I both need to remove duplicates and retain insertion order like list.append().