* The 2.6-backported Mapping ABC has the 3.0 dict API, that is, it uses keys() that returns a view etc.
Curious to hear what Guido thinks about this one. A nice use of the Mapping ABC is to be able to get 3.0 behaviors. I thought that was the whole point of all these backports. If the ABC gets altered, then it just makes the 2-to-3 conversion harder.
* The 2.6 UserDict is not registered as a mapping.
Since the API's are not currently the same, it makes sense that UserDict is not registered. If the Mapping ABC does get changed, only IterableUserDict should get registered. A regular UserDict does not comply.
* collections.deque isn't registered as a MutableSequence.
Deque's do not support count(), insert() or __iadd__(). They should not be registered. General purpose indexing into a deque is typically a mis-use of the data structure. It was provided only to make it easier to substitute for lists in apps the operate only one ends (i.e.d[0], d[1], d[-1], d[-2] but not d[i] to somewhere in the middle).
If there are no objections, I will correct these issues in the 2.6 and 3.0 branches.
I think none of these changes should be made. Raymond