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Py3.0 updte: UserDict is going to survive into Py3.0 and will be moved 
into the collections module. DictMixin is replaced by the abstract 
base classes for mappings.
I think the discussion here has grown far beyond the original bug 
report. I recommend this one be closed and that you start a thread on 
python-3000 or python-dev if you want to propose altering basic APIs or 
moving the ABC code into C.
I can't say that I support your "UserDict is a kludge ..." comment. 
The class has its uses and the standard library will continue to use it 
where appropriate. 
FWIW, at one time I also thought all uses of UserDict would ultimately 
be supplanted by dict subclassing, but I found that it was somewhat 
difficult to remove in some circumstances and that its API had some 
advantages (i.e. it's easier to write subclass code like self.data
[computedkey]=computedvalue than to dispatch to dict.__setitem__(self, 
computedkey, computedvalue) -- the latter form is somewhat errorprone 
when the subclass methods become more complex).
Will look at 1367711 as you suggest.