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Hmm... I like this!

Of course, I am wary of adding *yet another* special double-
underscore function to satisfy a single special purpose, but 
this would satisfy all of *my* needs, and without any 
performance impact for lookups that are FOUND. Lookups that 
are NOT found would have a slight performance degrade (I 
know better than to speculate about the size of the effect 
without measuring it). I'm not really sure what percentage of 
dict lookups succeed.

At any rate, what are these "other contexts" you mention in 
which a __keyerror__ would "also be useful"? Because if we 
can find other places where it is useful, that significantly 
improves the usefulness.

OTOH, if the tests can be done ONLY for eval (I don't really 
think that's possible), then I'm certain no one cares about the 
performance of eval.