OK, sorry, I was under the impression that the global binding was still 
available (I can't find anything to the contrary here 
http://docs.python.org/reference/simple_stmts.html#assignment-statements 
) but it's obviously using a static definition of scope.

The error message isn't super helpful, though. :) Would it make sense to 
add a "(non-local "tmp" is shadowed)"? I guess that's easy to detect?