This looks like a documentation issue:  it's well documented that in the exec statement, the globals dictionary must be a dict.  What's not so clear from the documentation (AFAICT) is that it must actually have *type* dict, rather than merely being an instance of dict.  (Or, from experimentation, it *can* be an instance of a dict subclass, but the underlying C-implemented dict methods are called directly, so overloads for __getitem__ and the like don't have any effect.)