Yes, that's the way it works (and is intended to work, for performance reasons).  The documentation on this could be improved...while it does say globals must be a dict and that locals can be any mapping object, it does it in a sentence that is a bit confusing in this context, and it doesn't make it clear that it has to be a "real" dict, not a subclass.  (I wouldn't be surprised if that sentence was written back when you couldn't subclass dict.)