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eval() can set items in the locals, e.g. by using list comprehension.  Moreover the ability to exec in custom dicts would be extremely useful too, e.g. to catch definitions while they appear.

If you really want to avoid any performance impact, using PyDict_CheckExact() in all critical parts looks good (do not use PyDict_Check(), it's both slower and not what we want because it prevents subclasses of dicts to override __xxxitem__).