Oh, yes. I noticed that the pep3119 defines return value of method MutableMapping.keys() as Set, as well as method items(). So the implementation of dumb.keys() and dump.items() are not correct since they all return lists while the class inherits MutableMapping. 

The implementations in my patch should also be corrected since I made the same mistake. Besides, since issue6045 has already added get(), I need to update my patch. I will do it later.

And who can tell the specification of MutableMapping.update()? The pep3119 lacks of it. Should I follow the implementation in the ABC class MutableMapping?