Another reason this issue is really important, is that the lack of a
consistent iter() interface for dbm.* makes shelve iteration not
scalable; i.e. trying to iterate on a Shelf will run self.dict.keys() to
load the entire index into memory.  This seems contrary to a primary
purpose of shelve, namely to store the index on-disk so as to avoid
having to keep the whole index in memory.  

I suspect that for most users, shelve is the main way they will access
the dbm.* interfaces.  Therefore, fixing the dbm.* interfaces so that
shelve is scalable seems like an important need.

Once dbm and gdbm support the iterator protocol, it will be trivial to
add an __iter__() method to shelve.Shelf, that simply returns
iter(self.dict).