[Florent Guillaume]
> In storage.py there's a comment saying that only a few types of pickles
> for the class descriptions are written, even if more are read. But
> actually in DB.py there's this code that creates a new root if oid z64
> doesn't exist:
>              root = PersistentMapping()
>              ...
>              p.dump((root.__class__, None))
>              p.dump(root.__getstate__())
> This is actually format #5 and according to serialize.py it's not used
> anymore for writing. I guess the root is an exception.
> I haven't checked if just justing p.dump(root.__class__) works but it
> should.

I agree with Jim:  change it.  I expect that keeping this served a purpose
long ago, to make sure that databases written by a then-new ZODB could still
be read by an older ZODB.  That's the same reason we keep the (objectively
idiotic) "distinction" between PersistentDict and PersistentMapping (which
are conceptually identical, although their implementations keep getting out
of synch :-( -- PersistentDict should go away).

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