[Python-3000] How much should non-dict mappings behave like dict?

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    >> select key from dict order by key
    >> select value from dict order by key

    Guido> What's the purpose of the "order by key" clauses here? Doesn't
    Guido> that force the return order? Perhaps you meant to leave those
    Guido> out?

It's simply to guarantee that the order of the elements of values() is the
same as the order of the elements of keys().  Again, I was thinking that
this property: zip(d.keys(), d.values()) == d.items() was a desirable
property of mappings, not just of the CPython dict implementation.

So is there a definition of what it means to be a mapping?  Maybe this page
in the C API doc?

    http://docs.python.org/api/mapping.html

>From that I infer that a mapping must offer these methods: keys, values,
items, __len__, __contains__, __getitem__, __setitem__ and __delitem__.  No
guarantee about the ordering of keys, values and items is made.  Can we
settle on something like this and spell it out explicitly somewhere in the
3.0 docs?

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