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

[skip at pobox.com]

In issue 3783 (http://bugs.python.org/issue3783) the question was raised
about whether or not it's worthwhile making this guarantee:

    zip(d.keys(), d.values()) == d.items()

in the face of no changes to the mapping object.  At issue is whether the
SQL query should force a predictable order on the keys and values fetched
from the database or if that's just wasted CPU cycles.  Making it concrete,
should these two SELECT statements force a consistent ordering on the keys
and values retrieved from the database:

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

Currently SQLite does return the keys and values in the same, predictable,
order, but doesn't guarantee that behavior (so it could change in the
future).

While the discussion in the issue is related to this nascent dbm.sqlite
module, I think it's worth considering the more general issue of how
behavior non-dict mapping types should be required to share with the dict
type.

In the section "Mapping Types -- dict" in the 2.5.2 library reference:

    http://docs.python.org/lib/typesmapping.html

there is a footnote about ordering of keys and values:

    Keys and values are listed in an arbitrary order which is non-random,
    varies across Python implementations, and depends on the dictionary's
    history of insertions and deletions. If items(), keys(), values(),
    iteritems(), iterkeys(), and itervalues() are called with no intervening
    modifications to the dictionary, the lists will directly
    correspond. This allows the creation of (value, key) pairs using zip():
    "pairs = zip(a.values(), a.keys())". The same relationship holds for the
    iterkeys() and itervalues() methods: "pairs = zip(a.itervalues(),
    a.iterkeys())" provides the same value for pairs. Another way to create
    the same list is "pairs = [(v, k) for (k, v) in a.iteritems()]".

It's not entirely clear if this page is meant to apply just to dictionaries
or if (to the extent possible) it should apply to all mapping types.  I'm of
the opinion it should apply more broadly.  Others are not of that opinion.
Should the documentation be more explicit about this?

Comments?  

Thx,

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