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, Skip _______________________________________________ Python-3000 mailing list Python-3000@python.org http://mail.python.org/mailman/listinfo/python-3000 Unsubscribe: http://mail.python.org/mailman/options/python-3000/archive%40mail-archive.com