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

[Josiah Carlson]

On Fri, Sep 12, 2008 at 10:33 AM, Guido van Rossum <guido at python.org> wrote:
> On Fri, Sep 12, 2008 at 10:13 AM,  <skip at pobox.com> wrote:
>>
>>    >> 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.
>
> But in SQL this would force alphabetical ordering so of course it
> would both return them in corresponding order. Maybe we should just
> drop this, it seems hardly relevant.

If the desire is to behave like a bsddb.btree instance, alphabetical
is ok.  Replacing the 'order by key' with 'order by rowid' is
reasonably sane, if alphabetical is explicitly undesired.  Really
there are 3 options: alphabetical ordering, rowid ordering, no
guaranteed ordering (which seems to be on-disk or rowid ordering, my
brief tests tell me nothing).

>> 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?
>
> That's a C API definition that hasn't been updated. If anything
> documents the concept of a mapping it would be the Mapping ABC in the
> collections module.

According to PEP 3119 on the mapping ABC: "i.e. iterating over the
items, keys and values should return results in the same order."

So... key ordered, or rowid ordered?

 - Josiah