[Python-ideas] Adding collections.abc.Ordered

[Guido van Rossum]

I'm warming up slightly to the idea of this ABC.

I've re-read Amir's post, and if I found myself in his situation I would
have used a combination of documenting that __slots__ needs to be ordered
and at runtime only checking for a few common definitely-bad cases. E.g.
for exactly set or dict (not subclasses, since OrderedDict inherits from
dict) would cover most of the common mistakes: most people will use a
literal in their __slots__ definition so we just need to watch out for the
common literals. Those users who are sophisticated enough to use some
advanced mapping type in their __slots__ should just be expected to deal
with the consequences.

But the Ordered ABC, while not essential (unless you're a perfectionist, in
which case you're in the wrong language community anyways :-) still fills a
niche.

The Ordered ABC should have no additional methods, and no default
implementations. I think it should apply to collections but not to
iterators. It should apply at the level of the read-only interface.
Sequence is always Ordered. Mapping and Set are not by default, but can
have it added. OrderedDict is the prime (maybe only) example -- it's a
MutableMapping and Ordered. We might eventually get an OrderedSet.

A sorted set or mapping (e.g. one implemented using some kind of tree)
should also be considered Ordered, even though otherwise this is a totally
different topic -- while some other languages or branches of math use
"order" to refer to sorting (e.g. "partial ordering"), in Python we make a
distinction: on the one hand there's sorted() and list.sort(), and on the
other hand there's OrderedDict.

So, I think that the Ordered ABC proposed here is totally well-defined and
mildly useful. It may be somewhat confusing (because many people when they
first encounter the term "ordered" they think it's about sorting -- witness
some posts in this thread). The use cases are not very important. I guess
I'm -0 on adding it -- if better use cases than Amir's are developed I
might change to +0. (I don't care much about Serhiy's use cases.)

Sorry for the rambling.

-- 
--Guido van Rossum (python.org/~guido)
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