[Python-ideas] Where to put non-collection ABCs (was: Deprecating the old-style sequence protocol)

[Brett Cannon]

Speaking of using ABCs more, where should we put ABCs which have nothing to
do with collections? As of right now all ABCs seem to get shoved into
collections.abc, but e.g. Awaitable and Coroutine are not types of
collections. I personally want to add a context manager ABC with a default
__exit__.

I opened http://bugs.python.org/issue25637 to discuss this, but I figured a
wider discussion wouldn't hurt. Some suggest just putting the ABCs into the
abc module. We could create an interfaces module (top-level or a submodule
of ABC). The other option is to put the ABCs in subject-specific modules,
so my context manager one would go into contextlib (either top-level or an
abc submodule); don't know where the coroutine ones would go since it might
be overloading asyncio if we out them there.

Anyway, the key point is collections.abc is starting to get non-collections
stuff and if we are going to start pushing ABCs more we should decide how
we want to organize them in general in the stdlib and instead of dumping
them into collections.abc.

On Sun, Dec 27, 2015, 09:35 Guido van Rossum <guido at python.org> wrote:

> I think there's a lot of interesting stuff in this thread. Personally I
> don't think we should strive to distinguish between mappings and sequences
> structurally. We should instead continue to encourage inheriting from (or
> registering with) the corresponding ABCs. The goal is to ensure that
> there's one best-practice way to distinguish mappings from sequences, and
> it's by using isinstance(x, Sequence) or isinstance(x, Mapping).
>
> If we want some way to turn something that just defines __getitem__ and
> __len__ into a proper sequence, it should just be made to inherit from
> Sequence, which supplies the default __iter__ and __reversed__.
> (Registration is *not* good enough here.) If we really want a way to turn
> something that just supports __getitem__ into an Iterable maybe we can
> provide an additional ABC for that purpose; let's call it a HalfSequence
> until we've come up with a better name. (We can't use Iterable for this
> because Iterable should not reference __getitem__.)
>
> I also think it's fine to introduce Reversible as another ABC and
> carefully fit it into the existing hierarchy. It should be a one-trick pony
> and be another base class for Sequence; it should not have a default
> implementation. (But this has been beaten to death in other threads -- it's
> time to just file an issue with a patch.)
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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