[Python-Dev] ABC issues

[Guido van Rossum]

On Tue, May 27, 2008 at 10:44 AM, Raymond Hettinger <python at rcn.com> wrote:
>>>> * The 2.6-backported Mapping ABC has the 3.0 dict API,
>>>>  that is, it uses keys() that returns a view etc.
>>>
>>> Curious to hear what Guido thinks about this one.
>>> A nice use of the Mapping ABC is to be able to
>>> get 3.0 behaviors.  I thought that was the whole
>>> point of all these backports.  If the ABC gets altered,
>>> then it just makes the 2-to-3 conversion harder.
>>
>> It's wrong if the ABC doesn't describe the behavior of actual
>> implementations; that is its primary purpose, the mixin class is a
>> nice side benefit.
>
> ISTM, the one purpose of backporting is to make 2.6 closer to 3.0.  Altering
> the API will just make them further apart.

Well, the ABCs have two sides -- they describe the API and they
provide a mix-in. I feel strongly that the primary function of ABCs is
to describe the API (in fact many ABCs do only that, e.g. look at
Sized and Container). I want isinstance({}, collections.Mapping) to be
true.

If you want to use the 3.0 mixins in 2.6, perhaps an alternate set of
APIs could be imported from the future? E.g. from future_collections
import Mapping. IIRC a similar mechanism was proposed for some
built-in functions, even though I see no traces of an implementation
yet.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)