[Python-Dev] PEP 246: lossless and stateless

[James Y Knight]

On Jan 15, 2005, at 6:02 PM, Simon Percivall wrote:

> On 2005-01-15, at 23.50, Just van Rossum wrote:
>> Phillip J. Eby wrote:
>>
>>>> But it _does_ perform an implicit adaptation, via PyObject_GetIter.
>>>
>>> First, that's not implicit.  Second, it's not adaptation, either.
>>> PyObject_GetIter invokes the '__iter__' method of its target -- a
>>> method that is part of the *iterable* interface.  It has to have
>>> something that's *already* iterable; it can't "adapt" a non-iterable
>>> into an iterable.
>>>
>>> Further, if calling a method of an interface that you already have in
>>> order to get another object that you don't is adaptation, then what
>>> *isn't* adaptation?  Is it adaptation when you call 'next()' on an
>>> iterator? Are you then "adapting" the iterator to its next yielded
>>> value?
>>
>> That's one (contrived) way of looking at it. Another is that
>>
>>   y = iter(x)
>>
>> adapts the iterable protocol to the iterator protocol.
>
> Especially since an iterable can also be an object without an __iter__
> method but with a __getitem__ method. Calling __iter__ might get an
> iterator, but calling __getitem__ does not. That seems like adaptation.
> No? It's still not clear to me, as this shows, exactly what counts as
> what in this game.

I think that's wrong. To spell iter() in an adapter/interface world, 
I'd spell iter(obj) as:
   adapt(obj, IIterable).iterator()

Then, list, tuple, dict objects would specify that they implement 
IIterable. There is a default adapter from object->IIterable which 
provides a .iterate() method which creates an iterator that uses 
__getitem__ on the adaptee.

In my opinion, adapters provide a different view of an object. I can 
see treating list "as a" iterable, but not "as a" iterator.

James