[Python-Dev] PEP 246: lossless and stateless

[Phillip J. Eby]

At 06:56 PM 1/14/05 +0100, Just van Rossum wrote:
>Phillip J. Eby wrote:
>
> > For example, if there were a weak reference dictionary mapping
> > objects to their (stateful) adapters, then adapt() could always
> > return the same adapter instance for a given source object, thus
> > guaranteeing a single state.
>
>Wouldn't that tie the lifetime of the adapter object to that of the
>source object?

Well, you also need to keep the object alive if the adapter is still 
hanging around.  I'll get to implementation details and alternatives in the 
PEP.


>Possibly naive question: is using adaptation to go from iterable to
>iterator abuse? That would be a clear example of per-adapter state.

I don't know if it's abuse per se, but I do know that speciifying whether a 
routine takes an iterable or can accept an iterator is often something 
important to point out, and it's a requirement that back-propagates through 
code, forcing explicit management of the iterator's state.

So, if you were going to do some kind of adaptation with iterators, it 
would be much more useful IMO to adapt the *other* way, to turn an iterator 
into a reiterable.  Coincidentally, a reiterable would create per-object 
state.  :)

In other words, if you *did* consider iterators to be adaptation, it seems 
to me an example of wanting to be explicit about when the adapter gets 
created, if its state is per-adapter.  And the reverse scenario 
(iterator->reiterable) is an example of adaptation where shared state could 
solve a problem for you if it's done implicitly.  (E.g. by declaring that 
you take a reiterable, but allowing people to pass in iterators.)