[Python-ideas] __iter__ implies __contains__?

[Guido van Rossum]

On Sun, Oct 2, 2011 at 10:20 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Terry Reedy wrote:
>
>> I had the same reaction as Guido. Iteration is the *only* generic way to
>> tell if an item is in a sequence or other collection.
>
> I think the root cause of this problem is our rather cavalier
> attitude to the distinction between iterables and iterators.
> They're really quite different things, but we started out with
> the notion that "for x in stuff" should be equally applicable
> to both, and hence decided to give every iterator an __iter__
> method that returns itself. By doing that, we made it
> impossible for any generic protocol function to reliably tell
> them apart.
>
> If I were designing the iterator protocol over again, I think
> I would start by recognising that starting a new iteration
> and continuing with an existing one are very different
> operations, and that you almost always intend the former
> rather than the latter. So I would declare that
> "for x in stuff" always implies a *new* iteration, and
> devise another syntax for continuing an existing one,
> such as "for x from stuff". I would define iterables and
> iterators as disjoint categories of object, and give __iter__
> methods only to iterables, not iterators.
>
> However, at least until Py4k comes around, we're stuck with
> the present situation, which seems to include accepting that
> "x in y" will occasionally gobble an iterator that you were
> saving for later.

I think that's a fine analysis.

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