[Python-Dev] PEP 340 -- loose ends

[Delaney, Timothy C (Timothy)]

Phillip J. Eby wrote:

> Specifically, I propose that PEP 340 *not* allow the use of "normal"
> iterators.  Instead, the __next__ and __exit__ methods would be an
> unrelated protocol.  This would eliminate the need for a 'next()'
> builtin, 
> and avoid any confusion between today's iterators and a template
> function 
> for use with blocks.

PEP 340 does not address "normal" iterators very well, but a
properly-constructed iterator will behave correctly.

The PEP though is very generator-focussed. The issues I see for "normal"
iterators (and that need to be addressed/stated in the PEP) are:

    1. No automatic handling of parameters passed to __next__ and
__exit__.
       In a generator, these will raise at the yield-statement or
-expression.
       A "normal" iterator will have to take care of this manually.

This could be an argument to only allow generator-iterators to be used
with PEP 340 semantics (i.e. continue <EXPR>, block), but I don't think
it's a very compelling one.

Although perhaps the initial implementation could be restricted to
generator-iterators. So if a for-loop used `continue <EXPR>` it would
have a check (at the start of the for loop) that the iterator is a
generator-iterator. Likewise, a block-statement would always include
this check.

As another option, it might be worthwhile creating a base iterator type
with "correct" semantics.

Tim Delaney