[Python-Dev] PEP 340 - For loop cleanup, and feature separation

[Phillip J. Eby]

At 01:58 PM 5/6/2005 +1000, Delaney, Timothy C (Timothy) wrote:
>Personally, I'm of the opinion that we should make a significant break
>(no pun intended ;) and have for-loops attempt to ensure that iterators
>are exhausted.

This is simply not backward compatible with existing, perfectly valid and 
sensible code.
Therefore, this can't happen till Py3K.

The only way I could see to allow this is if:

1. Calling __iter__ on the target of the for loop returns the same object
2. The for loop owns the only reference to that iterator.

However, #2 is problematic for non-CPython implementations, and in any case 
the whole thing seems terribly fragile.

So how about this: calling __exit__(StopIteration) on a generator that 
doesn't have any active blocks could simply *not* exhaust the 
iterator.  This would ensure that any iterator whose purpose is just 
iteration (i.e. all generators written to date) still behave in a resumable 
fashion.

Ugh.  It's still fragile, though, as adding a block to an iterator will 
then make it behave differently.  It seems likely to provoke subtle errors, 
arguing again in favor of a complete separation between iteration and block 
protocols.