[Python-Dev] PEP 340 -- loose ends

[Nick Coghlan]

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.

I would extend this to say that invoking the blocktemplate decorator should 
eliminate the conventional iteration interface, preventing the following 
problematically silent bug:

   for l in synchronized(mylock):
     # This lock is not released promptly!
     break

> My argument is that this is both Explicit (i.e., better than implicit) and 
> One Obvious Way (because using existing iterators just Another Way to do a 
> "for" loop).  It also doesn't allow Errors (using an iterator with no 
> special semantics) to Pass Silently.

While I agree these are advantages, a bigger issue for me would be the one 
above: keeping a block template which expects prompt finalisation from being 
inadvertently used in a conventional for loop which won't finalise on early 
termination of the loop.

I'd also suggest that the blocktemplate decorator accept any iterator, not just 
generators.

> Of course, since Practicality Beats Purity, I could give this all up.  But 
> I don't think the Implementation is Hard to Explain, as it should be just 
> as easy as Guido's proposal.

I think it would be marginally easier to explain, since the confusion between 
iterators and block templates would be less of a distraction.

>  Really, the only thing that changes is that you get a 
> TypeError when a template function returns an iterator instead of a block 
> template, and you have to use the decorator on your generators to 
> explicitly label them safe for use with blocks.

I'd add raising a TypeError when a block template is passed to the iter() 
builtin to the list of differences from the current incarnation of the PEP.

As for Phillip, I think using different API's is a good way to more clearly 
emphasise the difference in purpose between conventional for loops and the new 
block statement, but I'm also a little concerned about incorrectly passing a 
block template to a for loop.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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