[Python-Dev] Merging PEP 310 and PEP 340-redux?

[Steven Bethard]

On 5/13/05, Guido van Rossum <gvanrossum at gmail.com> wrote:
> So then the all-important question I want to pose is: do we like the
> idea of using a (degenerate, decorated) generator as a "template" for
> the do-statement enough to accept the slightly increased complexity?

+0.  I'm not thoroughly convinced that generators are that much easier
to read than a class.  But I don't find them hard to read, and I think
it would only take a little effort to learn that generators might not
always be intended to build iterators.

If we do support generators in do-statements, I'd like their
__enter__() and __exit__() methods (if possible) to have semantics
like Nick Coghlan suggested[1], so that:
 * __enter__() raises an exception if next() has already been called, and
 * __exit__() raises an exception if StopIteration is not raised
The first makes sure that the generator is only used once, and the
second makes sure that there is only one yield on the given control
path through the generator.  In all but the most sick and twisted
code, raising exceptions like this will be identifying errors in how
the generator was written.

> Straight up-or-down votes in the full senate are appreciated at this point.

+1 on the PEP 340 redux semantics.

> On to the secondary questions:
> 
> - Today I like the 'do' keyword better; 'with' might confuse folks
> coming from Pascal or VB

+1 on using 'do'.

> - I have a more elaborate proposal for __exit__'s arguments. Let the
> translation be as follows:
[snip]
> abc.__exit__(True) - normal completion of BLOCK
> 
> abc.__exit__(False) - BLOCK was left by a non-local goto (break/continue/return)
> 
> abc.__exit__(False, t, v, tb) - BLOCK was left by an exception

-1. This looks like a fair bit of added complexity for not much gain. 
The only example that even *might* make use of this was the
transactional one, and I haven't yet seen a use case where it actually
*is*.

The simpler semantics give you the difference between a normal exit
and an exceptional exit.  I'd like to see an example that needs to
know the difference between block completion exit and a
break/continue/return exit before I'd want to make PEP 340 redux this
much more complex.

STeVe

[1] http://members.iinet.net.au/~ncoghlan/public/pep-3XX.html
-- 
You can wordify anything if you just verb it.
        --- Bucky Katt, Get Fuzzy