[Python-Dev] PEP 340 - possible new name for block-statement

[Josiah Carlson]

Nick Coghlan <ncoghlan at gmail.com> wrote:
> Then, in Py3K, finalisation could simply become the default for loop behaviour. 
> However, the '__finalise__' flag would result in some impressive code bloat, as 
> any for loop would need to expand to:
> 
>      itr = iter(EXPR1)
>      if getattr(itr, "__finalise__", False):
>          # Finalised semantics
>          #    I'm trying to channel Guido here.
>          #    This would really look like whatever the PEP 340 block statement
>          #    semantics end up being
>          val = arg = None
>          ret = broke = False
>          while True:
>              try:
>                  VAR1 = next(itr, arg)
>              except StopIteration:
>                  BLOCK2
>                  break
>              try:
>                  val = arg = None
>                  ret = False
>                  BLOCK1
>              except Exception, val:
>                  itr.__error__(val)
>              if ret:
>                  try:
>                      itr.__error__(StopIteration())
>                  except StopIteration:
>                      pass
>                  return val


The problem is that BLOCK2 is executed within the while loop (the same
problem I had with a fix I offered), which may contain a break for
breaking out of a higher-level loop construct.  Here's one that works as
you intended (though perhaps I'm being a bit to paranoid about the
__error__ attribute)...

            val = arg = None
            ret = ex_block_2 = False
            while True:
                try:
                    VAR1 = next(itr, arg)
                except StopIteration:
                    ex_block_2 = True
                    break
                try:
                    val = arg = None
                    ret = False
                    BLOCK1
                except Exception, val:
                    if hasattr(itr, '__error__):
                        itr.__error__(val)
                if ret:
                    try:
                        if hasattr(itr, '__error__'):
                            itr.__error__(StopIteration())
                    except StopIteration:
                        pass
                    return val
            if ex_block_2:
                BLOCK2


> P.S. I think PEP 340's proposed for loop semantics are currently incorrect, as 
> BLOCK2 is unreachable. It should look more like the non-finalised semantics 
> above (with BLOCK2 before the break in the except clause)

Indeed, I also mentioned this on Wednesday.

 - Josiah