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

[Steven Bethard]

[Guido]
> Going for all-out simplicity, I would like to be able to write these examples:
> 
> class locking:
>     def __init__(self, lock): self.lock = lock
>     def __enter__(self): self.lock.acquire()
>     def __exit__(self, *args): self.lock.release()
> 
> class opening:
>     def __init__(self, filename): self.filename = filename
>     def __enter__(self): self.f = open(self.filename); return self.f
>     def __exit__(self, *args): self.f.close()\
> 
> And do EXPR as VAR: BLOCK would mentally be translated into
> 
> itr = EXPR
> VAR = itr.__enter__()
> try: BLOCK
> finally: itr.__exit__(*sys.exc_info()) # Except sys.exc_info() isn't
> defined by finally

Yeah, that's what I wanted to do too.  That should be about what my
second suggestion did.  Slightly updated, it looks like:

   stmt = EXPR1
   VAR1 = stmt.__enter__()
   exc = () # or (None, None, None) if you prefer
   try:
       try:
           BLOCK1
       except:
           exc = sys.exc_info()
   finally:
       if stmt.__exit__(*exc) is not None:
           raise exc[0], exc[1], exc[2]

The only difference should be that with the above semantics if you
return a (non-None) value from __exit__, the exception will be
suppressed (that is, it will not be reraised).  Means that if you want
to suppress an exception, you have to add a return statement (but if
you want exceptions to be reraised, you don't have to do anything.)

I suggest this only because there were a few suggested use-cases for
suppressing exceptions.  OTOH, almost all my uses are just
try/finally's so I'm certainly not going to cry if that last finally
instead looks like:

   finally:
       stmt.__exit__(*exc)
       raise exc[0], exc[1], exc[2]

=)

STeVe
-- 
You can wordify anything if you just verb it.
        --- Bucky Katt, Get Fuzzy