[Python-Dev] PEP 343 - Abstract Block Redux

[Phillip J. Eby]

At 05:13 PM 5/13/2005 -0700, Guido van Rossum wrote:
>I've written up the specs for my "PEP 340 redux" proposal as a
>separate PEP, PEP 343.
>
>http://python.org/peps/pep-0343.html
>
>Those who have been following the thread "Merging PEP 310 and PEP
>340-redux?" will recognize my proposal in that thread, which received
>mostly positive responses there.
>
>Please review and ask for clarifications of anything that's unclear.

May I suggest this alternative translation in the "Specification" section:

         abc = EXPR
         __args = ()  # pseudo-variable, not visible to the user

         try:
             VAR = abc.__enter__()
             try:
                 BLOCK
             except:
                 __args = sys.exc_info()
         finally:
             abc.__exit__(*__args)

While slighly more complex than the current translation, the current 
translation seems a bit misleading to me.  OTOH, that may simply be because 
I see the *sys.exc_info() part and immediately wonder what happens when 
there was no exception, and sys.exc_info() contains some arbitrary previous 
data...

Also, one question: will the "do protocol" be added to built-in "resource" 
types?  That is, locks, files, sockets, and so on?  Or will there instead 
be "macros" like the "opening" and "locking" templates?  I notice that 
grammatically, "do gerund" works a lot better than "do noun", so all of 
your examples are words like locking, blocking, opening, redirecting, and 
so on.  This makes it seem awkward for e.g. "do self.__lock", which doesn't 
make any sense.  But the extra call needed to make it "do 
locking(self.__lock)" seems sort of gratuitous.

It makes me wonder if "with" or "using" or some similar word that works 
better with nouns might be more appropriate, as then it would let us just 
add the resource protocol to common objects, and still read well.  For 
example, a Decimal Context object might implement __enter__ by setting 
itself as the thread-local context, and __exit__ by restoring the previous 
context.    "do aDecimalContext" doesn't make much sense, but "with 
aDecimalContext" or "using aDecimalContext" reads quite nicely.