[Python-ideas] Possible PEP 380 tweak

[Guido van Rossum]

On Mon, Oct 25, 2010 at 6:35 PM, Jacob Holm <jh at improva.dk> wrote:
> I had some later suggestions for how to change the expansion, see e.g.
> http://mail.python.org/pipermail/python-ideas/2009-April/004195.html  (I
> find that version easier to reason about even now 1½ years later)

I like that style too. Here it is with some annotations.

     _i = iter(EXPR)
     _m, _a = next, (_i,)
     # _m is a function or a bound method;
     #  _a is a tuple of arguments to call _m with;
     # both are set to other values further down
     while 1:
         # Move the generator along
         try:
             _y = _m(*_a)
         except StopIteration as _e:
             _r = _e.value
             break

         # Yield _y and process what came back in
         try:
             _s = yield _y
         except GeneratorExit as _e:
             # Request to exit
             try:
                 # NOTE: This _m is unrelated to the other
                 _m = _i.close
             except AttributeError:
                 pass
             else:
                 _m()
             raise _e  # Always exit
         except BaseException as _e:
             # An exception was thrown in; pass it along
             _a = sys.exc_info()
             try:
                 _m = _i.throw
             except AttributeError:
                 # Can't throw it in; throw it back out
                 raise _e
         else:
             # A value was sent in; pass it along
             if _s is None:
                 _m, _a = next, (_i,)
             else:
                 _m, _a = _i.send, (_s,)

     RESULT = _r

I do note that this is a bit subtle; I don't like the reusing of _m
and it's hard to verify that _m and _a are set on every path that goes
back to the top of the loop.

-- 
--Guido van Rossum (python.org/~guido)