On Thu, Nov 27, 2014 at 10:08 AM, Victor Stinner <victor.stin...@gmail.com> wrote:
> I'm trying to follow the discussion about the PEP 479 (Change > StopIteration handling inside generators), but it's hard to read all > messages. I'm concerned by trollius and asyncio which heavily rely on > StopIteration. > > Trollius currently supports running asyncio coroutines: a trollius > coroutine can executes an asyncio coroutine, and and asyncio coroutine > can execute a trollius coroutine. > > I modified the Return class of Trollius to not inherit from > StopIteration. All trollius tests pass on Python 3.3 except on one > (which makes me happy, the test suite is wide enough to detect bugs > ;-)): test_trollius_in_asyncio. > > This specific test executes an asyncio which executes a trollius coroutine. > > https://bitbucket.org/enovance/trollius/src/873d21ac0badec36835ed24d13e2aeda24f2dc64/tests/test_asyncio.py?at=trollius#cl-60 > > The problem is that an asyncio coroutine cannot execute a Trollius > coroutine anymore: "yield from coro" raises a Return exception instead > of simply "stopping" the generator and return the result (value passed > to Return). > > I don't see how an asyncio coroutine calling "yield from > trollius_coroutine" can handle the Return exception if it doesn't > inherit from StopIteration. It means that I have to drop this feature > in Python 3.5 (or later when the PEP 479 becomes effective)? > > I'm talking about the current behaviour of Python 3.3, I didn't try > the PEP 479 (I don't know if an exception exists). > The issue here is that asyncio only interprets StopIteration as returning from the generator (with a possible value), while a Trollius coroutine must use "raise Return(<value>)" to specify a return value; this works as long as Return is a subclass of StopIteration, but PEP 479 will break this by replacing the StopIteration with RuntimeError. It's an interesting puzzle. The only way out I can think of is to have asyncio special-case the Return exception -- we could do that by defining a new exception (e.g. AlternateReturn) in asyncio that gets treated the same way as StopIteration, so that Trollius can inherit from AlternateReturn (if it exists). What do you think? -- --Guido van Rossum (python.org/~guido)
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