[Python-Dev] Problems with GeneratorExit deriving from Exception

[Guido van Rossum]

On Dec 1, 2007 2:38 PM, Chad Austin <chad at imvu.com> wrote:
> Here at IMVU, we love Python 2.5's generators-as-coroutines.  That feature has
> let us succinctly express algorithms that intermix asynchronous network requests
> and UI operations without writing complicated state machines, and, perhaps most
> importantly, get high-quality unit tests around these algorithms.
>
> However, we've been having a problem with the way GeneratorExit interacts with
> our coroutine system.  Let's take a bit of simplified example code from our system:
>
>         @task
>         def pollForChatInvites(chatGateway, userId):
>                 while True:
>                         try:
>                                 # Network call.
>                                 result = yield chatGateway.checkForInvite({'userId': userId})
>                         except Exception:  # An XML-RPC call can fail for many reasons.
>                                 result = None
>                         # ... handle result here
>                         yield Sleep(10)
>
> If a task (coroutine) is cancelled while it's waiting for the result from
> checkForInvite, a GeneratorExit will be raised from that point in the generator,
> which will be caught and ignored by the "except Exception:" clause, causing a
> RuntimeError to be raised from whoever tried to close the generator.  Moreover,
> any finally: clauses or with-statement contexts don't run.
>
> We have also run into problems where a task tries to "return" (yield Return())
> from within a try: except Exception: block.  Since returning from a coroutine is
> roughly equivalent to "raise GeneratorExit", the exception can be caught and
> ignored, with the same consequences as above.
>
> This problem could be solved in several ways:
>
> 1) Make GeneratorExit derive from BaseException, just like SystemExit.
>
> 2) Add "except GeneratorExit: raise" to every usage of except Exception: in tasks.
>
> 3) Change the semantics of except clauses so that you can use any container as
> an exception filter.  You could have a custom exception filter object that would
> catch any Exception-derived exception except for GeneratorExit.  Then we'd have
> to teach the team to use "except ImvuExceptionFilter:" rather than "except
> Exception:".
>
> I prefer option #1, because it matches SystemExit and I haven't ever seen a case
> where I wanted to catch GeneratorExit.  When a generator is closed, I just want
> finally: clauses to run, like a normal return statement would.  In fact, we have
> already implemented option #1 locally, but would like it to be standard.
>
> Option #2 would add needless noise throughout the system,
>
> You could argue that it's bad style to catch Exception, but there are many
> situations where that's exactly what I want.  I don't actually care _how_ the
> xml-rpc call failed, just that the error is logged and the call is retried
> later.  Same with loading caches from disk.
>
> Proposals for changing GeneratorExit to be a BaseException have come up on this
> list in the past [1] [2], but were rejected as being too "theoretical".  A
> significant portion of the IMVU client is now specified with coroutines, so I
> hope to resume this conversation.
>
> Thoughts?
>
> Chad
>
> [1] http://mail.python.org/pipermail/python-dev/2006-March/062654.html
> [2] http://mail.python.org/pipermail/python-dev/2006-March/062825.html

Well argued. I suggest to go for option (1) -- make GeneratorExit
inherit from BaseException. We can do this starting 2.6. Feel free to
upload a patch to bugs.python.org.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)