On Dec 1, 2007 2:38 PM, Chad Austin 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/)