Hello Python-Dev,
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
--
http://imvu.com/technology
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