[Python-Dev] Twisted Isn't Specific (was Re: Trial balloon: microthreads library in stdlib) (original) (raw)

Jean-Paul Calderone exarkun at divmod.com
Thu Feb 15 17:01:17 CET 2007

On Thu, 15 Feb 2007 10:46:05 -0500, "A.M. Kuchling" <amk at amk.ca> wrote:

On Thu, Feb 15, 2007 at 09:19:30AM -0500, Jean-Paul Calderone wrote:

>That feels like 6 layers too many, given that > logrun(selectable, drdw, selectable, method, dict) > return context.call({ILogContext: newCtx}, func, *args, **kw) > return self.currentContext().callWithContext(ctx, func, *args, **kw) > return func(*args, **kw) > getattr(selectable, method()) > klass(number, string) > >are all generic calls.

I know function calls are expensive in Python, and method calls even more so... but I still don't understand this issue. Twisted's call stack is too deep? It is fair to say it is deep, I guess, but I don't see how that is a problem. If it is, I don't see how it is specific to this discussion. It's hard to debug the resulting problem. Which level of the 12 levels in the stack trace is responsible for a bug? Which of the 6 generic calls is calling the wrong thing because a handler was set up incorrectly or the wrong object provided? The code is so 'meta' that it becomes effectively undebuggable.

I've debugged plenty of Twisted applications. So it's not undebuggable. :)

Application code tends to reside at the bottom of the call stack, so Python's traceback order puts it right where you're looking, which makes it easy to find. For any bug which causes something to be set up incorrectly and only later manifests as a traceback, I would posit that whether there is 1 frame or 12, you aren't going to get anything useful out of the traceback. Standard practice here is just to make exception text informative, I think, but this is another general problem with Python programs and event loops, not one specific to either Twisted itself or the particular APIs Twisted exposes.

As a personal anecdote, I've never once had to chase a bug through any of the 6 "generic calls" singled out. I can't think of a case where I've helped any one else who had to do this, either. That part of Twisted is very old, it is very close to bug-free, and application code doesn't have very much control over it at all. Perhaps in order to avoid scaring people, there should be a way to elide frames from a traceback (I don't much like this myself, I worry about it going wrong and chopping out too much information, but I have heard other people ask for it)?

Jean-Paul

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