On Tue, Oct 9, 2012 at 2:11 AM, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
Ben Darnell wrote:
StackContext doesn't quite give you better tracebacks, although I think it could be adapted to do that. ExceptionStackContext is essentially a try/except block that follows you around across asynchronous operations - on entry it sets a thread-local state, and all the tornado asynchronous functions know to save this state when they are passed a callback, and restore it when they execute it.
This is something that generator-based coroutines using yield-from ought to handle a lot more cleanly. You should be able to just use an ordinary try-except block in your generator code and have it do the right thing.
Indeed, in NDB this works great. However tracebacks don't work so great: If you don't catch the exception right away, it takes work to make the tracebacks look right when you catch it a few generator calls down on the (conceptual) stack. I fixed this to some extent in NDB, by passing the traceback explicitly along when setting an exception on a Future; before I did this, tracebacks looked awful. But there are still StackContextquite a few situations in NDB where an uncaught exception prints a baffling traceback, showing lots of frames from the event loop and other async machinery but not the user code that was actually waiting for anything. I have to study Tornado's to see if there are ideas there for improving this.
I hope that the new async core will be designed so that generator-based coroutines can be plugged into it directly and efficiently, without the need for a lot of decorators, callbacks, Futures, etc. in between.
That has been my hope too. But so far when thinking about this recently I have found the goal elusive -- somehow it seems there *has* to be a distinction between an operation you just *yield* (this would be waiting for a specific low-level I/O operation) and something you use with yield-from, which returns a value through StopIteration. I keep getting a headache when I think about this, so there must be a Monad in there somewhere... :-( Perhaps you can clear things up by showing some detailed (but still simple enough) example code to handle e.g. a simple web client? -- --Guido van Rossum (python.org/~guido)