[Python-Dev] with_traceback

glyph at divmod.com glyph at divmod.com
Thu Mar 1 07:53:19 CET 2007

On Wed, 28 Feb 2007 18:10:21 -0800, Guido van Rossum <guido at python.org> wrote:
>I am beginning to think that there are serious problems with attaching
>the traceback to the exception; I really don't like the answer that
>pre-creating an exception is unpythonic in Py3k.

In Twisted, to deal with asynchronous exceptions, we needed an object to specifically represent a "raised exception", i.e. an Exception instance with its attached traceback and methods to manipulate it.

You can find its API here:


Perhaps the use-cases for attaching the traceback object to the exception would be better satisfied by simply having sys.exc_info() return an object with methods like Failure?  Reading the "motivation" section of PEP 344, it describes  "passing these three things in parallel" as "tedious and error-prone".  Having one object one could call methods on instead of a 3-tuple which needed to be selectively passed on would simplify things.

For example, chaining could be accomplished by doing something like this:


I can't think of a good name for the new object type, since "traceback", "error", "exception" and "stack" all already mean things in Python.

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