[Python-Dev] with_traceback

[James Y Knight]

On Mar 1, 2007, at 3:27 AM, Greg Ewing wrote:
> James Y Knight wrote:
>> The traceback won't necessarily be *useful*,
>
> Almost completely use*less*, I would have thought.
> The traceback is mostly used to find out where
> something went wrong, not where it went right (i.e.
> successful creation of the exception).

The advantages are that it's an easily understandable and explainable  
behavior, and the traceback points you (the programmer) to the exact  
location where you went wrong: creating the exception at module level.

Creating an exception with a non-exceptional stacktrace isn't always  
useless: sometimes you have exceptions where you know you never care  
about the stacktrace (internal flow control/etc).

> This would be a major and surprising change to Python
> users.

It's only a major change if you don't raise the exception in the same  
place you create it. (which other people are claiming is extremely  
rare).

> It would also be considerably *more* expensive to implement
> than the current scheme, because it would require copying the
> entire stack, instead of just linking stack frames together
> as they are unwound during the search for an exception
> handler.

Yes of course, you're right, I withdraw the proposal. I had forgotten  
that python doesn't currently save the entire stack, only that  
between the 'raise' and the 'except'. (I had forgotten, because  
Twisted's "Failure" objects do save and print the entire stacktrace,  
both above and below the catch-point).

James