[Python-3000] exception info [was: Discussions with no PEPs] (original) (raw)

Guido van Rossum guido at python.org
Tue Mar 13 17:56:55 CET 2007

On 3/13/07, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Guido van Rossum wrote:

> The internal mechanism for bubbling an exception up the stack until an > except clause catches it could continue to use the (class, instance, > traceback) triple, Hmmm, so most of that complexity will still be there at the C level. I was hoping there might be some simplification there as well.

We can still gain some simplification there, as we shouldn't have to keep shadow copies of these triples in each frame and in each thread state.

However...

> and if raise is passed a class instead of an > instance, the instance would be NULL; when forced to instantiate the > exception, the traceback collected up to that point is attached to it. Would this apply to Python code as well? I.e. if you use a raise statement with a class, it doesn't get instantiated immediately? And if you catch it with an except clause which doesn't capture the exception, it's never instantiated? That would be a bonus.

I think that's how it works now, and I don't intend to break this.

> If an instance was passed to raise, the traceback pointer in the > instance is updated each time we leave a stack frame.

So it seems we're back to declaring pre-instantiated exceptions to be bad style, which you said you didn't like earlier -- have you changed your mind?

Yes. After learning that there isn't much of a performance gain and there aren't many users, and after looking into other solutions, I really think that having the tb attached to the exc is the best approach.

What about thread safety? Do we just document that using pre-instantiated exceptions is not thread-safe?

It's more than non-thread-safe. It's unsafe, period. And yes, we document this.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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