[Python-Dev] Traceback problem (original) (raw)

Christian Tismer tismer@tismer.com
Tue, 25 Feb 2003 23:55:01 +0100

Kevin Jacobs wrote: ...

I've uploaded a patch to SF:

http://www.python.org/sf/693195 It turns out that ceval.c:resetexcinfo was a red herring and did not do what we wanted. Luckily, the correct behavior is fairly trivial (although the interaction between setexcinfo and resetexcinfo had me seeing cross-eyed for a few minutes).

I had a look at your patch.

Here a small analysis:

In essence, it is doing exactly the same as PyErr_Restore(NULL, NULL, NULL) from errors.c, with the difference that it is acting on sys.exc_(type, value, traceback) instead of sys.curexc_(type, value, traceback).

I just want to make sure that this is what we want to do.

The rest of reset_exc_info restores an exception that has been captured in the frame, before.

The counterpart is set_exc_info, which is called only if an exception handler is executed. The frame then saves a prior exception from tstate to local variables and sets a new exception into tstate. I have no real clue what should happen if this occours multiple times -- the frame will only accept one such exception to save. Ah, I see: This is meant to be an exception which originated in the caller of the function, and the frame is just supposed to save the caller's exception, in case it is itself called in an exception handler!

The action on return is to drop the current exception, since it has been handled, and to restore the caller's exception, if this exists.

There is also a small trick here: set_exc_info assigns Py_None to tstate->exc_type, if it has been NULL. This is then assigned to the local variables of the frame. The side effect of this trick is, that after set_exc_info has been called once, reset_exc_info always will think that it has to restore, even if there is just a None-Exception. The side effect is, that the frame's variables always will be cleared out on a return.

Well, this code could stand a few more comments ;-)

Ok, what I understood now is, that this extra action of reset_exc_info is really not what we need here, this is about restoring the caller's exception. But we are not concerned with the caller here, since we stay in the current frame.

We just want to clear the current exception in tstate, and I think your code does the right thing.

+1

cheers -- chris

p.s.: I would probably have done the exception saving in the caller's frame, where it belongs, IMHO.

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