[Python-Dev] bpo-36829: Add sys.unraisablehook() (original) (raw)

Steve Dower steve.dower at python.org
Thu May 16 19:00:07 EDT 2019

On 16May2019 1441, Victor Stinner wrote:

Le jeu. 16 mai 2019 à 23:17, Steve Dower <steve.dower at python.org> a écrit :

You go on to say "pass an error message" and "keep repr(obj) if you want", but how is this different from creating an exception that contains the custom message, the repr of the object, and chains the exception that triggered it? Well, "unraisable exceptions" are raised when something goes wrong. I'm not comfortable with creating a new exception instance and chaining it to the previous exception, because it could go wrong and raise a 3rd exception. Issue a new error while trying to log an error can be annoying :-(

I mean, aren't they all? :) That's why they're exceptional.

If we're not in a position to raise a new exception, then it's not safe to call into user's Python code. If it's safe to call into their Python code, then we have to be in a position to raise a new exception.

You said earlier that we can safely do this, and now you're saying we can't safely do it. Which is it?

Moreover, when I looked at details of the "unraisable exception" (see my reply to Petr Viktorin), I saw that the exception is not well defined as you might expect. (exctype, excvalue, exctb) can not be replaced with (type(excvalue), excvalue, excvalue.traceback). Sometimes, excvalue is None. Sometimes, exctb is a traceback object, but excvalue.traceback is None. So I'm not comfortable neither to chain such badly shaped exception into a new exception.

Yeah, this is the normal optimization for raising and handling exceptions that never reach Python code. Exceptions are supposed to be normalized before re-entering the eval loop.

I prefer to pass "raw" values and let the hook decides how to handle them :-)

This is fine, it's how we handle exceptions in most other places (such as exit, logging, sys, traceback and all through the C API), and skipping normalization for a last-chance "the-world-may-be-burning" hook isn't the worst thing ever.

Cheers, Steve

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