[Python-Dev] Low-level exception invariants?

[Guido van Rossum]

+1, if you can also prove that the traceback will never be null. I
failed at that myself last time I tried, but I didn't try very long or
hard.

--Guido

On 5/26/06, Tim Peters <tim.peters at gmail.com> wrote:
> In various places we store triples of exception info, like a
> PyFrameObject's f_exc_type, f_exc_value, and f_exc_traceback PyObject*
> members.
>
> No invariants are documented, and that's a shame.  Patch 1145039 aims
> to speed ceval a bit by relying on a weak guessed invariant, but I'd
> like to make the stronger claim (which allows for stronger speedups)
> that, at any moment,
>
>     all three are NULL
> or
>     all three are not NULL
>
> Now I know that's not true, although Python's test suite doesn't
> provoke a violation.  In particular, PyErr_SetNone() forces
> tstate->curexc_value to NULL.  Other code then special-cases the snot
> out of XYZ_value, replacing NULL with Py_None on the fly, like
>
>         PyErr_Fetch(&exc, &val, &tb);
>         if (val == NULL) {
>                 val = Py_None;
>                 Py_INCREF(val);
>         }
>
> in the main eval loop.  I'd much rather change PyErr_SetNone() to set
> the value to Py_None to begin with -- that function is almost never
> called, so special-casing in that is less damaging than special-casing
> NULL everywhere else.
>
> The docs are confused about this now too, claiming:
>
> void PyErr_SetNone(PyObject *type)
>     This is a shorthand for "PyErr_SetObject(type, Py_None)".
>
> It's actually a shorthand for PyErr_SetObject(type, NULL) now.
>
> Does anyone see a real problem with _establishing_ an "all NULL or
> none NULL" invariant?  It makes outstanding ;-) _conceptual_ sense to
> me, and would allow removing some masses of distributed test-branch
> special-casing in code trying to use these guys.
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)