[Python-Dev] Exception masking/chaining

[Guido van Rossum]

> This could be done automatically, i.e. whenever you call
> PyErr_SetString() etc. the currently active exception will be normalized
> and put into a fourth global variable (exc_cause). In Python code
> whenever a exception handler raises a new exception the exception
> that is currently handled could be set as the cause for this
> exception automatically by the raise statement.

More globals of course defeat the purpose (one of the reasons for
putting the traceback in the exception is to get rid of
sys.exc_traceback).

I also think that doing this automatically whenever PyErr_Set*() is
called would cause a lot of spurious tracebacks.  (Unless the idea is
to skip the chaining if the original exception has an empty traceback;
that will almost always be the case for exceptions raised at the C
level.)  I worry about backwards compatibility.

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