[issue3112] implement PEP 3134 exception reporting

[Adam Olsen]

Adam Olsen <rhamph at gmail.com> added the comment:

On Sun, Jun 22, 2008 at 1:04 PM, Antoine Pitrou <report at bugs.python.org> wrote:
>
> Antoine Pitrou <pitrou at free.fr> added the comment:
>
> Le dimanche 22 juin 2008 à 17:17 +0000, Adam Olsen a écrit :
>> I meant only that trivial cycles should be detected.  However, I
>> hadn't read your patch, so I didn't realize you already knew of a way
>> to create a non-trivial cycle.
>>
>> This has placed a niggling doubt in my mind about chaining the
>> exceptions, rather than the tracebacks.  Hrm.
>
> Chaining the tracebacks rather than the exceptions loses important
> information: what is the nature of the exception which is the cause or
> context of the current exception?

I assumed each leg of the traceback would reference the relevant exception.

Although.. this is effectively the same as creating a new exception
instance when reraised, rather than modifying the old one.  Reusing
the old is done for performance I believe.

> It is improbable to create such a cycle involuntarily, it means you
> raise an old exception in replacement of a newer one caused by the
> older, which I think is quite contorted. It is also quite easy to avoid
> creating the cycle, simply by re-raising outside of any except handler.

I'm not convinced.

    try:
        ...  # Lookup
    except A as a:  # Lookup failed
        try:
            ...  # Fallback
        except B as b:  # Fallback failed
            raise a  # The original exception is of the type we want

For this behaviour, this is the most natural way to write it.
Conceptually, there shouldn't be a cycle - the traceback should be the
lookup, then the fallback, then whatever code is about this - exactly
the order the code executed in.

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<http://bugs.python.org/issue3112>
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