[Python-Dev] Chained Exceptions

[Delaney, Timothy C (Timothy)]

James Y Knight wrote:

> Of course you can already do similar with current python, it just
> can't be spelled as nicely, and the default traceback printer won't
> use the info:
> 
> try:
>    raise AError
> except:
>    newException = BError()
>    newException.cause=sys.exc_info()
>    raise newException

Well, one thing you can do is (somewhat evil ;)

::

    import sys

    try:
        raise AError, 'message'
    except:
        exc_type, exc_value, exc_traceback - sys.exc_info()
        raise BError, exc_value, exc_traceback

with the result:

    Traceback (most recent call last):
        File ...
            raise AError, 'message'
    BError: message

So you store the original exception as the argument to the new exception
(so it's accessible). This has the nice side effect that message is
displayed in the traceback - but the type has changed.

Whilst in the above example, it's particularly evil, in the case where
the original exception came from a function call and you want to
translate the type it works very nicely.

Tim Delaney