James Y Knight
foom at fuhm.net
Thu Mar 1 05:15:24 CET 2007
On Feb 28, 2007, at 9:10 PM, Guido van Rossum wrote:
> I am beginning to think that there are serious problems with attaching
> the traceback to the exception; I really don't like the answer that
> pre-creating an exception is unpythonic in Py3k.
I'll say up front that I haven't been paying as much attention to the
topic of exception behavior as perhaps I should before attempting to
contribute to a thread about it...but...
It seems to me that a stack trace should always be attached to an
exception object at creation time of the exception, and never at any
other time. Then, if someone pre-creates an exception object, they
get consistent and easily explainable behavior (the traceback to the
creation point). The traceback won't necessarily be *useful*, but
presumably someone pre-creating an exception object did so to save
run-time, and creating the traceback is generally very expensive, so
doing that only once, too, seems like a win to me.
FWIW, that's basically how exceptions work in Java.
> Instances of two subclasses, Error and Exception, are
> conventionally used to indicate that exceptional situations have
> occurred. Typically, these instances are freshly created in the
> context of the exceptional situation so as to include relevant
> information (such as stack trace data).
> A throwable contains a snapshot of the execution stack of its
> thread at the time it was created. It can also contain a message
> string that gives more information about the error. Finally, it can
> contain a cause: another throwable that caused this throwable to
> get thrown. The cause facility is new in release 1.4. It is also
> known as the chained exception facility, as the cause can, itself,
> have a cause, and so on, leading to a "chain" of exceptions, each
> caused by another.
There's probably a million reasons why this doesn't work for python,
but they don't immediately jump out at me. :)
Migration from 2.X to 3.X would consist of recommending not to create
an exception outside of a raise line, unless you're okay with the
traceback location changing from the raise point to the creation point.
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