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. From http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Throwable.html: > 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. James _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com