[Tutor] Exceptions and wotnot

Michael P. Reilly arcege@speakeasy.net
Wed, 20 Feb 2002 08:29:08 -0500

On Wed, Feb 20, 2002 at 08:46:00PM +0800, Andy W wrote:
> >   But it's not really what I want.  Surely there must be some way of
> getting
> > the traceback, out of the except Excpeption: construct.  I dir()ed the
> The "sys" module has a reference to the last traceback, strangely enough
> called "last_traceback" ;)
> You might also want to look at the "traceback" module, as it does some work
> for you.

Actually, last_traceback is the previous traceback, not the current one.
If you put your call in a try-expect: statement, then it would be in
exc_traceback.  Because of a "feature" of the interactive interpreter,
the exc_* values get put into last_* at the end of the executed statement.

Some things to know:

1.  Import traceback before you need it (before the exception occurs).

2.  If you are using threads, use sys.exc_info().  There are thread
    specific exceptions and sys.exc_info will get those for you.  The
    sys.exc_* values will be of either the main thread or last exception
    (I can't remember which, and the behavior is undefined anyway).

    (You probably don't want to have a thread display a popup to a
    Tkinter window, but there are ways to do that too.)

3.  There are a lot of functions in the traceback module, pick the right

import string, sys, traceback
def main():
    # display the usual Python exception output, but to a window
    exc, val, tb = sys.exc_info()
    formatted_lines = traceback.format_exception(exc, val, tb)
    exc_output = string.join(formatted_lines)

Using the last_traceback value would get you the wrong info.