dropping into the debugger on an exception

Thomas Heller theller at python.net
Wed Jun 9 09:13:00 CEST 2004

Jon Perez <jbperez808 at yahoo.com> writes:

> (sorry for the duplicate post, just wanted to make the subject
> line clearer)
> How do you set up pdb such that you will automatically
> get dropped into its prompt if an unanticipated exception
> occurs in a script you are using?
> ASPN Python cookbook gives you the following method
> which you can add to your script and hook into sys.excepthook.
> But is there a way to do it without adding stuff to your
> script?  (It's okay if this means having to invoke the script
> from within pdb, but #1, I don't know how to get it stay inside
> pdb in the case of an /unanticipated/ exception.  And #2, I
> don't know how to pass [the equivalent of] command-line
> arguments to a script invoked from within pdb.)

Is the description in the cookbook unclear?  You do *not* have to add
anything to your script - save the code as a file
C:\Python23\sitecustomize.py and everything will work.  And you don't
have to start the script from within pdb. The sitecustomize module is
automatically imported when Python starts - if it is found.

I'm appending the version I currently use, maybe it has some
improvements not mentioned in the online version.


# code snippet, to be included in 'sitecustomize.py'
import sys
import traceback, pdb
##import pywin.debugger

def info(type, value, tb,
##         debug=pywin.debugger.pm):
   if not __debug__ \
          or not sys \
          or hasattr(sys, 'ps1') \
          or not sys.stderr.isatty() \
          or not sys.stdin.isatty() \
          or not hasattr(sys, "last_traceback"):
      # call the default hook if one of the following conditions
      # is satisfied:
      # - Python is running with the -O flag (__debug__ == 0)
      # - sys is None (python is shutting down, and the sys module
      #   already has been cleared)
      # - we are in interactive mode (sys has a 'ps1' attribute)
      # - sys.stderr is not a tty: we are not connected to a terminal
      # - no last_traceback attribute in sys (SyntaxError)
      excepthook(type, value, tb)
      # print the exception...
      print_exc(type, value, tb)
      # ...then start the debugger in post-mortem mode.
sys.excepthook = info

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