How do I set sys.excepthook to invoke pdb globally in python?

Question

From Python docs:

sys.excepthook(type, value, traceback)

This function prints out a given traceback and exception to sys.stderr.

When an exception is raised and uncaught, the interpreter calls sys.excepthook with three arguments, the exception class, exception instance, and a traceback object. In an interactive session this happens just before control is returned to the prompt; in a Python program this happens just before the program exits. The handling of such top-level exceptions can be customized by assigning another three-argument function to sys.excepthook.

http://docs.python.org/library/sys.html

How do I modify this globally so the default action is to always invoke pdb? Is there a configuration file I can change? I don't want to wrap my code to do this.

Answer 1

Here's what you need

http://ynniv.com/blog/2007/11/debugging-python.html

Three ways, the first is simple but crude (Thomas Heller) - add the following to site-packages/sitecustomize.py:

import pdb, sys, traceback
def info(type, value, tb):
    traceback.print_exception(type, value, tb)
    pdb.pm()
sys.excepthook = info

The second is more sophisticated, and checks for interactive mode (weirdly skipping the debugging in interactive mode), from the cookbook:

# code snippet, to be included in 'sitecustomize.py'
import sys

def info(type, value, tb):
   if hasattr(sys, 'ps1') or not sys.stderr.isatty():
      # we are in interactive mode or we don't have a tty-like
      # device, so we call the default hook
      sys.__excepthook__(type, value, tb)
   else:
      import traceback, pdb
      # we are NOT in interactive mode, print the exception...
      traceback.print_exception(type, value, tb)
      print
      # ...then start the debugger in post-mortem mode.
      pdb.pm()

sys.excepthook = info

And the third (which always start the debugger unless stdin or stderr are redirected) by ynniv

# code snippet, to be included in 'sitecustomize.py'
import sys

def info(type, value, tb):
   if (#hasattr(sys, "ps1") or
       not sys.stderr.isatty() or 
       not sys.stdin.isatty()):
       # stdin or stderr is redirected, just do the normal thing
       original_hook(type, value, tb)
   else:
       # a terminal is attached and stderr is not redirected, debug 
       import traceback, pdb
       traceback.print_exception(type, value, tb)
       print
       pdb.pm()
       #traceback.print_stack()

original_hook = sys.excepthook
if sys.excepthook == sys.__excepthook__:
    # if someone already patched excepthook, let them win
    sys.excepthook = info

Answer 2

Another option is to use ipython, which I consider a must-have tool for any python developer anyway. Instead of running your script from the shell, run it from ipython with %run. When an exception occurs, you can type %debug to debug it. (There's also an option to automatically debug any exception that occurs, but I forget what it is.)