cannot override sys.excepthook

Question

I try to customize behavior of sys.excepthook as described by the recipe.

in ipython:

:import pdb, sys, traceback
:def info(type, value, tb):
:    traceback.print_exception(type, value, tb)
:    pdb.pm()
:sys.excepthook = info
:--
>>> x[10] = 5
-------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
NameError: name 'x' is not defined
>>>

pdb.pm() is not being called. It seems that sys.excepthook = info doesn't work in my python 2.5 installation.

Answer 1

ipython, which you're using instead of the normal Python interactive shell, traps all exceptions itself and does NOT use sys.excepthook. Run it as ipython -pdb instead of just ipython, and it will automatically invoke pdb upon uncaught exceptions, just as you are trying to do with your excepthook.

Answer 2

Five years after you wrote this, IPython still works this way, so I guess a solution might be useful to people googling this.

IPython replaces sys.excepthook every time you execute a line of code, so your overriding of sys.excepthook has no effect. Furthermore, IPython doesn't even call sys.excepthook, it catches all exceptions and handles them itself before things get that far.

To override the exception handler whilst IPython is running, you can monkeypatch over their shell's showtraceback method. For example, here's how I override to give what looks like an ordinary Python traceback (because I don't like how verbose IPython's are):

def showtraceback(self):
    traceback_lines = traceback.format_exception(*sys.exc_info())
    del traceback_lines[1]
    message = ''.join(traceback_lines)
    sys.stderr.write(message)

import sys
import traceback
import IPython
IPython.core.interactiveshell.InteractiveShell.showtraceback = showtraceback

This works in both the normal terminal console and the Qt console.

Answer 3

expanding on Chris answer, you can use another function like a decorator to add your own functionality to jupyters showbacktrace:

from IPython.core.interactiveshell import InteractiveShell
from functools import wraps
import traceback
import sys

def change_function(func):
    @wraps(func)
    def showtraceback(*args, **kwargs):
        # extract exception type, value and traceback
        etype, evalue, tb = sys.exc_info()
        if issubclass(etype, Exception):
            print('caught an exception')
        else:
            # otherwise run the original hook
            value = func(*args, **kwargs)
            return value
    return showtraceback

InteractiveShell.showtraceback = change_function(InteractiveShell.showtraceback)

raise IOError

Answer 4

See this SO question and make sure there isn't something in your sitecustomize.py that prevents debugging in interactive mode.