From: annada@tilde.green

Hi,

Recently I have been increasingly adding this piece of code as
a preamble to a lot of my code.

import (sys, os, ipdb)

def debug_hook(exc_type, exc_value, traceback):
if exc_type is KeyboardInterrupt:
sys.__excepthook__(exc_type, exc_value, traceback)
return
print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")
ipdb.post_mortem(traceback)

if os.environ.get('DEBUG'): sys.excepthook = debug_hook

This has been extemely helpful when debugging and even in prod. When
degugging, this helps me go up and down my backtrace like I am in gdb.
In fact, it is better than gdb. I can use evaluate any python expression directly, and verify the shape of my tensors and exactly what caused
the error. It's like freezing the entire program right at the time the
program failed. This way I don't have to second guess what exactly
failed in the entire training loop and the next time I can fix it.

if True or os.environ.get('DEBUG'): sys.excepthook = debug_hook

Even when not debugging, it helps me salvage the results if a 3 week
running experiment fails at 90% completion.

Annada

--- SoupGate-Linux v1.05
* Origin: Dragon's Lair ---:- FidoNet<>Usenet Gateway -:--- (3:633/10)

From: ram@zedat.fu-berlin.de

Annada Behera <annada@tilde.green> wrote or quoted:

Recently I have been increasingly adding this piece of code as
a preamble to a lot of my code.

import (sys, os, ipdb)

def debug_hook(exc_type, exc_value, traceback):
if exc_type is KeyboardInterrupt:
sys.__excepthook__(exc_type, exc_value, traceback)
return
print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")
ipdb.post_mortem(traceback)

if os.environ.get('DEBUG'): sys.excepthook = debug_hook

Thanks!

I have changed it a bit to work without "ipdb" and added demo code.

import sys
import os
import pdb

i = 1

def debug_exception_hook(exc_type, exc_value, tb):
i = 2
if issubclass(exc_type, KeyboardInterrupt):
# Call the default excepthook for KeyboardInterrupt to allow clean exit
sys.__excepthook__(exc_type, exc_value, tb)
return
print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")
pdb.post_mortem(tb)

def cause_exception():
# Function to demonstrate an unhandled exception
i = 3
return 1 / 0 # Will raise ZeroDivisionError

def f():
# Function to demonstrate an unhandled exception
i = 4
cause_exception()
return i

if __name__ == "__main__": # demo code
import os; os.environ['DEBUG'] = 'true'

if os.environ.get('DEBUG'):
sys.excepthook = debug_exception_hook
print("Debug mode enabled: Using pdb post-mortem on uncaught exceptions.")
else:
print("Debug mode not enabled: Regular exception handling.")

# Run demo to trigger exception
i = 5
f()

Then, I got this dialog:

Debug mode enabled: Using pdb post-mortem on uncaught exceptions.
Uncaught exception: ZeroDivisionError: division by zero

debug_hook.py(19)cause_exception()

return 1 / 0 # Will raise ZeroDivisionError

(Pdb) i
3
(Pdb)

.

--- SoupGate-Linux v1.05
* Origin: Dragon's Lair ---:- FidoNet<>Usenet Gateway -:--- (3:633/10)

From: ec1828@somewhere.edu

Annada Behera <annada@tilde.green> writes:

Hi,

Recently I have been increasingly adding this piece of code as
a preamble to a lot of my code.

import (sys, os, ipdb)

def debug_hook(exc_type, exc_value, traceback):
if exc_type is KeyboardInterrupt:
sys.__excepthook__(exc_type, exc_value, traceback)
return
print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")
ipdb.post_mortem(traceback)

if os.environ.get('DEBUG'): sys.excepthook = debug_hook

Pretty nice contribution! I had no idea such thing was possible with
Python. The more Common Lispy it gets, the better it feels! :>

--- SoupGate-Linux v1.05
* Origin: Dragon's Lair ---:- FidoNet<>Usenet Gateway -:--- (3:633/10)

From: annada@tilde.green

So, ipdb is the ipython version of pdb. In fact, post_mortem is a
pdb function. I use ipdb because its REPL is a bit nicer to work
with then pdb.

-----Original Message-----
From: Stefan Ram <ram@zedat.fu-berlin.de>
Subject: Re: Drop into REPL when your program crashes.
Date: 09/08/2025 06:04:16 PM
Newsgroups: comp.lang.python

Annada Behera <annada@tilde.green> wrote or quoted:

Recently I have been increasingly adding this piece of code as
a preamble to a lot of my code.

   import (sys, os, ipdb)

   def debug_hook(exc_type, exc_value, traceback):
       if exc_type is KeyboardInterrupt:
           sys.__excepthook__(exc_type, exc_value, traceback)            return
       print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")        ipdb.post_mortem(traceback)

   if os.environ.get('DEBUG'): sys.excepthook = debug_hook

  Thanks!

  I have changed it a bit to work without "ipdb" and added demo code.

import sys
import os
import pdb

i = 1

def debug_exception_hook(exc_type, exc_value, tb):
    i = 2
    if issubclass(exc_type, KeyboardInterrupt):
        # Call the default excepthook for KeyboardInterrupt to allow clean exit
        sys.__excepthook__(exc_type, exc_value, tb)
        return
    print(f"Uncaught exception: {exc_type.__name__}: {exc_value}")
    pdb.post_mortem(tb)

def cause_exception():
    # Function to demonstrate an unhandled exception
    i = 3
    return 1 / 0  # Will raise ZeroDivisionError

def f():
    # Function to demonstrate an unhandled exception
    i = 4
    cause_exception()
    return i

if __name__ == "__main__": # demo code
    import os; os.environ['DEBUG'] = 'true'

    if os.environ.get('DEBUG'):
        sys.excepthook = debug_exception_hook
        print("Debug mode enabled: Using pdb post-mortem on uncaught exceptions.")
    else:
        print("Debug mode not enabled: Regular exception handling.")    
    # Run demo to trigger exception
    i = 5
    f()

  Then, I got this dialog:

Debug mode enabled: Using pdb post-mortem on uncaught exceptions.
Uncaught exception: ZeroDivisionError: division by zero

debug_hook.py(19)cause_exception()

return 1 / 0  # Will raise ZeroDivisionError

(Pdb) i
3
(Pdb)

  .

--- SoupGate-Linux v1.05
* Origin: Dragon's Lair ---:- FidoNet<>Usenet Gateway -:--- (3:633/10)