Python - start interactive debugger when exception would be otherwise thrown

Question

Is there any way to make a python program start an interactive debugger, like what import pdb; pdb.set_trace() instead of actually throwing an exception?

I know the difficulty of making this work, but it would be much more valuable than a huge stack trace after which I have to use to figure out where to insert breakpoints and then restart the program to debug it. I know that simply making the debugger start instead of throwing an exception would not make sense because any exception can be caught at a level or another, so if I could just select a list of exceptions for which an interactive debug session would start instead of them being thrown (because I know the exceptions in this list would really be "errors" and no meaningful program behavior could follow afterwards)...

I've heard that Common Lisp has something like this, but I don't know how it works exactly, just that "true lispers" praise it a lot...

Answer 1

The simplest way is to wrap your entire code inside a try block like this:

if __name__ == '__main__':      try:         raise Exception()     except:         import pdb         pdb.set_trace() 

There is a more complicated solution which uses sys.excepthook to override the handling of uncaught exceptions, as described in this recipe:

## {{{ http://code.activestate.com/recipes/65287/ (r5) # 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 ## end of http://code.activestate.com/recipes/65287/ }}} 

The above code should be included in a file called sitecustomize.py inside site-packages directory, which is automatically imported by python. The debugger is only started when python is run in non-interactive mode.

Answer 2

This question is quite old, so this is mainly for future me

try:     ... except:     import traceback, pdb, sys     traceback.print_exc()     print ''     pdb.post_mortem()     sys.exit(1)