Starting python debugger automatically on error

Question

This is a question I have wondered about for quite some time, yet I have never found a suitable solution. If I run a script and I come across, let\'s say an IndexError, pyth

Answer 1

Put a breakpoint inside the constructor of topmost exception class in the hierarchy, and most of the times you will see where the error was raised.

Putting a breakpoint means whatever you want it to mean : you can use an IDE, or pdb.set_trace, or whatever

Answer 2

If you are using ipython, after launching type %pdb

In [1]: %pdb
Automatic pdb calling has been turned ON

Answer 3

If you are running a module:

python -m mymodule

And now you want to enter pdb when an exception occurs, do this:

PYTHONPATH="." python -m pdb -c c mymodule/__main__.py

(or extend your PYTHONPATH). The PYTHONPATH is needed so that the module is found in the path, since you are running the pdb module now.

Answer 4

IPython makes this simple on the command line:

python myscript.py arg1 arg2

can be rewritten to

ipython --pdb myscript.py -- arg1 arg2

Or, similarly, if calling a module:

python -m mymodule arg1 arg2

can be rewritten to

ipython --pdb -m mymodule -- arg1 arg2

Note the -- to stop IPython from reading the script's arguments as its own.

This also has the advantage of invoking the enhanced IPython debugger (ipdb) instead of pdb.

Answer 5

You can use traceback.print_exc to print the exceptions traceback. Then use sys.exc_info to extract the traceback and finally call pdb.post_mortem with that traceback

import pdb, traceback, sys

def bombs():
    a = []
    print a[0]

if __name__ == '__main__':
    try:
        bombs()
    except:
        extype, value, tb = sys.exc_info()
        traceback.print_exc()
        pdb.post_mortem(tb)

If you want to start an interactive command line with code.interact using the locals of the frame where the exception originated you can do

import traceback, sys, code

def bombs():
    a = []
    print a[0]

if __name__ == '__main__':
    try:
        bombs()
    except:
        type, value, tb = sys.exc_info()
        traceback.print_exc()
        last_frame = lambda tb=tb: last_frame(tb.tb_next) if tb.tb_next else tb
        frame = last_frame().tb_frame
        ns = dict(frame.f_globals)
        ns.update(frame.f_locals)
        code.interact(local=ns)

Answer 6

If you are using the IPython environment, you can just use the %debug and the shell will take you back to the offending line with the ipdb environment for inspections etc. Another option as pointed above is to use the iPython magic %pdb which effectively does the same.