When python raise an exception in the middle of a pygtk signal handling callback, the exception is catched by the gtk main loop, its value printed and the main loop just continue, ignoring it. If you want to debug, with something like pdb (python -m pdb myscript.py), you want that when the exception occure PDB jump on it and you can start debuging. Because of that it's not possible. How can i debug pygtk program then ?