Python Cookbook

Getting More Information from Tracebacks

Credit: Bryn Keller

Problem

You want to display all of the available information when an uncaught exception is raised.

Solution

A traceback object is basically a linked list of nodes, in which each node refers to a frame object. Frame objects, in turn, form their own linked list opposite the linked list of traceback nodes, so we can walk back and forth if needed. This recipe exploits this structure and the rich amount of information held by frame objects, including the dictionary of local variables for the function corresponding to each frame, in particular:

import sys, traceback def print_exc_plus( ): """ Print the usual traceback information, followed by a listing of all the local variables in each frame. """ tb = sys.exc_info( )[2] while 1: if not tb.tb_next: break tb = tb.tb_next stack = [] f = tb.tb_frame while f: stack.append(f) f = f.f_back stack.reverse( ) traceback.print_exc( ) print "Locals by frame, innermost last" for frame in stack: print print "Frame %s in %s at line %s" % (frame.f_code.co_name, frame.f_code.co_filename, frame.f_lineno) for key, value in frame.f_locals.items( ): print "\t%20s = " % key, # We have to be VERY careful not to cause a new error in our error # printer! Calling str( ) on an unknown object could cause an # error we don't want, so we must use try/except to catch it -- # we can't stop it from happening, but we can and should # stop it from propagating if it does happen! try: print value except: print "<ERROR ...