Like any modern language, Python comes along with a nice traceback module. This module gives you stack traces from the line of code where an exception is raised up to the next try-except clause. So, you can easily catch exceptions and write stack traces into a debug log. This debugging technique is pretty handy to drill down bugs and I use it a lot in prototyping.
Using the traceback module is straight forward for evident programming mistakes. However, real bugs are context-sensitive and they can hardly be reproduced without the actual data that was processed when an exception was raised. If you can reproduce a specific bug, you can add some logging code in front and inspect the variables the next time the bug is triggered. But if a bug occurs once in a blue moon, you’d be better in logging the data the first time an exception raises.
import tracebackturbo as traceback def erroneous_function(): ham = u"unicode string with umlauts äöü." eggs = "binary string with umlauts äöü." i = 23 if i>5: raise Exception("it's true!") try: erroneous_function() except: print traceback.format_exc(with_vars=True)
Here’s my solution; an improved Python traceback module the logs variables from the local scope next to the affected code. You can find a working copy in our Mercirual repository (see the below).
Traceback (most recent call last): File "test.py", line 11, in
Local variables: __builtins__ = __doc__ = None __file__ = "x" __name__ = "__main__" __package__ = None erroneous_function = traceback = <module 'tracebackturbo' from '/private/tmp/python-... erroneous_function() File "test.py", line 8, in erroneous_function Local variables: eggs = "binary string with umlauts \xc3\xa4\xc3\xb6\xc3\xbc." ham = u"unicode string with umlauts ???." i = 23 raise Exception("it's true!") Exception: it's true!
I am not sure if it is the “right” solution as sensitive information might be logged. This might have security implications for some real-world scenarios where webapps report stack traces to the end user (e.g. by using cgitb in production).
Credit: this code was inspired by format_exc_plus by Bryn Keller.
2010-01-28: there’s an active discussion on python-dev.
2011-06-25: I’ve renamed the module, enable print_vars by default and merge with upstream