Benjamin Schweizer.

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

→ github repository

This entry was posted on Wednesday, January 27th, 2010 at 11:39 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.