From https://github.com/testing-cabal/testtools/issues/111 - any code that is data dependent can be hard to diagnose from a backtrace alone. Many unittest and server environments address this by doing custom tracebacks that include locals.

To address this in unittest, we need to add a similar capacity to traceback, and then use it from unittest (as we can't just depend on a module from PyPI in the stdlib). This issue is for the unittest part of it. For the traceback dependency see http://bugs.python.org/issue22936 .