Mutmut is a mutation testing system for Python, with a strong focus on ease of
use. If you don't know what mutation testing is try starting with this article.

Some highlight features:
* Found mutants can be applied on disk with a simple command making it very easy
  to work with the results
* Remembers work that has been done, so you can work incrementally
* Supports all test runners (because mutmut only needs an exit code from the
  test command)
* Can use coverage data to only do mutation testing on covered lines
* Battle tested on real libraries by multiple companies