mutmut

mutmut is a mutation testing system for Python, with a strong focus on ease of use. It modifies the source code in small ways and runs the test suite for each mutant to check whether the tests detect the change.

It is run from the command line, typically with pytest, and remembers the results between runs so that work can be resumed. Surviving mutants can be listed and inspected as diffs, and an interactive browser helps review them and apply a mutation to the code to understand which test is missing. Results can be exported for use in CI.

mutmut is free and open source under the BSD 3-Clause licence.