mutmut

What mutmut is

mutmut is an open-source mutation testing system for Python that helps teams check whether their pytest tests catch changes to code. It creates mutants by incrementing integer literals, changing < to <=, or swapping break and continue. Its interactive terminal interface lets users browse mutants, write one to disk, add tests to address surviving changes, and rerun individual mutants. mutmut remembers completed work for incremental runs and selects which tests to execute for each mutant. Execution is described as parallel and fast. When configured, coverage.py can restrict mutations to covered lines, while mypy or pyrefly can filter mutants; pyright and ty are not supported for that filtering purpose. It can also export CI/CD statistics and generate mutation-score badge JSON for a Shields endpoint. Configuration in setup.cfg or pyproject.toml supports source paths and pytest selection arguments. mutmut requires Python 3.10 or later and a system with fork support. It runs on Linux and macOS; Windows users must run it inside WSL. Version 3 and later mutate code inside functions, while version 2 has a different model for code outside functions.