Mutatest vs pytest-gremlins

Mutatest is a free command-line tool for Python projects that checks whether tests catch small changes to code. It creates valid mutations from Python’s Abstract Syntax Tree grammar and applies them to local __pycache__ files rather than changing source files. If coverage data is available, it can limit targets to lines marked as covered. Mutations are sampled randomly, with a seed available to make runs reproducible. Command-line controls include run modes, mutation categories, file exclusions, timeouts, reports, and thresholds for surviving mutants; an API supports custom controls. The documentation describes using test commands such as pytest and coverage data to filter targets. Mutatest can raise an exception when surviving mutants meet a configured threshold, including in automated runs such as CI. It supports Linux, macOS, and Windows, and is free under the MIT License. Installation requires Python 3.7 or 3.8; multiprocessing is available on 3.8, but not 3.7. Repeated test runs can be slow, so it is intended for diagnostics rather than every commit across an entire codebase.

pytest-gremlins is a pytest plugin for checking whether an existing test suite catches small changes to code. It runs mutation tests against Python projects and can report results in the console, HTML, or JSON, including multiple formats in one run. Getting started requires Python 3.11 or later, pytest 7.0 or later, and existing tests; installation is documented with pip, uv, and Poetry. The project’s quick-start command is `pytest --gremlins`. To manage execution, it uses mutation switching, coverage-guided test selection, incremental analysis, and parallel execution, and it works with pytest-xdist. Options can be set in `pyproject.toml` or with command-line flags, while inline pragmas can pardon specific mutations. A GitHub Action can run the checks and fail a workflow if its score misses a threshold; CI examples also cover GitLab CI, CircleCI, and Dagger. It is a free, MIT-licensed package for Linux, classified as Alpha. The guide warns that an initial run may take 10–100 times as long as a normal test run, though caching can shorten later runs. It also notes that equivalent mutations can make a perfect score unattainable and unnecessary.