Cosmic Ray vs pytest-gremlins
Cosmic Ray
cosmic-ray.readthedocs.iopytest-gremlins
github.comSide by side
| Figure | Cosmic Ray | pytest-gremlins |
|---|---|---|
| Price from | — | Free |
| Free plan | — | Yes |
| Free trial | — | — |
| Open source | — | — |
| Linux | Yes | Yes |
| Self-hosted | Yes | — |
| Plans published | 0 | 1 |
| Spec score · Mutation Testing Tools | 7.5 (#13) | 7.7 (#10) |
| License | The project repository identifies the software as MIT licensed. | The project is MIT licensed. |
| Purpose | Cosmic Ray is a mutation testing tool for Python 3 that makes small changes to production code and runs the test suite against each change. | pytest-gremlins is a pytest plugin that injects small code mutations and checks whether tests catch them. |
| Reports | Cosmic Ray provides `cr-report` for session summaries and `cr-html` for generating an HTML report that shows the mutation used. | It supports console, HTML, and JSON reports, including multiple report formats in one run. |
Plans
In their own words
Cosmic Ray
Cosmic Ray has no write-up on Specifiction yet; its figures are above.
pytest-gremlins
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.