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irradiate

Fast mutation testing for Python, written in Rust.

What mutation testing catches

Mutation testing works by making small, deliberate changes to your code — like flipping a < to <=, swapping True for False, or replacing + with - — and then running your tests against each change. If a test fails, great: your tests caught the bug. If every test still passes, that's a gap — you have code that can break without any test noticing.

Code coverage tells you which lines ran. Mutation testing tells you which lines are actually tested. A function can have 100% line coverage but still have mutants that survive, meaning your tests execute the code without meaningfully checking what it does.

irradiate lets you:

  • Add tests where they matter — surviving mutants point to exact lines and conditions your tests don't verify, so you know precisely where to write the next test.
  • Tighten weak assertions — a test that runs the code but only checks assert result is not None will let most mutants through. Surviving mutants reveal where you need stricter checks.
  • Remove tests that aren't pulling their weight — if a test kills zero mutants that other tests don't already catch, it's redundant. Cut it or replace it with something sharper.
  • Find code that isn't reachable — if every mutant in a block survives and no test even exercises it, that code may be dead. Delete it or question why it exists.

Quick start

pip install irradiate

irradiate run src

For a complete walkthrough, see the Quick Start guide.

Features

  • Fast — pre-warmed pytest workers with fork-per-mutant execution. Pytest starts once. Tests run many times.
  • 38 mutation operators — arithmetic, comparison, boolean, string methods, return values, exception types, regex patterns, and more.
  • Incremental — --diff main tests only functions changed since a git ref.
  • Cached — content-addressed results survive rebases, branch switches, and touch.
  • CI-ready — --fail-under 80 for gating, GitHub Actions annotations, JSON and HTML reports.
  • Drop-in — works with any pytest project. pip install irradiate && irradiate run src.