Cosmic Ray: mutation testing for Python

“Four human beings – changed by space-born cosmic rays into something more than merely human.”

—The Fantastic Four

Cosmic Ray is a mutation testing tool for Python 3. It makes small changes to your production source code, running your test suite for each change. If a test suite passes on mutated code, then you have a mismatch between your tests and your functionality.

Like coverage analysis, mutation testing helps ensure that you’re testing all of your code. But while coverage only tells you if a line of code is executed, mutation testing will determine if your tests actually check the behavior of your code. This adds tremendous value to your test suite by helping it fulfill its primary role: making sure your code does what you expect it to do!

Cosmic Ray has been successfully used on a wide variety of projects ranging from assemblers to oil exploration software.

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