I Mutation-Tested 50 Python Libraries. Tests in the Young Ones Missed 41% of Planted Faults.

Mutation testing evaluates a test suite's effectiveness by deliberately introducing small bugs into the code to see if the tests can detect them. An analysis of 50 Python libraries revealed that tests for younger libraries missed 41% of these planted faults, whereas tests for mature libraries missed only 17%. This process not only highlights gaps in test coverage, especially around boundary conditions, but also uncovered four real-world bugs in the libraries studied. The content provides a 60-line Python script using the `ast` module to implement a basic mutation testing tool. This technique provides a more accurate measure of a test suite's quality than simply observing that all tests pass.