Tester's Digest

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TESTER’S DIGEST

ISSUE #42 - December 17, 2017

You use tests to make your code better. You use mutation testing to make your tests better.

Topic: Mutation Testing

What is mutation testing: the basic idea is to inject bugs into your source code to establish whether your unit tests would find them, with the purpose of then adding the missing tests.

https://blog.codecentric.de/en/2016/01/mutation-testing-watching-watchmen/

How to pick the right mutants to destroy in your code by adding only the useful missing tests:

https://blog.codecentric.de/en/2016/02/sensible-mutation-testing-dont-go-killing-spree-2/

Neither 100% coverage, nor 100% mutation coverage are a silver bullet against bugs. However, mutation testing clearly shows out pieces of code which need refactoring, and causes you to write more asserts and construct more detailed tests.

http://atodorov.org/blog/2016/12/27/mutation-testing-vs-coverage/

How mutation testing fits with agile process: use pairing, apply MT pragmatically to avoid shift from the useful mentality of “Did you think of this corner case?” to the much less useful “These 3% of expressions/mutants are not covered”, and use MT as a tool to promote refactoring in the Red-Green-Refactor cycle of TDD.

https://www.sep.com/sep-blog/2015/07/14/mutation-testing-totally-a-thing/

A neat story of debugging a Rails issue, found via mutation testing:

https://blog.arkency.com/constructor-for-a-included-module-in-ruby/

This is the list of current MT tools I’m aware of.

Mutation testing in Java with PIT:

http://pitest.org/

Mutation testing in Python with Cosmic Ray or Mutmut:

http://cosmic-ray.readthedocs.io/en/latest/

https://hackernoon.com/mutmut-a-python-mutation-testing-system-9b9639356c78

Mutation testing in Javascript with Stryker:

https://stryker-mutator.github.io/

Mutation testing in Ruby with Mutant or Mutest:

http://blog.arkency.com/2015/06/how-good-are-your-ruby-tests-testing-your-tests-with-mutant/

https://blog.cognitohq.com/how-to-write-better-code-using-mutation-testing/

How Mutant gem works in Ruby, in much detail, for those interested in the technical underpinnings:

https://troessner.svbtle.com/kill-all-the-mutants-a-deep-dive-into-mutation-testing-and-how-the-mutant-gem-works

Off-Topic

Worth learning: what happens when you press “play” on Netflix, from “what’s a CDN” to how machine learning fits in.

http://highscalability.com/blog/2017/12/11/netflix-what-happens-when-you-press-play.html


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