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Mutation tests for ruby

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Build Status Dependency Status Code Climate

Mutant is a mutation testing tool for ruby that aims to be better than existing mutation testers.

The idea is that if code can be changed and your tests do not notice, either that code isn't being covered or it does not have a speced side effect.

Mutant does currently only support 1.9 mode under Rubinius or MRI. Support for JRuby is planned.

Also it is easy to write a mutation killer for other test/spec frameworks than rspec2. Just create your own Mutant::Killer subclass, and make sure I get a PR!

See this ASCII-Cast for mutant in action! (v0.2.1)

Using Mutant

The following projects adopted mutant.

Feel free to ping me to add your project to the list!


Install the gem mutant via your preferred method.


CLI will be simplified in the next releases, but currently stick with this:

cd virtus
# Run mutant on virtus namespace (that uses the dm-2 style spec layout)
mutant -I lib -r virtus --rspec-dm2 ::Virtus
# Run mutant on specific virtus class 
mutant -I lib -r virtus --rspec-dm2 ::Virtus::Attribute
# Run mutant on specific virtus class method
mutant -I lib -r virtus --rspec-dm2
# Run mutant on specific virtus instance method
mutant -I lib -r virtus --rspec-dm2 ::Virtus::Attribute#name


Mutation testing is slow. To make it fast the selection of the correct set of tests to run is the key. Mutant currently supports the following buildin strategies for selecting tests/specs.


This strategy is the fastest but requires discipline in spec file naming.

The following specs are executed to kill a mutation on:

Public instance  methods: spec/unit/#{namespace}/#{class_name}/#{method_name}_spec.rb
Public singleton methods: spec/unit/#{namespace}/#{class_name}/class_methods/#{method_name}_spec.rb
Private instance  methods: spec/unit/#{namespace}/#{class_name}/*_spec.rb
Private singleton methods: spec/unit/#{namespace}/#{class_name}/class_methods/*_spec.rb


This strategy executes all specs under ./spec/unit for each mutation.


This strategy executes all specs under ./spec/integration for each mutation.


This strategy executes all specs under ./spec for each mutation.

It is also plannned to allow explicit selections on specs to run and to support other test frameworks. Custom project specific strategies are also on the roadmap.




  • Fork the project.
  • Make your feature addition or bug fix.
  • Add tests for it. This is important so I don't break it in a future version unintentionally.
  • Commit, do not mess with Rakefile or version (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
  • Send me a pull request. Bonus points for topic branches.


See LICENSE file.

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