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Lazy coverage-aware running of Cucumber acceptance tests


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Cucover is a thin wrapper for Cucumber which makes it lazy.

Question: What does it mean for Cucumber to be lazy?

Answer: It will only run a scenario if it needs to.

How does it decide whether it needs to run a scenario? Every time you run a feature using Cucover, it watches the code in your application that is executed, and remembers. The next time you run Cucover, it skips a scenario if the source files (or the feature itself) have not been changed since it was last run.


  • Within a feature, Cucover will only re-run the Scenarios that have been affected by your changes
  • Uses RCov to map features to covered source files
  • Patches Rails to also map scenarios to covered .erb view templates
  • Shows skipped Scenarios, for confidence
  • Re-runs failing features, even when nothing has changed, for that good old red-bar feel.
  • Allows you to see which lines of a source file are tested by which scenarios

Installation and Usage

sudo gem install mattwynne-cucover --source

To run your features lazily, use the cucover binary instead of cucumber:

cucover -- features/lamb_chops.feature

To see what Cucover has already recorded (in the file):

cucover --show-recordings

To find out which tests cover which lines of a given source file:

cucover --coverage_of path/to/some_source_file.rb


  • Anything that runs out of process will not be covered, and therefore cannot trigger a re-run, so if you use Cucumber to drive Selenium, for example, you're out of luck.
  • This is very new and experimental. There may be bugs. Feedback is welcome via github messages.


  • Proper args parsing and command-line help
  • Work on a way to include coverage from out-of-process ruby code run during a test
  • Speed up the whole thing by only writing the recordings to disk when the process exits
  • Run code coverage and remove any slop following refactoring
  • Speed up the Rails test - maybe strip some guff out of the environment load?

Similar 'Selective Testing' Tools


  • Matt Wynne
  • Joseph Wilk