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Class 8: Behavior-driven design (BDD) and Test-driven Development

Held: Wednesday, 8 February 2017

We consider two key ideas in software development and testing, one of which emphasizes high-level functionality and one of which emphasizes procedure-level functionality.



  • Behavior-driven design (BDD)
  • Cucumber basics
  • Test-driven development
  • Why test?
  • An example


News / Etc.

  • Please let me know when you notice problems on the Web site. I think I’ve fixed most of them.

Upcoming Work

  • No reading or reading journal for Thursday! Use the time to work on understanding Cucumber.
  • Programming assignment: Intro rails (due TONIGHT at 10:30 p.m.)
  • Programming assignment: Cucumber (due Tuesday the 26 at 10:30 p.m.)
    • Those working on Python/Django projects may choose to design their own equivalent assignment.

Good things to do

Note: I do not do extra credit in two-credit classes.


  • CS Table today
  • CS Extras Thursday


  • Happy Kareoke Fun Time activities
    • Musical, Friday 9pm, Gardner.
    • Workshop: Burn your fear. Saturday, 2pm. The Wall. Email [improv] to sign up.
    • Workshop: Creating through improv. Sunday, 2pm. The Wall. Email [improv] to sign up.
    • Musical, Sunday, 7pm, Gardner

Test-driven development

  • What is it?
  • How does it relate to the kinds of testing you’ve done in your other classes?
  • Why do we do it?

Behavior-Driven Design

  • What do you see as the key features of BDD?
  • What differences do you see between BDD and TDD?
  • What are some potential disadvatnages of BDD?

Cucumber Basics

  • A programmatic approach to Behavior-Driven Design
  • Often viewed in terms of a “testing stack”

      Step Definitions
      Support Code
      Automation Library
  • Features, scenarios, and steps give the high-level limited-form natural-language definition of aspects of the user story.
  • Step definitions indicate how to translate the steps to procedure calls. (Yay pattern matching!)
  • Support code helps implement some of those calls.
  • The automation library deals with common things, like fetching pages.
    • We often use Capybara to help with automation.

A Cucumber Example

Let’s walk through some scenarios and think about what work we might have to do to implement these Cucumber scenarios. Do they affect model, view, or controller (or all three)? (Suppose we had the repaired version of the original database, with no director.)

Feature: search for movies by director

  As a movie buff
  So that I can find movies with my favorite director
  I want to include and search on director information in movies I enter

Background: movies in database

  Given the following movies exist:
  | title        | rating | director     | release_date |
  | Star Wars    | PG     | George Lucas |   1977-05-25 |
  | Blade Runner | PG     | Ridley Scott |   1982-06-25 |
  | Alien        | R      |              |   1979-05-25 |
  | THX-1138     | R      | George Lucas |   1971-03-11 |

Scenario: add director to existing movie
  When I go to the edit page for "Alien"
  And  I fill in "Director" with "Ridley Scott"
  And  I press "Update Movie Info"
  Then the director of "Alien" should be "Ridley Scott"

Scenario: find movie with same director
  Given I am on the details page for "Star Wars"
  When  I follow "Find Movies With Same Director"
  Then  I should be on the Similar Movies page for "Star Wars"
  And   I should see "THX-1138"
  But   I should not see "Blade Runner"

Scenario: can't find similar movies if we don't know director (sad path)
  Given I am on the details page for "Alien"
  Then  I should not see "Ridley Scott"
  When  I follow "Find Movies With Same Director"
  Then  I should be on the home page
  And   I should see "'Alien' has no director info"

Cucumber in Practice

  • Our alums tell me that Cucumber has been less successful than people had expected.
  • Ideal: Clients can write user stories.
    • Reality: Clients can read user stories.
    • Reality: Programmers have to write user stories.
  • “Natural language” descriptions seem like a good idea, but managing three sets of files (use cases, patterns that translate to real test code, real test code) can be problematic.
  • Automatic database setup is nice, but there are other Gems, such as FactoryGirl, that also help with database setup.
  • I will not require you to write explicit Cucumber tests, but I will expect you to write use cases and to write tests (in RSpec or Minitest) that correspond to those use cases.