GivenWhenThen in BDD

Given-When-Then is a style of representing tests - or as its advocates would say - specifying a system's behavior using SpecificationByExample. It's an approach developed by Dan North and Chris Matts as part of Behavior-Driven Development (BDD). [1] It appears as a structuring approach for many testing frameworks such as Cucumber. You can also look at it as a reformulation of the Four-Phase Test pattern.

The essential idea is to break down writing a scenario (or test) into three sections:

• The given part describes the state of the world before you begin the behavior you're specifying in this scenario. You can think of it as the pre-conditions to the test.
• The when section is that behavior that you're specifying.
• Finally the then section describes the changes you expect due to the specified behavior.

Feature: User trades stocks
  Scenario: User requests a sell before close of trading
    Given I have 100 shares of MSFT stock
       And I have 150 shares of APPL stock
       And the time is before close of trading

    When I ask to sell 20 shares of MSFT stock
     
     Then I should have 80 shares of MSFT stock
      And I should have 150 shares of APPL stock
      And a sell order for 20 shares of MSFT stock should have been executed