Core concept

BDD stands for Behaviour driven development mostly used in Agile development, it follows the same principle as TDD (Test driven development), writing your test before the code.

The key point is not only to test using unit test, but also having the application tested end to end using acceptance tests. The process can be simply defined as:

  • Write a failing acceptance test
  • TDD cycle (Test fails ❌, make it pass ✅, refactor the code 🏗)
  • The acceptance test should pass

Acceptance tests make use of an English (or possibly alternative) language format called Gherkin syntax in the feature file, describing what the test is covering and the individual tests themselves.

Acceptance Tests with Gherkin Syntax

Acceptance tests were introduced with the Cucumber Framework 🥒. The syntax is quite easy to understand, and, in the Lettuce Python package, makes use of a range of keywords to define your features and tests:

  • Given
  • When
  • Then

… and more to make more english like when stacking them up.

With this framework, the acceptance tests will be saved in a .feature file. It’s decomposed into multiple blocks:

  • Feature:
    • Where you write your documentation for what this group of test is cover, no code executed here. Only for comprehension
  • Background:
    • Executed prior to every Scenario in the .feature file, like a setup.
  • Scenario:
    • Here you define the test, the first line is for documentation and then the test (with the given syntax).

Once the .feature has been created you need to have a that will create the test from the features (it’s not just magic). The uses RegEx with lettuce to read the input and execute the test.

Example in Python

Following the tutorial from code tutorial’s plus by David Sale

Project Structure

The project should be structured like:

  |_ app
  |   |_
  |   |_
  |_ tests
      |_ features
          |_ factorial.feature


For the process, once you have python 2.x installed (because lettuce is only compatible with Python 2.x):

  • Install lettuce that will be used as the Gherkin parser.
pip install lettuce
  • Install the same test module as in tdd such as Nose:
easy_install nose
# or
pip install nose

Writing a scenario

So here would be an example of a factorial.feature with one scenario:

Feature: Compute factorial
  In order to play with Lettuce
  We'll implement factorial in a calculator

  Given I am using the calculator

Scenario: Factorial of 4
  Given I have the number 4
  When I compute its factorial
  Then I see the number 24

The text in Feature is descriptive, the Background gives a step that will be used for every scenario. Then we have the Scenario which contains the actual test.

Usually the setup is done in the Given, then an action is triggered in the When and the result is validated in the Then phase.

Implementing a step

In the you can implement the code that will be executed on a Gherkin sentence like:

Given I have the number 4

The code behind it will look like:

from lettuce import world

@step('I have the number (\d+)')
def have_the_number(step, number):
    world.number = int(number)

The world is a built-in lettuce tool that allow you to link all the steps together. For example the calculator could be in world.calculator depending on how you set it.

You can see that the parsing is done via some regex in the @step the number variable passed into the method is caught via the (/d+) in the regex. You can pass as many variable as you want. Find online some BDD cheatsheet to create better steps.

Executing the features

You can either run just one feature file, or, if you pass a directory of feature files, you can run all of them.

lettuce tests/features/factorial.feature 

Then you will see each scenario and the corresponding result. Having it available only for Python 2.x is a bit of a let down, they may make it available with Python 3.x in the future.