Behave brings the Behaviour-driven development (BDD) to Python. BDD is an agile software development technique that guarantees code quality and makes easier the collaboration of non-tecnical participant. To succeed in this the Gherkin language is used. The Gherkin language is a text format to specify software behaviours through natural language without needing a non-technical vocabulary.

In BDD the software functionality is defined using scenarios. An scenario defines an specific aspect of behavior of the application. To do so it is composed by three descriptions (or steps):

1. A given description where the initial condition is setted.
2. States that trigger the scenario.
3. The expected outcome.

Scenario: New lists are empty
Given: a new list
When: 
Then: the list should be empty.

def test_empty_list_is_false(self):
   list = []
   assertEqual(bool(list), False)

Scenario: Lists with things in them are not empty.
Given: a new list
When: we add an object
Then: the list should not be empty.

def test_populated_list_is_true(self):
   list = []
   list.append('item')
   assertEqual(bool(list), True)

In BDD all the scenarios are placed into a user story. A user story describes a particular feature and the value delivered throught that feature. Scenarios will be executables independently, it allows to know the way in which the value is delivered at each level of the user story. So the sum of those values is the user required value. On the other hand the software responsability is divided in units of code, it minimises possible failures and reduce non-valuable code.