interpreter pattern this little bit of css code tells browser to render all the text inside the body tag of the web page that uses this css file in green the code needed by the browser to do this is lot more complex than this simple snippet of codeyet css is simple enough for most people to pick up in couple of hours html itself is dsl that is interpreted by the browser and displayed as formatted document with fairly intricate layout alternativelyyou may have used aloe in behavior-driven developmentwhere you define acceptance criteria for project in human-readable sentences install the aloe framework using pip pip install aloe nowwe can write some tests zero feature featurecompute factorial in order to play with aloe as beginners we'll implement factorial scenariofactorial of given have the number when compute its factorial then see the number nextyou map these steps to some python code so the aloe framework can understand the behavior described steps py from aloe import @step( ' have the number (\ +)'def have_the_number(stepnumber)world number int(number@step(' compute its factorial'def compute_its_factorial(step)world number factorial(world number