Imagine you are a Django developer building a social network for a lean startup. The CEO is pressuring your team for an MVP. The engineers have agreed to build the product using behavior-driven development (BDD) to deliver fast and efficient results. The product owner gives you the first feature request, and following the practice of all good programming methodologies, you begin the BDD process by writing a test. Next you code a bit of functionality to make your test pass and you consider your design. The last step requires you to analyze the feature itself. Does it belong in your app?

We can't answer that question for you, but we can teach you when to ask it. In the following tutorial, we walk you through the BDD development cycle by programming an example feature using Django and Aloe. Follow along to learn how you can use the BDD process to help catch and fix poor designs quickly while programming a stable app.

Contents

Objectives

By the time you complete this tutorial, you should be able to:

Describe and practice behavior-driven development (BDD)

Explain how to implement BDD in a new project

Test your Django applications using Aloe

Project Setup

Want to build this project as you read the post?

Start by:

Adding a project directory. Creating and activating a virtual environment.

Then, install the following dependencies and start a new Django project:

( venv ) $ pip install \ django == 3 .0.5 \ djangorestframework == 3 .11.0 \ aloe_django == 0 .2.0 ( venv ) $ django-admin startproject example_bdd ( venv ) $ cd example_bdd ( venv ) $ python manage.py startapp example

You may need to manually install setuptools-scm ( pip install setuptools-scm ) if you get this error when trying to install aloe_django :

distutils.errors.DistutilsError: Could not find suitable distribution for Requirement.parse ( 'setuptools_scm' )

Update the INSTALLED_APPS list in settings.py:

INSTALLED_APPS = [ ... 'aloe_django' , 'rest_framework' , 'example' , ]

Just looking for the code? Grab it from the repo.

Brief Overview of BDD

Behavior-driven development is a way of testing your code that challenges you to constantly revisit your design. When you write a test, you answer the question Does my code do what I expect it to do? through assertions. Failing tests expose the mistakes in your code. With BDD, you analyze a feature: Is the user experience what I expect it to be? There is nothing as concrete as a failing test to expose a bad feature, but the consequences of delivering a bad experience are tangible.

Execute BDD as part of your test development cycle. Draw the functional boundaries of a feature with tests. Create code that colors in the details. Step back and consider your design. And then do it all over again until the picture is complete.

Review the following post for a more in-depth explanation of BDD.

Your First Feature Request

"Users should be able to log into the app and see a list of their friends."

That's how your product manager starts the conversation about the app's first feature. It's not much but you can use it to write a test. She's actually requesting two pieces of functionality--user authentication and the ability to form relationships between users. Here's a rule of thumb: treat a conjunction like a beacon warning you against trying to test too many things at once. If you ever see an "and" or an "or" in a test statement, you should break that test into smaller ones.

With that truism in mind take the first half of the feature request and write a test scenario: a user can log into the app. In order to support user authentication, your app must store user credentials and give users a way to access their data with those credentials. Here's how you translate those criteria into an Aloe .feature file.

example/features/friendships.feature

Feature: Friendships Scenario: A user can log into the app Given I empty the "User" table And I create the following users: | id | email | username | password | | 1 | [email protected] | Annie | pAssw0rd! | When I log in with username "Annie" and password "pAssw0rd!" Then I am logged in

An Aloe test case is called a feature. You program features using two files--a Feature file and a Steps file.

The Feature file consists of statements written in plain English that describe how to configure, execute, and confirm the results of a test. Use the Feature keyword to label the feature and the Scenario keyword to define a user story that you are planning to test. In the example above, the scenario defines a series of steps that explain how to populate the User database table, log a user into the app, and validate the login. All step statements must begin with one of four keywords: Given , When , Then , or And . The Steps file contains Python functions that are mapped to the Feature file steps using regular expressions.

You may need to add an __init__.py file to the "features" directory for the interpreter to load the friendships_steps.py file correctly.

Run python manage.py harvest and see the following output.

nosetests --verbosity=1 Creating test database for alias 'default' ... E ====================================================================== ERROR : A user can log into the app ( example . features . friendships : Friendships ) ---------------------------------------------------------------------- Traceback ( most recent call last ): File "/blog-aloe-bdd/lib/python3.6/site-packages/aloe/registry.py" , line 151 , in wrapped return function ( * args , ** kwargs ) File "/blog-aloe-bdd/example_bdd/example/features/friendships.feature" , line 5 , in A user can log into the app Given I empty the "User" table File "/blog-aloe-bdd/lib/python3.6/site-packages/aloe/registry.py" , line 151 , in wrapped return function ( * args , ** kwargs ) File "/blog-aloe-bdd/lib/python3.6/site-packages/aloe/exceptions.py" , line 44 , in undefined_step raise NoDefinitionFound ( step ) aloe . exceptions . NoDefinitionFound : The step r "Given I empty the " User " table" is not defined ---------------------------------------------------------------------- Ran 1 test in 0 . 509 s FAILED ( errors = 1 ) Destroying test database for alias 'default' ...

The test fails because you haven't mapped the step statements to Python functions. Do so in the following file.

example/features/friendships_steps.py

from aloe import before , step , world from aloe.tools import guess_types from aloe_django.steps.models import get_model from django.contrib.auth.models import User from rest_framework.test import APIClient @before . each_feature def before_each_feature ( feature ): world . client = APIClient () @step ( 'I empty the "([^"]+)" table' ) def step_empty_table ( self , model_name ): get_model ( model_name ) . objects . all () . delete () @step ( 'I create the following users:' ) def step_create_users ( self ): for user in guess_types ( self . hashes ): User . objects . create_user ( ** user ) @step ( 'I log in with username "([^"]+)" and password "([^"]+)"' ) def step_log_in ( self , username , password ): world . is_logged_in = world . client . login ( username = username , password = password ) @step ( 'I am logged in' ) def step_confirm_log_in ( self ): assert world . is_logged_in

Each statement is mapped to a Python function via a @step() decorator. For example, Given I empty the "User" table will trigger the step_empty_table() function to run. In this case, the string "User" will be captured and passed to the function as the model_name parameter. The Aloe API includes a special global variable called world that can be used to store and retrieve data between test steps. Notice how the world.is_logged_in variable is created in step_log_in() and then accessed in step_confirm_log_in() . Aloe also defines a special @before decorator to execute functions before tests run.

One last thing--consider the structure of the following statement.

And I create the following users: | id | email | username | password | | 1 | [email protected] | Annie | pAssw0rd! |

With Aloe, you can represent lists of dictionaries using a tabular structure. Access the data using self.hashes . Wrapping self.hashes in the guess_types() function returns the list with the dictionary values correctly typed. In the case of this example, guess_types(self.hashes) returns this code.

Run the Aloe test suite with the following command and see all tests pass.

( venv ) $ python manage.py harvest

nosetests --verbosity=1 Creating test database for alias 'default' ... . ---------------------------------------------------------------------- Ran 1 test in 0 . 512 s OK Destroying test database for alias 'default' ...

Write a test scenario for the second part of the feature request: a user can see a list of friends.

example/features/friendships.feature

Scenario: A user can see a list of friends Given I empty the "Friendship" table When I get a list of friends Then I see the following response data: | id | email | username |

Before you run the Aloe test suite, modify the first scenario to use the keyword Background instead of Scenario . Background is a special type of scenario that is run once before every block defined by Scenario in the Feature file. Every scenario needs to start with a clean slate and using Background refreshes the data every time it is run.

example/features/friendships.feature

Feature: Friendships Background: Set up common data Given I empty the "User" table And I create the following users: | id | email | username | password | | 1 | [email protected] | Annie | pAssw0rd! | | 2 | [email protected] | Brian | pAssw0rd! | | 3 | [email protected] | Casey | pAssw0rd! | When I log in with username "Annie" and password "pAssw0rd!" Then I am logged in Scenario: A user can see a list of friends Given I empty the "Friendship" table And I create the following friendships: | id | user1 | user2 | | 1 | 1 | 2 | # Annie and Brian are now friends. When I get a list of friends Then I see the following response data: | id | email | username | | 2 | [email protected] | Brian |

Now that you're dealing with friendships between multiple users, add a couple new user records to the database to start. The new scenario clears all entries from a "Friendship" table and creates one new record to define a friendship between Annie and Brian. Then it calls an API to retrieve a list of Annie's friends and it confirms that the response data includes Brian.

The first step is to create a Friendship model. It's simple--it just links two users together.

example/models.py

from django.conf import settings from django.db import models class Friendship ( models . Model ): user1 = models . ForeignKey ( settings . AUTH_USER_MODEL , on_delete = models . CASCADE , related_name = 'user1_friendships' ) user2 = models . ForeignKey ( settings . AUTH_USER_MODEL , on_delete = models . CASCADE , related_name = 'user2_friendships' )

Make a migration and run it.

( venv ) $ python manage.py makemigrations ( venv ) $ python manage.py migrate

Next, create a new test step for the I create the following friendships: statement.

example/features/friendships_steps.py

@step ( 'I create the following friendships:' ) def step_create_friendships ( self ): Friendship . objects . bulk_create ([ Friendship ( id = data [ 'id' ], user1 = User . objects . get ( id = data [ 'user1' ]), user2 = User . objects . get ( id = data [ 'user2' ]) ) for data in guess_types ( self . hashes ) ])

Add the Friendship model import to the file.

from ..models import Friendship

Create an API to get a list of the logged-in user's friends. Create a serializer to handle the representation of the User resource.

example/serializers.py

from django.contrib.auth.models import User from rest_framework import serializers class UserSerializer ( serializers . ModelSerializer ): class Meta : model = User fields = ( 'id' , 'email' , 'username' ,) read_only_fields = fields

Create a manager to handle table-level functionality for your Friendship model.

example/models.py

# New import! from django.db.models import Q class FriendshipManager ( models . Manager ): def friends ( self , user ): """Get all users that are friends with the specified user.""" # Get all friendships that involve the specified user. friendships = self . get_queryset () . select_related ( 'user1' , 'user2' ) . filter ( Q ( user1 = user ) | Q ( user2 = user ) ) def other_user ( friendship ): if friendship . user1 == user : return friendship . user2 return friendship . user1 return map ( other_user , friendships )

The friends() function retrieves all of the friendships that the specified user shares with other users. Then it returns a list of those other users. Add objects = FriendshipManager() to the Friendship model.

Create a simple ListAPIView to return a JSON-serialized list of your User resources.

example/views.py

from rest_framework.generics import ListAPIView from .models import Friendship from .serializers import UserSerializer class FriendsView ( ListAPIView ): serializer_class = UserSerializer def get_queryset ( self ): return Friendship . objects . friends ( self . request . user )

Finally, add a URL path.

example_bdd/urls.py

from django.urls import path from example.views import FriendsView urlpatterns = [ path ( 'friends/' , FriendsView . as_view (), name = 'friends' ), ]

Create the remaining Python step functions--one to call your new API and another generic function to confirm response payload data. (We can reuse this function to check any payload.)

example/features/friendships_steps.py

@step ( 'I get a list of friends' ) def step_get_friends ( self ): world . response = world . client . get ( '/friends/' ) @step ( 'I see the following response data:' ) def step_confirm_response_data ( self ): response = world . response . json () if isinstance ( response , list ): assert guess_types ( self . hashes ) == response else : assert guess_types ( self . hashes )[ 0 ] == response

Run the tests and watch them pass.

( venv ) $ python manage.py harvest

Think of another test scenario. Users with no friends should see an empty list when they call the API.

example/features/friendships.feature

Scenario: A user with no friends sees an empty list Given I empty the "Friendship" table # Annie has no friends. When I get a list of friends Then I see the following response data: | id | email | username |

No new Python functions are required. You can reuse all of your steps! Tests pass without any intervention.

You need one last piece of functionality to get this feature off the ground. Users can get a list of their friends, but how do they make new friends? Here's a new scenario: "a user should be able to add another user as a friend." Users should be able to call an API to create a friendship with another user. You know the API works if a record gets created in the database.

example/features/friendships.feature

Scenario: A user can add a friend Given I empty the "Friendship" table When I add the following friendship: | user1 | user2 | | 1 | 2 | Then I see the following rows in the "Friendship" table: | user1 | user2 | | 1 | 2 |

Create the new step functions.

example/features/friendships_steps.py

@step ( 'I add the following friendship:' ) def step_add_friendship ( self ): world . response = world . client . post ( '/friendships/' , data = guess_types ( self . hashes [ 0 ])) @step ( 'I see the following rows in the "([^"]+)" table:' ) def step_confirm_table ( self , model_name ): model_class = get_model ( model_name ) for data in guess_types ( self . hashes ): has_row = model_class . objects . filter ( ** data ) . exists () assert has_row

Extend the manager and do some refactoring.

example/models.py

class FriendshipManager ( models . Manager ): def friendships ( self , user ): """Get all friendships that involve the specified user.""" return self . get_queryset () . select_related ( 'user1' , 'user2' ) . filter ( Q ( user1 = user ) | Q ( user2 = user ) ) def friends ( self , user ): """Get all users that are friends with the specified user.""" friendships = self . friendships ( user ) def other_user ( friendship ): if friendship . user1 == user : return friendship . user2 return friendship . user1 return map ( other_user , friendships )

Add a new serializer to render the Friendship resources.

example/serializers.py

class FriendshipSerializer ( serializers . ModelSerializer ): class Meta : model = Friendship fields = ( 'id' , 'user1' , 'user2' ,) read_only_fields = ( 'id' ,)

Add a new view.

example/views.py

class FriendshipsView ( ModelViewSet ): serializer_class = FriendshipSerializer def get_queryset ( self ): return Friendship . objects . friendships ( self . request . user )

Add a new URL.

example/urls.py

path ( 'friendships/' , FriendshipsView . as_view ({ 'post' : 'create' })),

Your code works and the tests pass!

Analyzing the Feature

Now that you've successfully programmed and tested your feature, it's time to analyze it. Two users become friends when one user adds the other one. This is not ideal behavior. Maybe the other user doesn't want to be friends--don't they get a say? A user should request a friendship with another user, and the other user should be able to accept or reject that friendship.

Revise the scenario where a user adds another user as a friend: "a user should be able to request a friendship with another user."

Replace Scenario: A user can add a friend with this one.

example/features/friendships.feature

Scenario: A user can request a friendship with another user Given I empty the "Friendship" table When I request the following friendship: | user1 | user2 | | 1 | 2 | Then I see the following response data: | id | user1 | user2 | status | | 3 | 1 | 2 | PENDING |

Refactor your test step to use a new API, /friendship-requests/ .

example/features/friendships_steps.py

@step ( 'I request the following friendship:' ) def step_request_friendship ( self ): world . response = world . client . post ( '/friendship-requests/' , data = guess_types ( self . hashes [ 0 ]))

Start by adding a new status field to the Friendship model.

example/models.py

class Friendship ( models . Model ): PENDING = 'PENDING' ACCEPTED = 'ACCEPTED' REJECTED = 'REJECTED' STATUSES = ( ( PENDING , PENDING ), ( ACCEPTED , ACCEPTED ), ( REJECTED , REJECTED ), ) objects = FriendshipManager () user1 = models . ForeignKey ( settings . AUTH_USER_MODEL , on_delete = models . CASCADE , related_name = 'user1_friendships' ) user2 = models . ForeignKey ( settings . AUTH_USER_MODEL , on_delete = models . CASCADE , related_name = 'user2_friendships' ) status = models . CharField ( max_length = 8 , choices = STATUSES , default = PENDING )

Friendships can be ACCEPTED or REJECTED . If the other user has not taken action, then the default status is PENDING .

Make a migration and migrate the database.

( venv ) $ python manage.py makemigrations ( venv ) $ python manage.py migrate

Rename the FriendshipsView to FriendshipRequestsView .

example/views.py

class FriendshipRequestsView ( ModelViewSet ): serializer_class = FriendshipSerializer def get_queryset ( self ): return Friendship . objects . friendships ( self . request . user )

Replace the old URL path with the new one.

example/urls.py

path ( 'friendship-requests/' , FriendshipRequestsView . as_view ({ 'post' : 'create' }))

Add new test scenarios to test the accept and reject actions.

example/features/friendships.feature

Scenario: A user can accept a friendship request Given I empty the "Friendship" table And I create the following friendships: | id | user1 | user2 | status | | 1 | 2 | 1 | PENDING | When I accept the friendship request with ID "1" Then I see the following response data: | id | user1 | user2 | status | | 1 | 2 | 1 | ACCEPTED | Scenario: A user can reject a friendship request Given I empty the "Friendship" table And I create the following friendships: | id | user1 | user2 | status | | 1 | 2 | 1 | PENDING | When I reject the friendship request with ID "1" Then I see the following response data: | id | user1 | user2 | status | | 1 | 2 | 1 | REJECTED |

Add new test steps.

example/features/friendships_steps.py

@step ( 'I accept the friendship request with ID "([^"]+)"' ) def step_accept_friendship_request ( self , pk ): world . response = world . client . put ( f '/friendship-requests/ {pk} /' , data = { 'status' : Friendship . ACCEPTED }) @step ( 'I reject the friendship request with ID "([^"]+)"' ) def step_reject_friendship_request ( self , pk ): world . response = world . client . put ( f '/friendship-requests/ {pk} /' , data = { 'status' : Friendship . REJECTED })

Add one more URL path. Users need to target the specific friendship they want to accept or reject.

example/urls.py

path ( 'friendship-requests/<int:pk>/' , FriendshipRequestsView . as_view ({ 'put' : 'partial_update' }))

Update Scenario: A user can see a list of friends to include the new status field.

example/features/friendships.feature

Scenario: A user can see a list of friends Given I empty the "Friendship" table And I create the following friendships: | id | user1 | user2 | status | | 1 | 1 | 2 | ACCEPTED | # Annie and Brian are now friends. When I get a list of friends Then I see the following response data: | id | email | username | | 2 | [email protected] | Brian |

Add one more scenario after Scenario: A user can see a list of friends to test filtering on the status. A user's friends consist of people who have accepted friendship requests from the user. Those who have not taken action or who have rejected the requests are not considered.

example/features/friendships.feature

Scenario: A user with no accepted friendship requests sees an empty list Given I empty the "Friendship" table And I create the following friendships: | id | user1 | user2 | status | | 1 | 1 | 2 | PENDING | | 2 | 1 | 3 | REJECTED | When I get a list of friends Then I see the following response data: | id | email | username |

Edit the step_create_friendships() function to implement the status field on the Friendship model.

example/features/friendships_steps.py

@step ( 'I create the following friendships:' ) def step_create_friendships ( self ): Friendship . objects . bulk_create ([ Friendship ( id = data [ 'id' ], user1 = User . objects . get ( id = data [ 'user1' ]), user2 = User . objects . get ( id = data [ 'user2' ]), status = data [ 'status' ] ) for data in guess_types ( self . hashes ) ])

And also edit the FriendshipSerializer serializer to implement the status field on the Friendship model.

example/serializers.py

class FriendshipSerializer ( serializers . ModelSerializer ): class Meta : model = Friendship fields = ( 'id' , 'user1' , 'user2' , 'status' ,) read_only_fields = ( 'id' ,)

Complete the filtering by adjusting the friends() method on the manager.

example/models.py

def friends ( self , user ): """Get all users that are friends with the specified user.""" friendships = self . friendships ( user ) . filter ( status = Friendship . ACCEPTED ) def other_user ( friendship ): if friendship . user1 == user : return friendship . user2 return friendship . user1 return map ( other_user , friendships )

Feature complete!

Conclusion

If you take one thing from this post, I hope it's this: behavior-driven development is as much about feature analysis as it is about writing, testing, and designing code. Without that crucial step, you're not creating software, you're just programming. BDD is not the only way to produce software, but it's a good one. And if you're practicing BDD with a Django project, give Aloe a try.

Grab the code from the repo.