buy the book ribbon

Appendix E: Behaviour-Driven Development (BDD)

Now I haven’t used BDD "in anger," so I can’t claim any sort of expertise, but I really like what I have seen of it, and I thought that you deserved at least a whirlwind tour. In this appendix, we’ll take some of the tests we wrote in a "normal" FT, and convert them to using BDD tools.

What Is BDD?

BDD, strictly speaking, is a methodology rather than a toolset—​it’s the approach of testing your application by testing the behaviour that we expect it to display to a user (the Wikipedia entry has quite a good overview). So, in some ways, the Selenium-based FTs that I’ve shown in the rest of the book could be called BDD.

But the term has become closely associated with a particular set of tools for doing BDD, most importantly the Gherkin syntax, which is a human-readable DSL for writing functional (or acceptance) tests. Gherkin originally came out of the Ruby world, where it’s associated with a test runner called Cucumber.

In the Python world, we have a couple of equivalent test running tools, Lettuce and Behave. Of these, only Behave was compatible with Python 3 at the time of writing, so that’s what we’ll use. We’ll also use a plugin called behave-django.

Getting the Code for These Examples

I’m going to use the example from [chapter_outside_in]. We have a basic to-do lists site, and we want to add a new feature: logged-in users should be able to view the lists they’ve authored in one place. Up until this point, all lists are effectively anonymous.

If you’ve been following along with the book, I’m going to assume you can skip back to the code for that point. If you want to pull it from my repo, the place to go is the chapter_17 branch.

Basic Housekeeping

We make a directory for our BDD "features," add a steps directory (we’ll find out what these are shortly!), and placeholder for our first feature:

$ mkdir -p features/steps
$ touch features/my_lists.feature
$ touch features/steps/my_lists.py
$ tree features
features
├── my_lists.feature
└── steps
    └── my_lists.py

We install behave-django, and add it to settings.py:

$ pip install behave-django
superlists/settings.py
--- a/superlists/settings.py
+++ b/superlists/settings.py
@@ -40,6 +40,7 @@ INSTALLED_APPS = [
     'lists',
     'accounts',
     'functional_tests',
+    'behave_django',
 ]

And then run python manage.py behave as a sanity check:

$ python manage.py behave
Creating test database for alias 'default'...
0 features passed, 0 failed, 0 skipped
0 scenarios passed, 0 failed, 0 skipped
0 steps passed, 0 failed, 0 skipped, 0 undefined
Took 0m0.000s
Destroying test database for alias 'default'...

Writing an FT as a "Feature" Using Gherkin Syntax

Up until now, we’ve been writing our FTs using human-readable comments that describe the new feature in terms of a user story, interspersed with the Selenium code required to execute each step in the story.

BDD enforces a distinction between those two—​we write our human-readable story using a human-readable (if occasionally somewhat awkward) syntax called "Gherkin", and that is called the "Feature". Later, we’ll map each line of Gherkin to a function that contains the Selenium code necessary to implement that "step."

Here’s what a Feature for our new "My lists" page could look like:

features/my_lists.feature
Feature: My Lists
    As a logged-in user
    I want to be able to see all my lists in one page
    So that I can find them all after I've written them

    Scenario: Create two lists and see them on the My Lists page

        Given I am a logged-in user

        When I create a list with first item "Reticulate Splines"
            And I add an item "Immanentize Eschaton"
            And I create a list with first item "Buy milk"

        Then I will see a link to "My lists"

        When I click the link to "My lists"
        Then I will see a link to "Reticulate Splines"
        And I will see a link to "Buy milk"

        When I click the link to "Reticulate Splines"
        Then I will be on the "Reticulate Splines" list page

As-a /I want to/So that

At the top you’ll notice the As-a/I want to/So that clause. This is optional, and it has no executable counterpart—​it’s just a slightly formalised way of capturing the "who and why?" aspects of a user story, gently encouraging the team to think about the justifications for each feature.

Given/When/Then

Given/When/Then is the real core of a BDD test. This trilobite formulation matches the setup/exercise/assert pattern we’ve seen in our unit tests, and it represents the setup and assumptions phase, an exercise/action phase, and a subsequent assertion/observation phase. There’s more info on the Cucumber wiki.

Not Always a Perfect Fit!

As you can see, it’s not always easy to shoe-horn a user story into exactly three steps! We can use the And clause to expand on a step, and I’ve added multiple When steps and subsequent Then's to illustrate further aspects of our "My lists" page.

Coding the Step Functions

We now build the counterpart to our Gherkin-syntax feature, which are the "step" functions that will actually implement them in code.

Generating Placeholder Steps

When we run behave, it helpfully tells us about all the steps we need to implement:

$ python manage.py behave
Feature: My Lists # features/my_lists.feature:1
  As a logged-in user
  I want to be able to see all my lists in one page
  So that I can find them all after I've written them
  Scenario: Create two lists and see them on the My Lists page  #
features/my_lists.feature:6
    Given I am a logged-in user                                 # None
    Given I am a logged-in user                                 # None
    When I create a list with first item "Reticulate Splines"   # None
    And I add an item "Immanentize Eschaton"                    # None
    And I create a list with first item "Buy milk"              # None
    Then I will see a link to "My lists"                        # None
    When I click the link to "My lists"                         # None
    Then I will see a link to "Reticulate Splines"              # None
    And I will see a link to "Buy milk"                         # None
    When I click the link to "Reticulate Splines"               # None
    Then I will be on the "Reticulate Splines" list page        # None


Failing scenarios:
  features/my_lists.feature:6  Create two lists and see them on the My Lists
page


0 features passed, 1 failed, 0 skipped
0 scenarios passed, 1 failed, 0 skipped
0 steps passed, 0 failed, 0 skipped, 10 undefined
Took 0m0.000s

You can implement step definitions for undefined steps with these snippets:

@given(u'I am a logged-in user')
def step_impl(context):
    raise NotImplementedError(u'STEP: Given I am a logged-in user')

@when(u'I create a list with first item "Reticulate Splines"')
def step_impl(context):
[...]

And you’ll notice all this output is nicely coloured, as shown in Behave with coloured console ouptut.

Colourful console output
Figure 1. Behave with coloured console ouptut

It’s encouraging us to copy and paste these snippets, and use them as starting points to build our steps.

First Step Definition

Here’s a first stab at making a step for our "Given I am a logged-in user" step. I started by stealing the code for self.create_pre_authenticated_session from functional_tests/test_my_lists.py, and adapting it slightly (removing the server-side version, for example, although it would be easy to re-add later).

features/steps/my_lists.py
from behave import given, when, then
from functional_tests.management.commands.create_session import \
    create_pre_authenticated_session
from django.conf import settings


@given('I am a logged-in user')
def given_i_am_logged_in(context):
    session_key = create_pre_authenticated_session(email='[email protected]')
    ## to set a cookie we need to first visit the domain.
    ## 404 pages load the quickest!
    context.browser.get(context.get_url("/404_no_such_url/"))
    context.browser.add_cookie(dict(
        name=settings.SESSION_COOKIE_NAME,
        value=session_key,
        path='/',
    ))

The context variable needs a little explaining—it’s a sort of global variable, in the sense that it’s passed to each step that’s executed, and it can be used to store information that we need to share between steps. Here we’ve assumed we’ll be storing a browser object on it, and the server_url. We end up using it a lot like we used self when we were writing unittest FTs.

setUp and tearDown Equivalents in environment.py

Steps can make changes to state in the context, but the place to do preliminary set-up, the equivalent of setUp, is in a file called environment.py:

features/environment.py
from selenium import webdriver

def before_all(context):
    context.browser = webdriver.Firefox()

def after_all(context):
    context.browser.quit()

def before_feature(context, feature):
    pass

Another Run

As a sanity check, we can do another run, to see if the new step works and that we really can start a browser:

$ python manage.py behave
[...]
1 step passed, 0 failed, 0 skipped, 9 undefined

The usual reams of output, but we can see that it seems to have made it through the first step; let’s define the rest of them.

Capturing Parameters in Steps

We’ll see how Behave allows you to capture parameters from step descriptions. Our next step says:

features/my_lists.feature
    When I create a list with first item "Reticulate Splines"

And the autogenerated step definition looked like this:

features/steps/my_lists.py
@given('I create a list with first item "Reticulate Splines"')
def step_impl(context):
    raise NotImplementedError(
        u'STEP: When I create a list with first item "Reticulate Splines"'
    )

We want to be able to create lists with arbitrary first items, so it would be nice to somehow capture whatever is between those quotes, and pass them in as an argument to a more generic function. That’s a common requirement in BDD, and Behave has a nice syntax for it, reminiscent of the new-style Python string formatting syntax:

features/steps/my_lists.py (ch35l006)
[...]

@when('I create a list with first item "{first_item_text}"')
def create_a_list(context, first_item_text):
    context.browser.get(context.get_url('/'))
    context.browser.find_element_by_id('id_text').send_keys(first_item_text)
    context.browser.find_element_by_id('id_text').send_keys(Keys.ENTER)
    wait_for_list_item(context, first_item_text)

Neat, huh?

Capturing parameters for steps is one of the most powerful features of the BDD syntax.

As usual with Selenium tests, we will need an explicit wait. Let’s re-use our @wait decorator from base.py:

features/steps/my_lists.py (ch35l007)
from functional_tests.base import wait
[...]


@wait
def wait_for_list_item(context, item_text):
    context.test.assertIn(
        item_text,
        context.browser.find_element_by_css_selector('#id_list_table').text
    )

Similarly, we can add to an existing list, and see or click on links:

features/steps/my_lists.py (ch35l008)
from selenium.webdriver.common.keys import Keys
[...]


@when('I add an item "{item_text}"')
def add_an_item(context, item_text):
    context.browser.find_element_by_id('id_text').send_keys(item_text)
    context.browser.find_element_by_id('id_text').send_keys(Keys.ENTER)
    wait_for_list_item(context, item_text)


@then('I will see a link to "{link_text}"')
@wait
def see_a_link(context, link_text):
    context.browser.find_element_by_link_text(link_text)


@when('I click the link to "{link_text}"')
def click_link(context, link_text):
    context.browser.find_element_by_link_text(link_text).click()

Notice we can even use our @wait decorator on steps themselves.

And finally the slightly more complex step that says I am on the page for a particular list:

features/steps/my_lists.py (ch35l009)
@then('I will be on the "{first_item_text}" list page')
@wait
def on_list_page(context, first_item_text):
    first_row = context.browser.find_element_by_css_selector(
        '#id_list_table tr:first-child'
    )
    expected_row_text = '1: ' + first_item_text
    context.test.assertEqual(first_row.text, expected_row_text)

Now we can run it and see our first expected failure:

$ python manage.py behave

Feature: My Lists # features/my_lists.feature:1
  As a logged-in user
  I want to be able to see all my lists in one page
  So that I can find them all after I've written them
  Scenario: Create two lists and see them on the My Lists page  #
features/my_lists.feature:6
    Given I am a logged-in user                                 #
features/steps/my_lists.py:19
    When I create a list with first item "Reticulate Splines"   #
features/steps/my_lists.py:31
    And I add an item "Immanentize Eschaton"                    #
features/steps/my_lists.py:39
    And I create a list with first item "Buy milk"              #
features/steps/my_lists.py:31
    Then I will see a link to "My lists"                        #
functional_tests/base.py:12
      Traceback (most recent call last):
[...]
        File "features/steps/my_lists.py", line 49, in see_a_link
          context.browser.find_element_by_link_text(link_text)
[...]
      selenium.common.exceptions.NoSuchElementException: Message: Unable to
locate element: My lists

[...]

Failing scenarios:
  features/my_lists.feature:6  Create two lists and see them on the My Lists
page

0 features passed, 1 failed, 0 skipped
0 scenarios passed, 1 failed, 0 skipped
4 steps passed, 1 failed, 5 skipped, 0 undefined

You can see how the output really gives you a sense of how far through the "story" of the test we got: we manage to create our two lists successfully, but the "My lists" link does not appear.

Comparing the Inline-Style FT

I’m not going to run through the implementation of the feature, but you can see how the test will drive development just as well as the inline-style FT would have.

Let’s have a look at it, for comparison:

functional_tests/test_my_lists.py
def test_logged_in_users_lists_are_saved_as_my_lists(self):
    # Edith is a logged-in user
    self.create_pre_authenticated_session('[email protected]')

    # She goes to the home page and starts a list
    self.browser.get(self.live_server_url)
    self.add_list_item('Reticulate splines')
    self.add_list_item('Immanentize eschaton')
    first_list_url = self.browser.current_url

    # She notices a "My lists" link, for the first time.
    self.browser.find_element_by_link_text('My lists').click()

    # She sees that her list is in there, named according to its
    # first list item
    self.wait_for(
        lambda: self.browser.find_element_by_link_text('Reticulate splines')
    )
    self.browser.find_element_by_link_text('Reticulate splines').click()
    self.wait_for(
        lambda: self.assertEqual(self.browser.current_url, first_list_url)
    )

    # She decides to start another list, just to see
    self.browser.get(self.live_server_url)
    self.add_list_item('Click cows')
    second_list_url = self.browser.current_url

    # Under "my lists", her new list appears
    self.browser.find_element_by_link_text('My lists').click()
    self.wait_for(
        lambda: self.browser.find_element_by_link_text('Click cows')
    )
    self.browser.find_element_by_link_text('Click cows').click()
    self.wait_for(
        lambda: self.assertEqual(self.browser.current_url, second_list_url)
    )

    # She logs out.  The "My lists" option disappears
    self.browser.find_element_by_link_text('Log out').click()
    self.wait_for(lambda: self.assertEqual(
        self.browser.find_elements_by_link_text('My lists'),
        []
    ))

It’s not entirely an apples-to-apples comparison, but we can look at the number of lines of code in Lines of code comparison.

Table 1. Lines of code comparison
BDD Standard FT

Feature file: 20 (3 optional)

test function body: 45

Steps file: 56 lines

helper functions: 23

The comparison isn’t perfect, but you might say that the feature file and the body of a "standard FT" test function are equivalent in that they present the main "story" of a test, while the steps and helper functions represent the "hidden" implementation details. If you add them up, the total numbers are pretty similar, but notice that they’re spread out differently: the BDD tests have made the story more concise, and pushed more work out into the hidden implementation details.

BDD Encourages Structured Test Code

This is the real appeal, for me: the BDD tool has forced us to structure our test code. In the inline-style FT, we’re free to use as many lines as we want to implement a step, as described by its comment line. It’s very hard to resist the urge to just copy-and-paste code from elsewhere, or just from earlier on in the test. You can see that, by this point in the book, I’ve built just a couple of helper functions (like get_item_input_box).

In contrast, the BDD syntax has immediately forced me to have a separate function for each step, so I’ve already built some very reusable code to:

  • Start a new list

  • Add an item to an existing list

  • Click on a link with particular text

  • Assert that I’m looking at a particular list’s page

BDD really encourages you to write test code that seems to match well with the business domain, and to use a layer of abstraction between the story of your FT and its implementation in code.

The ultimate expression of this is that, theoretically, if you wanted to change programming languages, you could keep all your features in Gherkin syntax exactly as they are, and throw away the Python steps and replace them with steps implemented in another language.

The Page Pattern as an Alternative

In [chapter_page_pattern] of the book, I present an example of the "Page pattern", which is an object-oriented approach to structuring your Selenium tests. Here’s a reminder of what it looks like:

functional_tests/test_sharing.py
from .my_lists_page import MyListsPage
[...]

class SharingTest(FunctionalTest):

    def test_can_share_a_list_with_another_user(self):
        # [...]
        self.browser.get(self.live_server_url)
        list_page = ListPage(self).add_list_item('Get help')

        # She notices a "Share this list" option
        share_box = list_page.get_share_box()
        self.assertEqual(
            share_box.get_attribute('placeholder'),
            '[email protected]'
        )

        # She shares her list.
        # The page updates to say that it's shared with Oniciferous:
        list_page.share_list_with('[email protected]')

And the Page class looks like this:

functional_tests/lists_pages.py
class ListPage(object):

    def __init__(self, test):
        self.test = test


    def get_table_rows(self):
        return self.test.browser.find_elements_by_css_selector('#id_list_table tr')


    @wait
    def wait_for_row_in_list_table(self, item_text, item_number):
        row_text = '{}: {}'.format(item_number, item_text)
        rows = self.get_table_rows()
        self.test.assertIn(row_text, [row.text for row in rows])


    def get_item_input_box(self):
        return self.test.browser.find_element_by_id('id_text')

So it’s definitely possible to implement a similar layer of abstraction, and a sort of DSL, in inline-style FTs, whether it’s by using the Page pattern or whatever structure you prefer—​but now it’s a matter of self-discipline, rather than having a framework that pushes you towards it.

In fact, you can actually use the Page pattern with BDD as well, as a resource for your steps to use when navigating the pages of your site.

BDD Might Be Less Expressive than Inline Comments

On the other hand, I can also see potential for the Gherkin syntax to feel somewhat restrictive. Compare how expressive and readable the inline-style comments are, with the slightly awkward BDD feature:

functional_tests/test_my_lists.py
    # Edith is a logged-in user
    # She goes to the home page and starts a list
    # She notices a "My lists" link, for the first time.
    # She sees that her list is in there, named according to its
    # first list item
    # She decides to start another list, just to see
    # Under "my lists", her new list appears
    # She logs out.  The "My lists" option disappears
[...]

That’s much more readable and natural than our slightly forced Given/Then/When incantations, and, in a way, might encourage more user-centric thinking. (There is a syntax in Gherkin for including "comments" in a feature file, which would mitigate this somewhat, but I gather that it’s not widely used.)

Will Nonprogrammers Write Tests?

I haven’t touched on one of the original promises of BDD, which is that nonprogrammers—​business or client representatives perhaps—​might actually write the Gherkin syntax. I’m quite skeptical about whether this would actually work in the real world, but I don’t think that detracts from the other potential benefits of BDD.

Some Tentative Conclusions

I’ve only dipped my toes into the BDD world, so I’m hesitant to draw any firm conclusions. I find the "forced" structuring of FTs into steps very appealing though—​in that it looks like it has the potential to encourage a lot of reuse in your FT code, and that it neatly separates concerns between describing the story and implementing it, and that it forces us to think about things in terms of the business domain, rather than in terms of "what we need to do with Selenium."

But there’s no free lunch. The Gherkin syntax is restrictive, compared to the total freedom offered by inline FT comments.

I also would like to see how BDD scales once you have not just one or two features, and four or five steps, but several dozen features and hundreds of lines of steps code.

Overall, I would say it’s definitely worth investigating, and I will probably use BDD for my next personal project.

My thanks to Daniel Pope, Rachel Willmer, and Jared Contrascere for their feedback on this chapter.

BDD Conclusions
Encourages structured, reusable test code

By separating concerns, breaking your FTs out into the human-readable, Gherkin syntax "feature" file and a separate implementation of steps functions, BDD has the potential to encourage more reusable and manageable test code.

It may come at the expense of readability

The Gherkin syntax, for all its attempt to be human-readable, is ultimately a constraint on human language, and so it may not capture nuance and intention as well as inline comments do.

Try it! I will

As I keep saying, I haven’t used BDD on a real project, so you should take my words with a heavy pinch of salt, but I’d like to give it a hearty endorsement. I’m going to try it out on the next project I can, and I’d encourage you to do so as well.

Comments