pytest-bdd 0.6.1

BDD library for the py.test runner

pytest-bdd implements a subset of Gherkin language for the automation of the project requirements testing and easier behavioral driven development.

Unlike many other BDD tools it doesn’t require a separate runner and benefits from the power and flexibility of the pytest. It allows to unify your unit and functional tests, easier continuous integration server configuration and maximal reuse of the tests setup.

Pytest fixtures written for the unit tests can be reused for the setup and actions mentioned in the feature steps with dependency injection, which allows a true BDD just-enough specification of the requirements without maintaining any context object containing the side effects of the Gherkin imperative declarations.

Install pytest-bdd

pip install pytest-bdd

Example

publish_article.feature:

Scenario: Publishing the article
    Given I'm an author user
    And I have an article
    When I go to the article page
    And I press the publish button
    Then I should not see the error message
    And the article should be published  # Note: will query the database

test_publish_article.py:

from pytest_bdd import scenario, given, when, then

test_publish = scenario('publish_article.feature', 'Publishing the article')


@given('I have an article')
def article(author):
    return create_test_article(author=author)


@when('I go to the article page')
def go_to_article(article, browser):
    browser.visit(urljoin(browser.url, '/manage/articles/{0}/'.format(article.id)))


@when('I press the publish button')
def publish_article(browser):
    browser.find_by_css('button[name=publish]').first.click()


@then('I should not see the error message')
def no_error_message(browser):
    with pytest.raises(ElementDoesNotExist):
        browser.find_by_css('.message.error').first


@then('And the article should be published')
def article_is_published(article):
    article.refresh()  # Refresh the object in the SQLAlchemy session
    assert article.is_published

Step aliases

Sometimes it is needed to declare the same fixtures or steps with the different names for better readability. In order to use the same step function with multiple step names simply decorate it multiple times:

@given('I have an article')
@given('there\'s an article')
def article(author):
    return create_test_article(author=author)

Note that the given step aliases are independent and will be executed when mentioned.

For example if you associate your resource to some owner or not. Admin user can’t be an author of the article, but articles should have a default author.

Scenario: I'm the author
    Given I'm an author
    And I have an article


Scenario: I'm the admin
    Given I'm the admin
    And there is an article

Step arguments

Often it’s possible to reuse steps giving them a parameter(s). This allows to have single implementation and multiple use, so less code. Also opens the possibility to use same step twice in single scenario and with different arguments! Important thing that argumented step names are not just strings but regular expressions.

Example:

Scenario: Arguments for given, when, thens
    Given there are 5 cucumbers

    When I eat 3 cucumbers
    And I eat 2 cucumbers

    Then I should have 0 cucumbers

The code will look like:

import re
from pytest_bdd import scenario, given, when, then

test_arguments = scenario('arguments.feature', 'Arguments for given, when, thens')

@given(re.compile('there are (?P<start>\d+) cucumbers'))
def start_cucumbers(start):
    # note that you always get step arguments as strings, convert them on demand
    start = int(start)
    return dict(start=start, eat=0)


@when(re.compile('I eat (?P<eat>\d+) cucumbers'))
def eat_cucumbers(start_cucumbers, eat):
    eat = int(eat)
    start_cucumbers['eat'] += eat


@then(re.compile('I should have (?P<left>\d+) cucumbers'))
def should_have_left_cucumbers(start_cucumbers, start, left):
    start, left = int(start), int(left)
    assert start_cucumbers['start'] == start
    assert start - start_cucumbers['eat'] == left

Step parameters

Scenarios can be parametrized to cover few cases. In Gherkin the variable templates are written using corner braces as <somevalue>.

Example:

Scenario: Parametrized given, when, thens
    Given there are <start> cucumbers
    When I eat <eat> cucumbers
    Then I should have <left> cucumbers

Unlike other tools, pytest-bdd implements the scenario outline not in the feature files, but in the python code using pytest parametrization. The reason for this is that it is very often that some simple pythonic type is needed in the parameters like a datetime or a dictionary, which makes it more difficult to express in the text files and preserve the correct format.

The code will look like:

import pytest
from pytest_bdd import scenario, given, when, then

# Here we use pytest to parametrize the test with the parameters table
@pytest.mark.parametrize(
    ['start', 'eat', 'left'],
    [(12, 5, 7)])
@scenario(
    'parametrized.feature',
    'Parametrized given, when, thens',
)
# Note that we should take the same arguments in the test function that we use
# for the test parametrization either directly or indirectly (fixtures depend on them).
def test_parametrized(start, eat, left):
    """We don't need to do anything here, everything will be managed by the scenario decorator."""


@given('there are <start> cucumbers')
def start_cucumbers(start):
    return dict(start=start)


@when('I eat <eat> cucumbers')
def eat_cucumbers(start_cucumbers, start, eat):
    start_cucumbers['eat'] = eat


@then('I should have <left> cucumbers')
def should_have_left_cucumbers(start_cucumbers, start, eat, left):
    assert start - eat == left
    assert start_cucumbers['start'] == start
    assert start_cucumbers['eat'] == eat

Test setup

Test setup is implemented within the Given section. Even though these steps are executed imperatively to apply possible side-effects, pytest-bdd is trying to benefit of the PyTest fixtures which is based on the dependency injection and makes the setup more declarative style.

@given('I have a beautiful article')
def article():
    return Article(is_beautiful=True)

This also declares a PyTest fixture “article” and any other step can depend on it.

Given I have a beautiful article
When I publish this article

When step is referring the article to publish it.

@when('I publish this article')
def publish_article(article):
    article.publish()

Many other BDD toolkits operate a global context and put the side effects there. This makes it very difficult to implement the steps, because the dependencies appear only as the side-effects in the run-time and not declared in the code. The publish article step has to trust that the article is already in the context, has to know the name of the attribute it is stored there, the type etc.

In pytest-bdd you just declare an argument of the step function that it depends on and the PyTest will make sure to provide it.

Still side effects can be applied in the imperative style by design of the BDD.

Given I have a beautiful article
And my article is published

Functional tests can reuse your fixture libraries created for the unit-tests and upgrade them by applying the side effects.

given('I have a beautiful article', fixture='article')

@given('my article is published')
def published_article(article):
    article.publish()
    return article

This way side-effects were applied to our article and PyTest makes sure that all steps that require the “article” fixture will receive the same object. The value of the “published_article” and the “article” fixtures is the same object.

Fixtures are evaluated only once within the PyTest scope and their values are cached. In case of Given steps and the step arguments mentioning the same given step makes no sense. It won’t be executed second time.

Given I have a beautiful article
And some other thing
And I have a beautiful article  # Won't be executed, exception is raised

pytest-bdd will raise an exception even in the case of the steps that use regular expression patterns to get arguments.

Given I have 1 cucumbers
And I have 2 cucumbers  # Exception is raised

Will raise an exception if the step is using the regular expression pattern.

@given(re.compile('I have (?P<n>\d+) cucumbers'))
def cucumbers(n):
    return create_cucumbers(n)

Reusing fixtures

Sometimes scenarios define new names for the fixture that can be inherited. Fixtures can be reused with other names using given():

given('I have beautiful article', fixture='article')

Reusing steps

It is possible to define some common steps in the parent conftest.py and simply expect them in the child test file.

common_steps.feature:

Scenario: All steps are declared in the conftest
    Given I have a bar
    Then bar should have value "bar"

conftest.py:

from pytest_bdd import given, then


@given('I have a bar')
def bar():
    return 'bar'


@then('bar should have value "bar"')
def bar_is_bar(bar):
    assert bar == 'bar'

test_common.py:

test_conftest = scenario('common_steps.feature', 'All steps are declared in the conftest')

There are no definitions of the steps in the test file. They were collected from the parent conftests.

Feature file paths

But default, pytest-bdd will use current module’s path as base path for finding feature files, but this behaviour can be changed by having fixture named ‘pytestbdd_feature_base_dir’ which should return the new base path.

test_publish_article.py:

import pytest
from pytest_bdd import scenario


@pytest.fixture
def pytestbdd_feature_base_dir():
    return '/home/user/projects/foo.bar/features'

test_publish = scenario('publish_article.feature', 'Publishing the article')

Avoid retyping the feature file name

If you want to avoid retyping the feature file name when defining your scenarios in a test file, use functools.partial. This will make your life much easier when defining multiple scenarios in a test file.

For example:

test_publish_article.py:

from functools import partial

import pytest_bdd


scenario = partial(pytest_bdd.scenario, '/path/to/publish_article.feature')

test_publish = scenario('Publishing the article')
test_publish_unprivileged = scenario('Publishing the article as unprivileged user')

You can learn more about functools.partial in the Python docs.

Subplugins

The pytest BDD has plugin support, and the main purpose of plugins (subplugins) is to provide useful and specialized fixtures.

List of known subplugins:

• pytest-bdd-splinter - collection of fixtures for the real browser BDD testing

License

This software is licensed under the MIT license.

© 2013 Oleg Pidsadnyi

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