Trey Hunner

programming, teaching, open source

Visual Integration Tests for Django

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I recently added a new type of test to my testing arsenal: visual tests. Visual tests ensure the CSS, markup, and JavaScript produce a webpage that looks right.

Visual testing frameworks

Visual testing tools compare screenshots to ensure tested webpages look pixel perfect. Capturing webpage screenshots requires a full-featured web browser to render CSS and execute JavaScript. All three of the visual testing tools I found rely on Selenium or PhantomJS for rendering.

PhantomCSS

PhantomCSS uses PhantomJS for screenshot differencing. PhantomCSS won’t integrate directly with the Django live server or your Python test suite, so if you want to run a visual integration test, you’d need to manually start and stop the test server between tests. I might eventually try out PhantomCSS for CSS unit tests, but I wanted to visually test my full website so I needed integration with the Django live server.

Django-casper

Django-casper uses Django live server tests to execute CasperJS test files (which use PhantomJS) to compare screenshots. Each test requires an additional Python test which references a JavaScript file that executes the navigation and screenshotting code. I found this approach messy and difficult to setup.

Needle

The needle Python library uses Selenium to navigate your website and screenshot rendered pages. Unfortunately needle has poor test coverage, a seemingly failing test suite, and no change log. Despite these shortcomings, I went with needle for my visual integration tests because it got the job done.

Django and Needle

I used the following mixin to integrate the Django live server with needle. I used PhantomJS, but Firefox or another Selenium web driver should work as well.

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from django.test import LiveServerTestCase
from needle.cases import NeedleTestCase
from selenium.webdriver import PhantomJS


class DjangoNeedleTestCase(NeedleTestCase, LiveServerTestCase):

    """Needle test case for use with Django live server"""

    driver = PhantomJS

    @classmethod
    def get_web_driver(cls):
        return type('NeedleWebDriver', (NeedleWebDriverMixin, cls.driver), {})()

Unfortunately the above code only works with the version of needle on Github. The PyPI version does not yet include the NeedleWebDriverMixin (which I contributed recently for Django support). I have created an issue suggesting a new PyPI release be made to resolve this problem.

Room for improvement

Currently I only run my visual tests manually. Visual tests are very brittle and occasionally they just break without any changes. If I manage to stabilize my visual tests so that they pass consistently on different platforms, I may run them during continuous integration.

Do you have another solution for visual integration testing? Let me know in the comments.

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