Visual Diff Testing

The bok-choy framework uses Needle to provide the ability to capture portions of a rendered page in the browser and assert that the image captured matches that of a baseline.

The general methodology for creating a test with a screenshot assertion consists of the following steps.

Write Your Page Object and Test Case Code to Navigate the System Under Test

If you are not familiar with how to write a bok-choy page object and test case, first check out the Tutorial.

Here is an example of a test that navigates to the home page., which contains the page object.

from bok_choy.page_object import PageObject

class EdxHomePage(PageObject):
    url = ''

    def is_browser_on_page(self):
        return 'edx' in self.browser.title.lower(), which contains the test code.

from bok_choy.web_app_test import WebAppTest
from page import EdxHomePage

class TestEdxHomePage(WebAppTest):

    def test_page_existence(self):

Add the Call to assertScreenshot

assertScreenshot() takes two arguments: a CSS selector for the element to capture, and a filename for the image.

The following example uses the same test case shown in the previous section, with an assertion added to check that the site logo for the home page has not changed.

  • The first argument, is the css locator for the element that we want to capture and compare.
  • The second argument, edx_logo_header is the filename that will be used for both the baseline and the actual results. The .png extension is appended automatically.


For test reliability and synchronization purposes, a bok-choy best practice is to employ Promises to ensure that the page has been fully rendered before you take the screenshot. At the very least, you should first assert that the element you want to capture is present and visible on the screen., with the screenshot assertion.

from bok_choy.web_app_test import WebAppTest
from page import EdxHomePage

class TestEdxHomePage(WebAppTest):

    def test_page_existence(self):
        homepage = EdxHomePage(self.browser).visit()
        css_locator = ''
        self.assertScreenshot(css_locator, 'edx_logo_header')

Create the Initial Baseline Screenshot

To create an initial screenshot of the logo, run the test case in “baseline saving” mode by specifying the nose parameter --with-save-baseline.

$ nosetests --with-save-baseline

If using pytest, you can instead set the environment variable NEEDLE_SAVE_BASELINE.


The folder in which the baseline and actual (output) screenshots are saved is determined using the following environment variables.

  • NEEDLE_OUTPUT_DIR - defaults to “screenshots”
  • NEEDLE_BASELINE_DIR - defaults to “screenshots/baseline”

In our example, we would execute the test once with the save baseline parameter to create screenshots/baseline/edx_logo_header.png. We would then open it up and check that it looks okay.

Execute Your Test Cases After Changes to the System Under Test

Now if we run our tests, it will take the same screenshot and check it against the saved baseline screenshot on disk.

$ nosetests

If a regression causes them to become significantly different, then the test will fail.

Advanced Features

See the Needle documentation for more information on the following advanced features.

  • Setting the viewport’s size - This feature is particularly useful for predicting the size of the resulting screenshots when taking full screen captures, and for testing responsive sites.
  • Difference engine - Instead of PIL (the default), you might want to use PerceptualDiff. In addition to being much faster than PIL, PerceptualDiff generates a diff PNG file when a test fails, highlighting the differences between the baseline image and the new screenshot.
  • File cleanup - Each time you run tests, new screenshot images are saved to disk, for comparison with the baseline screenshots. You might want to set your configuration to delete these files for all successful tests.