Needle: Automated tests for your visuals¶
It checks that visuals (CSS/fonts/images/SVG/etc.) render correctly by taking screenshots of portions of a website and comparing them against known good screenshots. It also provides tools for testing calculated CSS values and the position of HTML elements.
If you haven’t got pip installed:
$ sudo easy_install pip
As root, or in a virtualenv:
$ pip install selenium $ pip install needle
test_bbc.py in an empty directory:
from needle.cases import NeedleTestCase class BBCNewsTest(NeedleTestCase): def test_masthead(self): self.driver.get('http://www.bbc.co.uk/news/') self.assertScreenshot('#blq-mast', 'bbc-masthead')
This is a test case which tells the Selenium web driver (by default Firefox)
to open BBC News and check the bar across the top of the page looks correct.
assertScreenshot() take two arguments: a
CSS selector for the element we are capturing and a filename for the image.
To create an initial screenshot of the logo, we need to run Needle in ‘baseline saving’ mode:
$ nosetests test_bbc.py --with-save-baseline
This will create
screenshots/baseline/bbc-masthead.png. Open it up and
check it looks okay.
Now if we run our tests, it will take the same screenshot and check it against the screenshot on disk:
$ nosetests test_bbc.py
If a regression in your CSS causes them to become significantly different, the test will fail.
Selecting a WebDriver¶
You may control which browser is used by Needle by overriding the
from needle.cases import NeedleTestCase from needle.driver import NeedlePhantomJS class MyTests(NeedleTestCase): @classmethod def get_web_driver(cls): return NeedlePhantomJS() def test_something(self): ...
By default Needle uses
NeedleFirefox, which is a wrapper of Selenium’s
selenium.webdriver.firefox.webdriver.WebDriver class. You may use
any of the following WebDrivers:
NeedleSafari. Refer to Selenium’s documentation to learn how to install and
configure any of those WebDrivers.
Setting the viewport’s size¶
You may set the size of the browser’s viewport using the
from needle.cases import NeedleTestCase class MyTests(NeedleTestCase): def test_something(self): self.set_viewport_size(width=1024, height=768) ...
This is particularly useful to predict the size of the resulting screenshots when taking fullscreen captures, or to test responsive sites.
You may also set the default viewport size for all your tests with the
viewport_height class attributes:
from needle.cases import NeedleTestCase class MyTests(NeedleTestCase): viewport_width = 1024 viewport_height = 768 ...
By default Needle uses the PIL engine (
to take screenshots. Instead of PIL, you may also use PerceptualDiff or
Example with PerceptualDiff:
from needle.cases import NeedleTestCase class MyTests(NeedleTestCase): engine_class = 'needle.engines.perceptualdiff_engine.Engine' def test_something(self): ...
Example with ImageMagick:
from needle.cases import NeedleTestCase class MyTests(NeedleTestCase): engine_class = 'needle.engines.imagemagick_engine.Engine' def test_something(self): ...
Besides being much faster than PIL, PerceptualDiff and ImageMagick also generate a diff PNG file when a test fails, highlighting the differences between the baseline image and the new screenshot.
Note that to use the PerceptualDiff engine you will first need to
perceptualdiff binary and
place it in your
To use the ImageMagick engine you will need to install a package on your
sudo apt-get install imagemagick on Ubuntu or
brew install imagemagick on OSX).
Each time you run tests, Needle will create new screenshot images on disk, for comparison with the baseline screenshots. It’s then up to you whether you want to delete them or archive them.
cleanup_on_success class attribute to
True to delete these
files for all successful tests. Any screenshots that differ from the baseline
will remain on disk for your inspection. Example:
from needle.cases import NeedleTestCase class MyTests(NeedleTestCase): cleanup_on_success = True def test_something(self): ...
You may also activate the file cleanup from the command line by using the
--with-needle-cleanup-on-success nose plugin:
$ nosetests –with-needle-cleanup-on-success