Introduction to Selenium for Python programmers

Selenium is an application that automates web browsers, helping you test your web application from a user perspective, in an automated manner. These properties make Selenium tests a perfect fit for validating your js-level functionality and implementing acceptance tests.

Of course, it has some drawbacks: you need to run your application from another process, which gives you some pain with checking the backend state of things. The tests might be quite slow, and - if you don't write them well - extremely fragile.

However, starting with basic Selenium tests is very simple, which I'm going to prove below. We will create a trivial website, with a single element only: a link to Google. Next, we will implement a Selenium test that makes sure this indeed happens.

Prepare the environment

We will work in a virtualenv called "seltest". If you don't know what virtualenv is, you likely want to read this first. Enter the directory of your choice and run the following commands:

mkdir seltest
cd seltest
virtualenv --no-site-packages .

We will work from within the seltest directory, so that we don't need to activate the virtualenv, and instead call our binaries by "bin/python" or "bin/pip".

Let's download the Selenium executable and its python bindings:

wget http://selenium.googlecode.com/files/selenium-server-standalone-2.0a4.jar
bin/pip install selenium

You can already play with Selenium. First, start another terminal window and run:

java -jar selenium-server-standalone-2.0a4.jar

Then from our main terminal:

$ bin/python
>>> from selenium.remote import connect
>>> from selenium import FIREFOX
>>> browser = connect(FIREFOX) # this will run the browser
>>> browser.get("http://www.yahoo.com") # you should see the browser navigating to yahoo
>>> browser.close() # this will close the session

Prepare and run the website

The website will consist of a single link, we can skip all the obligatory html boilerplate at this stage. Save the text

<a href="http://google.com">Go to Google</a>

into a file index.html, and from another terminal (yes, you will need three terminal windows) run:

Python -m SimpleHTTPServer

This will start serving your page on the port 8000, you can visit the page from your web browser on http://localhost:8000/

Implement the test

Open the file selenium_test.py in your editor of choice and dump the following

import unittest
from selenium.remote import connect
from selenium import FIREFOX

class SelTest(unittest.TestCase):
    def setUp(self):
        self.browser = connect(FIREFOX)
    def tearDown(self):
        self.browser.close()
    def test_simple(self):
        self.browser.get("http://localhost:8000/")
        link = self.browser.find_element_by_partial_link_text("Google")
        link.click()
        self.assertEqual(self.browser.get_title(), "Google")

if __name__ == "__main__":
    unittest.main()

The setUp and tearDown methods manage the browser session, and the actual test lives in the test_simple method. We are using four methods from the browser object: get, find_element_by_partial_link_text, click and get_title. In case you wonder where these come from, look for the WebDriver class definition. You can find it in lib/python2.6/site-packages/selenium/remote/webdriver.py in your environment.

Run the test

Now, you are ready to run your test.

bin/python selenium_test.py

You should see something along the lines of:

$ bin/python selenium_test.py 
.
----------------------------------------------------------------------
Ran 1 test in 5.521s

OK

Which indicates, that all your tests passed correctly.