While using py.test and Selenium on a recent project of mine, I needed to test that a spinner appears to indicate that some content is loading. As it is often the case when testing against a local server, the content is loaded too rapidly for the application to trigger the visual loading hint, and the tests kept failing.

To work around the issue, I artificially delayed the server-side response using a value stored in a cookie. Simple but effective!

Talk is cheap. Show me the code.

The implementation of this techique using Flask and Selenium takes just a handful of lines of code. First of all, we define a Flask before_request handler to throttle requests on the server when a certain cookie is set, as shown in lines 9 to 14 in the following snippet:

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# app.py

import time
from flask import Flask, request

app = Flask(__name__)
app.config['TESTING'] = True

DELAY_COOKIE = 'delay'

if app.config['TESTING']:
    @app.before_request
    def throttle():
        time.sleep(float(request.cookies.get(DELAY_COOKIE, 0)))

@app.route('/')
def hello_world():
    return 'Hello World!\n'

if __name__ == '__main__':
    app.run()

If we start the application right now, we can already test this behavior from the command line:

$ python app.py

In a separate session, run the following two commands, once without setting any cookies…

$ time curl localhost:5000
Hello World!
curl localhost:5000  0.00s user 0.00s system 62% cpu 0.005 total

And once by setting a delay of 5 seconds:

$ time curl --cookie delay=5 localhost:5000
Hello World!
curl --cookie delay=5 localhost:5000  0.00s user 0.00s system 0% cpu 5.013 total

As you can see from the total time, the throttling code is working properly.

A context manager for Selenium

We can take the whole concept a step further by applying it to Selenium tests run through py.test (check my previous post for some advice about integrating Selenium with py.test). The following code introduces a context manager which takes care to set and reset the delay cookie in the current browsing session and shows an example of its usage:

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import contextlib

from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.common.by import By


DELAY_COOKIE = 'delay'


@contextlib.contextmanager
def delay_responses(browser, time, cookie_name=DELAY_COOKIE):
    """
    Context manager to set the delay cookie on the given browser session
    on entering and restore its previous value on exiting.
    """
    prev = browser.get_cookie(cookie_name)
    browser.add_cookie({'name': cookie_name, 'value': str(time)})
    yield
    if prev:
        browser.add_cookie({'name': cookie_name, 'value': prev})
    else:
        browser.delete_cookie(cookie_name)


def test_spinner(browser):
    browser.get('/')

    # Trigger the action
    with delay_responses(browser, 3):
        browser.find_element_by_css_selector('button').click()

    # Check for spinner to appear
    WebDriverWait(browser, 1).until(EC.presence_of_element_located(
        (By.CSS_SELECTOR, '.spinner')
    ))

That’s it, happy (throttled) testing!