Dash Testing

dash.testing provides custom Dash pytest fixtures and a set of testing APIs for unit and end-to-end testing. This tutorial shows how to write and run tests for a Dash app.

Dash for R Testing

We released Dash for R in July 2019. It extends the Python package dash.testing to support Dash for R apps. The dashr fixture gives a test development experience in Dash for R that's nearly identical to the dash_duo fixture in Dash for Python. A dashr fixture is an instance of Python class which provides a reliable framework to execute the app and test one or more of its features via Selenium WebDriver. For more details, please visit pytest documentation.

Installing dash.testing

Install dash.testing with:

python -m pip install dash[testing]

In some shells (for example, Zsh), you may need to escape the opening bracket, [:

python -m pip install dash\[testing]

Example

Here is a simple example runnable with pytest. Your Dash App is written as a string inside the Python test code (the app argument can also be a valid path to a .R file), the app is then executed by Rscript within a Python subprocess and we can use the same set of API calls to test it exactly as we do Dash testing in Python Apps.

# the app is a raw string variable defining the Dash App in R
app = '''
library(dash)
library(dashHtmlComponents)
app <- Dash$new()
app$layout(htmlDiv(list(htmlDiv(id='container',children='Hello Dash for R testing'))))
app$run_server()
'''
# a test case is a simple Python function with the same prefix convention
# dashr is the default fixture combines the API for serving the app
# and Selenium tests.
def test_rstr001_r_with_string(dashr):
    # Since the app is the string chunk we defined above, the dashr
    # fixture creates an unique temporary folder to dump the content into
    # `app.R` and copies any possible subfolders (usually
    # assets or clientside source code) to the same temporary folder
    # start_server by default uses the temporary folder as current working
    # folder You can change this with `start_server(app, cwd='/my/cwd/')`
    dashr.start_server(app)
    assert dashr.find_element("#container").text == "Hello Dash for R Testing"

def test_rstr002_r_with_file_path(dashr):
    # Alternatively, the app can be a path to a separate file containing your Dash for R app
    # the `os.path.dirname(__file__)` is an useful Python trick to get the path
    # of the currently running script, so it will always work no matter where you start the
    # test runner.
    dashr.start_server(
        app=os.path.join(os.path.dirname(__file__), "assets/demo_hello.R))
    assert dashr.find_element("#container").text == "Hello Dash for R Testing"

Running Tests

There are many ways to run your tests, and you can change how your tests run by adding flags to the pytest command.

All Tests

You can run all tests in the current working directory (and its subdirectories) with:

pytest

Any tests in .py files with names that start with test_ or end with _test are run.

Specific Tests

In the example above, we gave our test cases IDs. We can use these to run a specific test:

pytest -k rstr001

WebDriver Options

There are different ways to configure your WebDriver when running end-to-end tests.

With an Alternative Web Driver

ChromeDriver is the default WebDriver, but dash.testing also supports geckodriver for Firefox. Add the --webdriver Firefox flag when running tests to use it:

pytest --webdriver Firefox -k 001

In Headless Mode

You can run tests in headless mode, if you don't need to see the tests in a browser UI:

pytest --headless -k 001

Using a Selenium Grid

You can use Selenium Grid to run tests across multiple machines. To run tests with a local hosted grid at http://localhost:4444/wd/hu:

pytest --remote -k 001

http://localhost:4444/wd/hu is the default value. To add an different remote, use the --remote-url flag:

pytest --webdriver Firefox --remote-url https://grid_provider_endpoints

Note: If you set --remote-url, and the value is different to the default value, http://localhost:4444/wd/hu, you don't need to include the --remote flag.

Caveats

We can't guarantee that the above examples work with every Selenium Grid. There may be limitations because of how your network is configured. For example, because of:

• A different hosting OS
• How docker-compose is configured

If you encounter issues, it may be because you need to do some auxiliary WebDriver options tuning to run the tests. Here are some things to try:

• Change the default logging level with --log-cli-level DEBUG.
• Customize your browser options. There is a back door for browser option customization by a pytest_setup_options hook defined in plugin.py. The example below is to use the headless mode with Chrome WebDriver in Windows. There is a workaround by adding --disable-gpu in the options.

from selenium.webdriver.chrome.options import Options

def pytest_setup_options():
	options = Options()
	options.add_argument('--disable-gpu')
	return options

Fixtures

To avoid accidental name collision with other pytest plugins, all Dash test fixtures start with the prefix dashr.

• dashr The default fixture for Dash for R integration tests. As dash_duo in Python, it contains a dashr_server and a Selenium WebDriver with the same Dash testing APIs.
• dashr_server Start your Dash for R App with Rscript in a python subprocess. Note that unlike python server fixtures, the start server arguments can only be configured inside the Dash for R App.

APIs

Selenium Overview

dashr exposes the Selenium WebDriver via the property driver, e.g. dash_duo.driver, which gives you full access to the Python Selenium API. (Note that this is not the official Selenium documentation site, but has somehow become the de facto Python community reference) One of the core components of Selenium testing is finding the web element with a locator, and performing some actions like click or send_keys on it, and waiting to verify if the expected state is met after those actions. The check is considered as an acceptance criterion, for which you can write in a standard Python assert statement.

Element Locators

There are several strategies to locate elements; CSS selector and XPATH are the two most versatile ways. We recommend using the CSS Selector in most cases due to its better performance and robustness across browsers. If you are new to using CSS Selectors, these SauceLab tips are a great start. Also, remember that Chrome Dev Tools Console is always your good friend and playground.

Waits

This link covers this topic nicely. For impatient readers, a quick take away is quoted as follows: The Selenium WebDriver provides two types of waits:

• explicit wait Makes WebDriver wait for a certain condition to occur before proceeding further with execution. All our APIs with wait_for_* falls into this category.
• implicit wait Makes WebDriver poll the DOM for a certain amount of time when trying to locate an element. We set a global two-second timeout at the driver level. Note all custom wait conditions are defined in dash.testing.wait and there are two extra APIs until and until_not which are similar to the explicit wait with WebDriver, but they are not binding to WebDriver context, i.e. they abstract a more generic mechanism to poll and wait for certain condition to happen

Browser APIs

This section lists a minimal set of browser testing APIs. They are convenient shortcuts to Selenium APIs and have been approved in our daily integration tests. The following table might grow as we start migrating more legacy tests in the near future. But we have no intention to build a comprehensive list, the goal is to make writing Dash tests concise and error-free. Please feel free to submit a community PR to add any missing ingredient, we would be happy to accept that if it's adequate for Dash testing.

Dash APIs

This section enumerates a full list of Dash App related properties and APIs apart from the previous browser ones.

Debugging

Verify Your Test Environment

If you run the integration in a virtual environment, make sure you are getting the latest commit in the master branch from each component, and that the installed pip versions are correct. Note: We have some enhancement initiatives tracking in this issue

Run the CI Job Locally

The CircleCI Local CLI is a handy tool to execute some jobs locally. It gives you an earlier warning before even pushing your commits to remote. For example, it's always recommended to pass lint and unit tests job first on your local machine. So we can make sure there are no simple mistakes in the commit.

# install the cli (first time only)
$ curl -fLSs https://circle.ci/cli | bash && circleci version
# run at least the lint & unit test job on both python 2 and 3
# note: the current config requires all tests pass on python 2.7, 3.6 and 3.7.
$ circleci local execute --job lint-unit-27 && $ circleci local execute --job lint-unit-37

Increase the Verbosity of pytest Logging Level

pytest --log-cli-level DEBUG -k bsly001

You can get more logging information from Selenium WebDriver, Flask server, and our test APIs.

14:05:41 | DEBUG | selenium.webdriver.remote.remote_connection:388 | DELETE http://127.0.0.1:53672/session/87b6f1ed3710173eff8037447e2b8f56 {"sessionId": "87b6f1ed3710173eff8037447e2b8f56"}
14:05:41 | DEBUG | urllib3.connectionpool:393 | http://127.0.0.1:53672 "DELETE /session/87b6f1ed3710173eff8037447e2b8f56 HTTP/1.1" 200 72
14:05:41 | DEBUG | selenium.webdriver.remote.remote_connection:440 | Finished Request
14:05:41 | INFO | dash.testing.application_runners:80 | killing the app runner
14:05:41 | DEBUG | urllib3.connectionpool:205 | Starting new HTTP connection (1): localhost:8050
14:05:41 | DEBUG | urllib3.connectionpool:393 | http://localhost:8050 "GET /_stop-3ef0e64e8688436caced44e9f39d4263 HTTP/1.1" 200 29

Selenium Snapshots

If you run your tests with CircleCI dockers (locally with CircleCI CLI and/or remotely with CircleCI). Inside a docker run or VM instance where there is no direct access to the video card, there is a known limitation that you cannot see anything from the Selenium browser on your screen. Automation developers use Xvfb as a workaround to solve this issue. It enables you to run graphical applications without a display (e.g., browser tests on a CI server) while also having the ability to take screenshots. We implemented an automatic hook at the test report stage, it checks if a test case failed with a Selenium test fixture. Before tearing down every instance, it will take a snapshot at the moment where your assertion is False or having a runtime error. refer to Browser APIs Note: you can also check the snapshot directly in CircleCI web page under Artifacts Tab

Percy Snapshots

There are two customized pytest arguments to tune Percy runner: 1. --nopercyfinalize disables the Percy finalize in dash fixtures. This is required if you run your tests in parallel, then you add an extra percy finalize --all step at the end. For more details, please visit Percy Documents. 2. --percy-assets lets Percy know where to collect additional assets such as CSS files.