Flask + Coverage Analysis

Flask + Coverage Analysis

This post demonstrates a simple web application written in Flask with coverage analysis for the tests. The main idea should be pretty translatable into most Python web application frameworks.


I've updated this scheme and described the update here.


If you're having difficulty getting coverage analysis to work with Flask then have a look at my example repository. The main take away is that you simply start the server in a process of its own using coverage to start it. However, in order for this to work you have to make sure you can shut the server process down from the test process. To do this we simply add a new "shutdown" route which is only available under testing. Your test code, whether written in Python or, say Javascript, can then make a request to this "shutdown" route once it completes its tests. This allows the server process to shutdown naturally and therefore allow 'coverage' to complete.


It's a good idea to test the web applications you author. Most web application frameworks provide relatively solid means to do this. However, if you're doing browser automated functional tests against a live server I've found that getting coverage to work to be non-trivial. A quick search will reveal similar difficulties such as this stack overflow question, which ultimately points to the coverage documentation on sub-processes.

Part of the reason for this might be that the Flask-Testing extension provides live server testing class that starts your server in testing mode as part of the start-up of the test. It then also shuts the server process down, but in so doing does not allow coverage to complete.

A simpler method is to start the server process yourself under coverage. You then only need a means to shutdown the server programatically. I do this by adding a shutdown route.

Example repository

If you just wish to look at the code check out the example repository.

The README should explain how to work this but roughly:

    $ git clone https://github.com/allanderek/flask-coverage-example.git
    $ cd flask-coverage-example
    $ . setup.fish # or source setup.sh
    $ python manage.py test

You should then be able to look in the htmlcov directory to see the source marked-up with all the lines ran. Specially open the file htmlcov/app_main_py.html.

To see an example of a missing line, locate the lines:

        check_fraction(50, 100, '50 of 100 is 50%')
        check_fraction(20, 30, '20 of 30 is 66%')
        check_fraction(50, 10, 'Invalid: Fraction greater than Total')
        check_fraction(0, 0, '0 of 0 is 0%')

Comment out some of them, say the bottom two, re-run python manage.py test and reload htmlcov/app_main_py.html in your browser.

Server process

Update: The code in this section has been updated in the update post and in the example repository.

In the example repository if you look in the manage.py file you'll see the code to start the server process. This is in the run_with_test_server function which starts the server and an additional process, that should be the process that you use to actually test the server. This process can be anything you like such as, in the example's case pytest app/main.py, or it could be an external call to casperJS instance if you want to write your browser tests in Javascript.

However, the main points are:

  • Starting the server process under coverage analysis
  • Waiting for the server process to indicate that it has started listening
  • Then starting your test process
  • Waiting for both processes to complete.
  • Finally running coverage report and coverage html

In a simpler form to that in the example repository this would be:

    import subprocess
    server_command = ['coverage', 'run', '--source', 'app.main',
                      'manage.py', 'run_test_server']
    server = subprocess.Popen(server_command, stderr=subprocess.PIPE)
    for line in server.stderr:
        if line.startswith(b' * Running on'):
    test_command = ['pytest', 'app/main.py']
    test_process = subprocess.Popen(test_command)
    server_return_code = server.wait(timeout=60)
    os.system("coverage report -m")
    os.system("coverage html")
    return server_return_code

Closing down the server

Notice that the above waits for the server process to end. Ordinarily the server process has to be killed since it expects to run indefinitely. To solve that we add a shutdown route. This is done in manage.py:

def shutdown():
    """Shutdown the Werkzeug dev server, if we're using it.
    From http://flask.pocoo.org/snippets/67/"""
    func = flask.request.environ.get('werkzeug.server.shutdown')
    if func is None:  # pragma: no cover
        raise RuntimeError('Not running with the Werkzeug Server')
    return 'Server shutting down...'

def run_test_server():
    """Used by the phantomjs tests to run a live testing server"""
    # running the server in debug mode during testing fails for some reason
    application.config['DEBUG'] = True
    application.config['TESTING'] = True
    port = application.config['TEST_SERVER_PORT']
    # Don't use the production database but a temporary test database.
    application.config['SQLALCHEMY_DATABASE_URI'] = "sqlite:///test.db"

    # Add a route that allows the test code to shutdown the server, this allows
    # us to quit the server without killing the process thus enabling coverage
    # to work.
    application.add_url_rule('/shutdown', 'shutdown', shutdown,
                             methods=['POST', 'GET'])

    application.run(port=port, use_reloader=False, threaded=True)


This code mostly speaks for itself. The shutdown method is obviously the one we wish to call when the tests are done. We add it to the application route handler via the call to application.add_url_rule. An alternative is to use a decorator to mark routes as 'test-only'. I quite like the method used here since it means the routes are only ever added at all when we start the test server.

Startup and Shutdown

For many test strategies you will want to do something before and after the server runs. In this example we setup the database to use a temporary database, in which we create all of the tables anew:

    # Don't use the production database but a temporary test database.
    application.config['SQLALCHEMY_DATABASE_URI'] = "sqlite:///test.db"

At the end of the test we remove the database session and drop all of the contents of the database.


In a real test-suite you may well have a way of factoring out this start-up and tear-down code, but I've left it as simple and test-framework agnostic for this example as I could.

At the end of your tests

Update: This section is now not required. See the update post.

Now for this shutdown to actually work, your test suite has to call access the shutdown route. There are many ways to write this and it will hugely depend on your test framework. However, in the example repository it is done via:


In the test_my_server method. Update: Not now it isn't as this is not required at all.


Go see the example repository.


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