Disable CSRF validation when unit-testing a Flask app!

In my current Flask project, I wanted to test a view method that includes a form with a POST request. But it wouldn't work.

The view method looks something like this:

# views.py
@tennis.route('/tournaments/add', methods=['GET', 'POST'])
def add_tournament():
    form = AddTournamentForm()
    
    if form.validate_on_submit():
        t = Tournament(name=form.name.data)
        db.session.add(t)
        db.session.commit()
        
        flash('A tournament was added to the database.')
        return redirect(url_for('tennis.index'))

    return render_template('tennis/add_tournament.html',
                       form=form)

Flask-Testing provides a TestCase class with several useful assert methods. So the test would look like this:

# test_views.py  
from flask_testing import TestCase

class TestTennisViews(TestCase):    
    […]   
      
    def test_tennis_add_tournament(self):
        # WHEN submitting correct form data to the add_tournament view
        r = self.client.post('/tennis/tournaments/add',
                             data=dict(
                                 name='Chicharito Open',
                             ))
    
        # THEN it redirects to the tennis overview page
        self.assertRedirects(r,'/tennis/')
        # and it shows a message
        self.assertMessageFlashed('A tournament was added to the database.')
        # and there is a tournament in the database
        assert Tournament.query.count() > 0
        # and it has the correct tournament
        t = Tournament.query.first()
        assert t.name == 'Chicharito Open'

assertRedirects checks if the status code is 301 or 302. In my tests, it was always 200, and the test failed. Of course, nothing was added to the database, either.

I googled a lot, without success. I tried all combinations of the following keywords, among others:

flask
unit test
wtforms
POST request
testing

The solution hit me when taking a break (it works!). It lies in the form.validate_on_submit() part of the view function. validate_on_submit() does two things:

1. It validates the form fields according to the validators specified in the form definition.
2. It validates a CSRF token.

Guess what: there is no CSRF token generated when executing a POST request directly in the test.

The solution is easy, as FLASK-WTF provides a configuration option to disable CSRF:

# config.py

[…]

class TestingConfig(Config):
    TESTING = True
    WTF_CSRF_ENABLED = False
    
    […]
    
class FunctionalTestingConfig(TestingConfig)
    WTF_CSRF_ENABLED = False

Of course, only disable CSRF in your test configuration! I created a separate configuration for my functional tests, so that CSRF is enabled in those. When using the browser with Selenium, the CSRF token is provided by the WTForms form on the web page.