How can I test a custom Flask error page?

Question

I'm attempting to test a custom error page in flask (404 in this case).

I've defined my custom 404 page as such:

@app.errorhandler(404)
def page_not_found(e):
    print "Custom 404!"
    return render_template('404.html'), 404

This works perfectly when hitting an unknown page in a browser (I see Custom 404! in stdout and my custom content is visible). However, when trying to trigger a 404 via unittest with nose, the standard/server 404 page renders. I get no log message or the custom content I am trying to test for.

My test case is defined like so:

class MyTestCase(TestCase):
    def setUp(self):
        self.app = create_app()
        self.app_context = self.app.app_context()
        self.app.config.from_object('config.TestConfiguration')
        self.app.debug = False # being explicit to debug what's going on...
        self.app_context.push()
        self.client = self.app.test_client()

    def tearDown(self):
        self.app_context.pop()

    def test_custom_404(self):
        path = '/non_existent_endpoint'
        response = self.client.get(path)
        self.assertEqual(response.status_code, 404)
        self.assertIn(path, response.data)

I have app.debug explicitly set to False on my test app. Is there something else I have to explicitly set?

Answer 1

After revisiting this with fresh eyes, it's obvious that the problem is in my initialization of the application and not in my test/configuration. My app's __init__.py basically looks like this:

def create_app():
    app = Flask(__name__)
    app.config.from_object('config.BaseConfiguration')
    app.secret_key = app.config.get('SECRET_KEY')
    app.register_blueprint(main.bp)
    return app

app = create_app()

# Custom error pages

@app.errorhandler(404)
def page_not_found(e):
    return render_template('404.html'), 404

Notice that the error handler is attached to @app outside of create_app(), the method I'm calling in my TestCase.setUp() method.

If I simply move that error handler up into the create_app() method, everything works fine... but it feels a bit gross? Maybe?

def create_app():
    app = Flask(__name__)
    app.config.from_object('config.BaseConfiguration')
    app.secret_key = app.config.get('SECRET_KEY')
    app.register_blueprint(main.bp)

    # Custom error pages
    @app.errorhandler(404)
    def page_not_found(e):
        return render_template('404.html'), 404

    return app

This ultimately answers my question and fixes my problem, but I'd love other thoughts on how to differently register those errorhandlers.