I love writing tests, mostly because I love refactoring, and testing makes me feel good about refactoring. However, testing web applications can be particularly hard because there is usually a lot more going on than in a math library where a test just means verifying that your add function correctly reproduces 1 + 1 = 2. Web apps have moving parts like database connections, servers, authentication, and request contexts. To further complicate matters, these components must work together with human input, and about the only thing predictable about user input is that if there is a way to break the app, a user will find it.

Recently, I’ve been writing a lot of Jasmine tests for Angular apps based in rxjs and ngrx (Redux), and I’ve found that mindset has bled over into my Python development as well. Something about the flow of setup/teardown with test fixtures resonates with me, and so it should be no surprise that I am a big fan of pytest when it comes to unit testing in Python. However, there is a bit of a split in the Flask community, as the official documentation recommends pytest, while the very popular flask-testing project favors unittest style tests with classes. I spent some time digging into the various ways to test Flask applications and have settled on a system that I like quite a bit and am now sharing with you.

The full code for this example project can be found on Github here

Test location

There are a number of schools of thought regarding where tests should be located. Most commonly tests are either a) placed alongside the code they are testing or b) lumped all together in a single tests/ directory. I have worked extensively with both practices, and I strongly advocate for placing test code alongside the production code. This has a number of advantages, a few of which I will mention:

    1. If you have a large project, it becomes progressively harder to find the test code for a piece of production code as you must essentially navigate two directory structures constantly.
    2. Placing test code alongside production code increases modularity. If I want to copy or remove an entire section of my app, I just have to do so for a single directory rather than hunting down many individual files.
    3. Testing gaps are more obvious when the test (or lack thereof) is right next to the production code. In a perfect world, for every widget.py there is a widget_test.py in the same location, and using this pattern makes it easy to see what lacks testing.
import pytest

from demo import create_app

def app():
    return create_app('test')

def client(app):
    return app.test_client()

def db(app):
    from demo import db
    with app.app_context():
        yield db

from demo.tests.fixtures import app

def test_app_creates(app):
    assert app