Flask - Part 2


This version_2 will explain how to structure your project to easily integrate your models, templates, ORM … and of course unit tests.

For this version_2, we will code the same app as version_1, no big deal.

1 - How to structure our app

To organize our routes, let’s start again from scratch :

# assuming you're in flask_learning
mkdir my_app_v2
cd my_app_v2
mkdir tests
mkdir app

initialize our venv :

# assuming you're in flask_learning/my_app_v2
virtualenv venv -p python3
source venv/bin/activate
# (venv)
pip install flask pytest
pip freeze > requirements.txt

and now lay down our achitecture :

# assuming you're in flask_learning/my_app_v2 (venv)
touch wsgi.py
touch app/__init__.py
mkdir app/api_v1
touch app/api_v1/__init__.py
touch app/api_v1/hello.py

With this architecture, we can easily update our api without breaking backward-compatibility.

$ my_app_v2
├── .editorconfig
├── app
│   ├── api_v1 
│   │   ├── __init__.py
│   │   └── hello.py 
│   └── __init__.py 
├── requirements.txt
├── tests
│   └── conftest.py 
├── venv
└── wsgi.py

2 - Creating our app

2.1 - Application factory

application factory is the file that contains the function create_app which returns our app.

Our application factory is app/__init__.py

# app/__init__.py
# application factory

from flask import Flask

def create_app():
    app = Flask(__name__)

    from .api_v1 import root_blueprint

    return app

Speaking about WSGI server, to facilitate the launch of our app, we created earlier a wsgi.py file. This file unique goal is to instantiate our app so flask can run it (yes that’s 2 lines).

# wsgi.py

from app import create_app

app = create_app()

2.2 - Our first API

Let’s create declate our root_blueprint in app/api_v1/__init__.py.

# app/api_v1/__init__.py

from flask import Blueprint

root_blueprint =  Blueprint('root', __name__)

# Import any endpoints here to make them available
from . import hello

2.3 - Our first route

Let’s code our fiirst route in api_v1/hello.py:

# app/api_v1/hello.py

from . import root_blueprint

@root_blueprint.route('/', methods=['GET'])
def hello_world():
    return 'Hello, World!'

As we saw earlier, this route belong to root_blueprint. This route will be accessible at /.

2.4 - Testing our app

It is important (crucial) to test your app.

Testing may seem difficult or boring but if you adopt unit-tests early, it will become a habit to create them.

First we need to create a client that will be injected in all our tests. Let’s create a file named conftest.py :

# assuming you're in flask_learning/my_app_v2 (venv)
touch tests/conftest.py

and add :

# tests/conftest.py

import pytest

from app import create_app

def global_data():
    return dict()

def client():
    test_app = create_app()
    test_app.config['TESTING'] = True
    client = test_app.test_client()
    yield client

Those 2 functions will be executed only once and they will inject their data inside every other test cases.

global_data allow us to share data between tests, and client represents the test_client to query our API.

2.4.1 - Quick thing about pytest scopes

and some useful links :

Then we create tests/test_root.py file. Every file need to be named test_*.py to be automaticaly discovered.

# assuming you're in flask_learning/my_app_v2 (venv)
touch tests/test_root.py

In this file we add our test :

# tests/test_root.py

def test_root_endpoint(client):
    rv = client.get('/')
    assert rv.status_code == 200
    assert b'Hello, World!' in rv.data

To run every test at once, we run this command:

# assuming you're in flask_learning/my_app_v2 (venv)
python -m pytest tests/

If everything went great, you should see this :

v2 unit testing

2.5 - Launching our app

To launch our app :

# assuming you're in flask_learning/my_app_v2 (venv)
FLASK_ENV=development FLASK_APP=wsgi.py flask run --host= --port=5000

v2 httpie example

v2 postman example

v2 postman example

Working great :-)


If you’re stuck or don’t understand something, feel free to drop me an email / dm on twitter / a comment below. You can also take a look at flask_learning/flask_cybermooc/version_2 to see the reference code. And use run.sh to launch it.

Otherwise, congratulations ! You just learned a lot of things :-)

Unit testing your app, creating a blueprint… Your app is now designed to be easily extended in the (near) future.

You’re now ready to go to part 3 to learn how to connect your app with a database.