Flask 3 cheatsheet

Flask 3: Blueprints, database and CORS

5 common Flask 3 tasks.

Group routes in a blueprint

Split a large app into modules and register them with the app.

# users.py
bp = Blueprint("users", __name__, url_prefix="/users")

@bp.get("/")
def list_users():
    return []

# app.py
app.register_blueprint(bp)

Use an application factory

Create the app in a function, which makes testing and configuration easier.

def create_app():
    app = Flask(__name__)
    app.config.from_prefixed_env()
    app.register_blueprint(bp)
    return app

Add SQLAlchemy

Install the extension and bind it to the app.

pip install flask-sqlalchemy

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///app.db"
db.init_app(app)

Manage migrations

Create and apply database migrations with Flask-Migrate.

pip install flask-migrate
flask --app app db init
flask --app app db migrate -m "Add users"
flask --app app db upgrade

Allow cross-origin requests

Let a browser app on another origin call your API.

pip install flask-cors

from flask_cors import CORS

CORS(app, origins=["https://app.example.com"])