Step 2: Enabling an extension

In step 1 you have created your first reusable package. Usually however, your packages will need access to e.g. a database backend, a cache etc. These features are enabled via extensions.

Flask-SQLAlchemy extension

Next up in our tutorial, we will enable the Flask-SQLAlchemy extension to provide database models for our application and reusable packages.

Here’s how the directory structure looks like:

myext/__init__.py  (empty)
myext/sqlalchemy.py

Flask-AppFactory enables extensions simply by calling a method setup_app(app) with the Flask application object (line 16). This pattern allows each extension to customize the Flask application as they see fit.

# myexts/sqlalchemy.py

import click
from flask_cli import with_appcontext
from flask.ext.sqlalchemy import SQLAlchemy
from flask_registry import ModuleAutoDiscoveryRegistry, RegistryProxy

db = SQLAlchemy()

models = RegistryProxy(
    'models',  # Registry namespace
    ModuleAutoDiscoveryRegistry,
    'models'   # Module name (i.e. models.py)
)

def setup_app(app):
    # Set default configuration
    app.config.setdefault(
        'SQLALCHEMY_DATABASE_URI',
        'sqlite:////tmp/test.db'
    )
    # Add extension CLI to application.
    app.cli.add_command(initdb)
    db.init_app(app)

@click.command()
@with_appcontext
def initdb():
    """Initialize database."""
    db.create_all()

Each extension can set default configuration (line 18). The extension should however not overwrite an existing value, hence app.config.setdefault is used. This is because any instance specific configuration has already been loaded at this point.

By default Flask-AppFactory only loads configuration, blueprints and CLI from each reusable package. Each enabled extension can however load further modules from each package. E.g. in the Flask-SQLAlchemy case, we may want to load database models from a models.py in all enabled reusable packages. This is achieved by creating a registry (line 10).

Each extension can natually also provide CLI commands (see line 26-30).

Using Flask-SQLAlchemy in packages

Now that the extension has been enabled, you’ll want to use it in your packages. E.g. you could add a models.py file to the reusable package you created in step 1:

# mymodule/models.py

from myexts.sqlalchemy import db

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    email = db.Column(db.String(120), unique=True)