Application Factory Pattern

The application factory pattern builds the app in a function:

• create_app()

This is the recommended pattern for larger Flask apps.

Why use a factory?

• enables easy testing (create a test app)
• supports multiple environments (dev/test/prod)
• avoids import-time side effects
• helps extensions initialize consistently

Example structure

text

myapp/
  __init__.py
  auth/
    routes.py
  models.py
config.py
wsgi.py

create_app example

myapp/__init__.py:

python

from flask import Flask
from .extensions import db, login_manager
from .auth.routes import auth_bp
 
 
def create_app(config_object="config.DevelopmentConfig"):
    app = Flask(__name__)
    app.config.from_object(config_object)
 
    db.init_app(app)
    login_manager.init_app(app)
 
    app.register_blueprint(auth_bp, url_prefix="/auth")
 
    return app

wsgi entrypoint

wsgi.py:

python

from myapp import create_app
 
app = create_app()

Now you can run:

• gunicorn "wsgi:app"

Key takeaway

Factories make your app predictable and keep global state under control.

Visualize it

Here’s the sequence of steps create_app() runs to build a fully configured Flask app.

diagram Application factory flow mermaid

  flowchart TD
  A["create_app()"] --> B["Create Flask app"]
  B --> C["Load config"]
  C --> D["Initialize extensions (db, login_manager)"]
  D --> E["Register blueprints"]
  E --> F["Return app"]

create_app() builds the Flask app, loads config, initializes extensions, registers blueprints, then returns the app