Flask Configuration & Application Factory

⚙️ Basic config

app.config["SECRET_KEY"] = "value"
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///app.db"
app.config["DEBUG"] = True

📄 Config classes

# config.py
import os
 
class Config:
    SECRET_KEY = os.environ.get("SECRET_KEY", "dev-key-change-me")
    SQLALCHEMY_TRACK_MODIFICATIONS = False
 
class DevelopmentConfig(Config):
    DEBUG = True
    SQLALCHEMY_DATABASE_URI = "sqlite:///dev.db"
 
class ProductionConfig(Config):
    DEBUG = False
    SQLALCHEMY_DATABASE_URI = os.environ.get("DATABASE_URL")
 
class TestingConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = "sqlite:///:memory:"

app.config.from_object(DevelopmentConfig)

.env file

pip install python-dotenv

from dotenv import load_dotenv
load_dotenv()

# .env
SECRET_KEY=genuinely-long-random-string
DATABASE_URL=postgresql://user:pass@localhost/mydb

Never commit .env. Add to .gitignore. Commit a .env.example instead.

🏭 Application factory pattern

# app/__init__.py
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
 
db = SQLAlchemy()   # created without app
 
def create_app(config_class=DevelopmentConfig):
    app = Flask(__name__)
    app.config.from_object(config_class)
 
    db.init_app(app)   # initialized separately
 
    from app.blog.routes import blog_bp
    app.register_blueprint(blog_bp, url_prefix="/blog")
 
    return app

# run.py
from app import create_app
app = create_app()
 
if __name__ == "__main__":
    app.run(debug=True)

Why bother: lets you spin up a fresh, differently-configured app per test/environment. Essential for testing (see Flask Testing & Deployment).

🗂️ Full factory-based layout

myproject/
├── run.py
├── config.py
├── requirements.txt
├── .env
├── .gitignore
└── app/
    ├── __init__.py
    ├── models.py
    ├── blog/
    │   ├── __init__.py
    │   ├── routes.py
    │   └── templates/blog/
    ├── auth/
    ├── templates/base.html
    └── static/

🌍 Selecting config by environment

import os
env = os.environ.get("FLASK_ENV", "development")
config_class = ProductionConfig if env == "production" else DevelopmentConfig
app = create_app(config_class)

🧾 Common config keys

Key	Purpose
SECRET_KEY	signs sessions/CSRF
SQLALCHEMY_DATABASE_URI	DB connection string
DEBUG	debug mode
TESTING	testing mode
SESSION_COOKIE_SECURE	HTTPS-only session cookie
PERMANENT_SESSION_LIFETIME	session duration
MAX_CONTENT_LENGTH	max request body size

🔗 Next

Flask Extensions & Middleware · Flask Testing & Deployment