Flask

Flask is a lightweight, Python-based web framework. It was created by Armin Ronacher and released in 2010 as part of the Pocoo projects. Flask aims to keep the core small and extensible, providing the essential features for web development while letting developers add libraries and components as needed.

The framework is built on top of Werkzeug, a WSGI utility library, and Jinja2, a templating engine. It emphasizes a minimal core and explicit configuration, enabling developers to structure applications with blueprints and, when appropriate, an application factory pattern instead of imposing a fixed project layout.

Key features include URL routing, request and response handling, a built-in development server with a debugger and automatic reloader, and a simple API surface for creating web interfaces. A typical Flask app creates an instance of the Flask class (for example, app = Flask(__name__)), defines routes with decorators, and uses render_template to produce HTML with Jinja2. Flask also provides session support via secure cookies.

Although Flask includes core utilities, it is intentionally unopinionated about databases, forms, and authentication. A large ecosystem of extensions—such as Flask-Login, Flask-WTF, Flask-Migrate, and Flask-SQLAlchemy—covers common needs. Blueprints enable modular applications, and the application factory pattern supports testing and scalable project structure.

Flask is licensed under the BSD license and maintained by the Pallets Projects. It is widely used for small to medium web applications and APIs, prototyping, and educational purposes. For production deployment, Flask apps are typically run behind a WSGI server such as Gunicorn or uWSGI.