Flask is a lightweight web application framework for Python, developed by the Pallets organization. A web framework is a library that receives HTTP requests and calls the code that builds each response. Flask is a WSGI framework: WSGI is the standard interface between Python web applications and web servers.
A developer creates a Flask application object and attaches Python functions to URL routes with a decorator such as @app.route("/"); each function returns the page or data for that address. Flask builds on the Werkzeug WSGI toolkit, the Jinja template engine for HTML pages and the Click toolkit for its command-line interface, and it includes a development server, a debugger, sessions and blueprints for splitting a large application into parts. It does not impose any database or project layout: the developer chooses the libraries, community extensions add features, and the documentation shows patterns for SQLAlchemy databases and WTForms form validation.
Flask is free and open source under the BSD-3-Clause license.