Flask Fundamentals

Flask is a lightweight WSGI web application framework for Python. It creates an application object that maps HTTP requests to Python view functions and turns their return values into HTTP responses. Flask builds on Werkzeug for WSGI and request handling, Jinja for templates, and Click for command-line integration.

The word micro describes Flask's core scope, not an upper limit on application size. Flask provides routing, request and application contexts, templates, sessions, configuration, error handling, testing tools, and a command-line interface. A database layer, authentication policy, form library, background worker, and deployment topology remain explicit choices. That makes Flask useful when a team wants a small HTTP core and can make those boundaries deliberately.

Follow one request

A production WSGI server calls the Flask application for an incoming request. Flask matches the request URL and method against registered rules, pushes the application and request contexts, runs request hooks, calls the selected view, converts its result to a response, runs response hooks, and removes the contexts. Errors can select registered error handlers before a response leaves the application.

client -> WSGI server -> Flask router -> request hooks -> view
                                      -> response hooks -> HTTP response

A route usually comes from a decorator. The decorator registers a URL rule and endpoint name against a view function. By default, a route accepts GET. Add explicit methods when the endpoint changes state or handles another HTTP method. Variable URL segments pass values into the view. Use url_for to build URLs from endpoint names instead of copying paths into templates; it keeps links coupled to the routing map.

Flask Fundamentals

Follow one request

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