Flask 3 Cheatsheet

Setup and CLI

• Install Flask — Install Flask into the active virtual environment.
• Hello world app — The smallest Flask application, saved as app.py.
• Run the development server — Start the app with the debugger and auto-reload. Never use debug mode in production.
• Run on another host or port — Listen on all interfaces on port 8000.
• List the routes — Show all URL rules and their endpoints.
• Open a shell with the app loaded — Start a Python shell with the application context active.
• Add a CLI command — Add your own command, run with flask --app app seed.

Routes and requests

• Capture part of the URL — Convert the value to an integer with a converter.
• Handle several methods — Accept GET and POST on one route.
• Read query string values — Read ?page=2 with a default.
• Read a JSON body — Parse the JSON request body.
• Return JSON — Return a dict or list and Flask converts it to JSON, with a status code.
• Read form fields — Read submitted form values.
• Redirect to another route — Send the user to another endpoint by name.
• Return an error — Stop the request with an HTTP error code.
• Customize an error page — Return a custom response for 404 errors.
• Run code before every request — Run a function before each request, for example to load the current user.

Templates, sessions and config

• Render a template — Render templates/index.html with variables. Jinja escapes values by default.
• Loop in a template — Use Jinja control structures in the template.
• Share a layout — Define blocks in a base template and fill them in child templates.
• Link to a static file — Build the URL of a file in the static folder.
• Remember a user between requests — Store values in the signed cookie session. It needs a secret key.
• Load configuration — Load settings from an object and the environment.
• Show a one-time message — Queue a message to show on the next page.

Blueprints, database and CORS

• Group routes in a blueprint — Split a large app into modules and register them with the app.
• Use an application factory — Create the app in a function, which makes testing and configuration easier.
• Add SQLAlchemy — Install the extension and bind it to the app.
• Manage migrations — Create and apply database migrations with Flask-Migrate.
• Allow cross-origin requests — Let a browser app on another origin call your API.

Testing and deployment

• Test with the test client — Call routes without running a server.
• Create a pytest fixture — Make a test client available to tests.
• Run with Gunicorn — Serve the app with a production WSGI server.
• Trust a reverse proxy — Read the real client IP and scheme when behind Nginx.

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