Flask

A small, explicit Python web framework.

Flask is taught here in 12 lessons, running from Your first Flask app through to Deployment: gunicorn, nginx and Docker. Each lesson takes one topic, shows the working code, and links onward to the next, so the course can be read straight through in order.

Lessons

1. Your first Flask appRoutes, view functions, the development server, and how Flask decides which function handles a request.
2. Templates with Jinja2Passing data into templates, inheritance with blocks, escaping (and the one line that disables it).
3. Forms, sessions and errorsReading request data, validating it, flashing messages, keeping a session, and returning sensible error responses.
4. Routing in depth: converters, methods and url_forUse URL converters well, handle HTTP methods and trailing slashes deliberately, and build every link from an endpoint name instead of a hard-coded path.
5. Blueprints and the application factorySplit a growing app into blueprints, build the app inside create_app() for testability, and initialise extensions without circular imports.
6. Configuration and project structureOrganise config as classes, read secrets from the environment, keep machine-specific files in the instance folder, and make development and production differ only where they must.
7. Databases with Flask-SQLAlchemyDefine models, query them through the session, handle transactions without leaving the session broken, and avoid the lazy-loading traps that make pages slow.
8. Schema migrations with Flask-MigrateInitialise Alembic through Flask-Migrate, review the scripts autogenerate produces, and apply schema and data changes in a controlled order.
9. Authentication with Flask-Login and password hashingGive users real passwords with Werkzeug hashing, keep them signed in with Flask-Login, protect routes, and gate features by role without breaking open redirects.
10. Building and testing a JSON APIReturn consistent JSON with honest status codes, then test the contract with the test client, fixtures and a database that is not the one you develop on.
11. File uploads, static files and sending emailAccept uploads without writing an attacker-controlled filename, cache static assets correctly, and send mail without blocking the request that triggered it.
12. Deployment: gunicorn, nginx and DockerRun Flask under a real WSGI server, put nginx in front for TLS and static files, and containerise the app without baking secrets into the image.

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Last refreshed 2026-09-18.