Your first Flask app

Flask is the minimal Python web framework — one file, three lines, and you have a server.

Anatomy

• Flask(__name__) — the app object. __name__ tells Flask where to find templates/static.
• @app.get("/path") / @app.post(...) — bind a URL to a Python function.
• <int:user_id> in the path — typed URL param, passed as function arg.
• request.get_json() — parse incoming JSON body.
• jsonify(...) — return a JSON response with correct Content-Type.

Run it

pip install flask
python app.py

Then hit http://localhost:5000/ in your browser.

debug=True

Auto-reloads on file change + prints tracebacks in the browser. Never leave it on in production — it exposes a Python REPL to anyone hitting the error page.

Where Flask fits

• Micro-services, internal tools, MVPs, prototypes.
• Not batteries-included — no built-in ORM, no admin panel. That's a feature; you pick your own stack.
• Grown-up alternative: FastAPI (next lesson) if you want async + auto-docs + type hints as validation.

Try it

1. Add a /health route that returns {"ok": True}.
2. Add /users (plural) that returns a list of 3 fake users.