054-flask/docs/wiki/Lecture-2-Hello-Flask.md

Lecture 2: The first Flask server

English · Dansk · Bahasa Malaysia

What the lecture covers

A library is called by your code, but a framework calls your code and decides the structure. The lecture creates the smallest Flask application in a file named hello.py (never flask.py, which would hide the library), installs Flask with pip, points FLASK_APP at the file and starts the server with flask run. The browser then shows "Hello, World!" from the local address. Ctrl+C stops the server so that two applications do not fight over the same port.

What we gained

• The difference between a library and a framework.
• A working web server in six lines: app = Flask(__name__), a route for / and a function that returns text.
• How FLASK_APP tells Flask which file to run.
• A habit of testing the route with Flask's test client instead of a browser.

In this repository

Code: src/flask_day_54/presentation/hello.py (route hello_world), the use case application/get_greeting.py and the constant GREETING. Tests: tests/e2e/test_hello_app.py.