First Flask App & Debug Mode

🐍 FlaskLesson 2Beginner

Getting started with Flask is incredibly fast. With just five lines of code, you can have a local development server running.

1 Writing Your First App

Python — app.py

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "Hello, World!"

if __name__ == "__main__":
    # Run server locally on port 5000 in debug mode
    app.run(debug=True)

2 Why Enable Debug Mode?

Auto-Reload: The server automatically restarts whenever you save changes to your code.
Interactive Debugger: If an exception occurs, an interactive debugger will display in the browser, letting you execute code dynamically at the error line.

Warning: Never enable debug mode in a production environment, as it allows arbitrary code execution on your server.

3 Code Challenge

Challenge: Install flask with pip install flask, create your first app.py script, and run it. Confirm that you can see "Hello, World!" by navigating to http://127.0.0.1:5000.