How do you create a basic Flask application?

Create an instance of the Flask class, then map URLs to view functions with the @app.route() decorator.

from flask import Flask

app = Flask(__name__)

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

if __name__ == '__main__':
    app.run(debug=True)

Flask(__name__) creates the application instance, the object a web server calls for each request. The @app.route('/') decorator binds / to hello_world, and the returned string becomes the response body.

app.run(debug=True) starts the development server when you run the file directly. From the terminal, flask --app hello run --debug does the same, where hello is the module name. Use the development server and its debugger only on your own machine, never in production.