Serving a web page

Starting on line (8), we use the Flask  @app.route() decorator to define a callback method that is invoked when the server receives an HTTP GET request from a client to the root URL /, that is, a request to http://localhost:5000:

# @app.route applies to the core Flask instance (app).
# Here we are serving a simple web page.
@app.route('/', methods=['GET'])                           # (8)
def index():
   """Make sure index_api_client.html is in the templates folder
   relative to this Python file."""
   return render_template('index_api_client.html',
                          pin=LED_GPIO_PIN)                # (9)

On line (9), render_template('index_api_client.html', pin=LED_GPIO_PIN) is a Flask method use to return a templated page to the requesting client. The pin=LED_GPIO_PIN parameter is an example of how to pass a variable from Python to the HTML page template for rendering. We will...