14.6. More About Flask

When you executed the flaskhello.py program in the previous section and used a web browser to access it with the url http://localhost:5000/, in addition to seeing a “Hello, world!” message in the browser, you should also have observed a log message like the following in the console: - - [21/Apr/2016 08:02:28] "GET / HTTP/1.1" 200 -

Every time the Flask server receives a request from a browser, it writes a log message to the console. The message contains information such as the IP address of the computer that sent the request ( is a special address indicating the request came from the browser on the same computer that the Flask server is running on); the date and time of the request; the path of the incoming request (“/” in this case); and the status of the result (here, 200 indicates the request was successfully processed).

The Flask server continues running until you press Ctrl-C to stop it. At that point, if you try to send a request to the application from the browser, the browser will display an error message indicating that it cannot contact the server. Go ahead and try this, so you can recognize what the error message looks like in your particular browser.

Recall that every URL has at least three components: the protocol, server, and the path. In our case the URL http://localhost:5000/ has the server name localhost, the path /, and an additional component: the port number, 5000. Let’s discuss some details about each of these.

Server name

When you use the name localhost in a URL, the browser attempts to connect to a server program running on your computer. This is the usual scenario when you are developing a web application: the browser and the server application are both running on the same computer. When you deploy the application to be hosted on an actual server, you will use the name of the server in the URL instead of the name localhost. If you want to experiment with deploying Flask applications to a public web server, check out pythonanywhere.com, which (at the time of writing) provides free hosting for Flask web applications.

Port number

Each server application running on a computer is assigned a distinct port number so that clients can connect to it. Port numbers range from 0 to 65,535. Web servers generally are assigned port number 80, and when the URL does not contain a port number, the web browser attempts to connect to a web server listening on port 80. But the default port number for Flask applications is 5000, so the URL must include that port number. You can specify a different port number for your Flask application in the line that launches the Flask server like this:

app.run(host="localhost", port=5001, debug=True)

Here, the Flask server binds to port 5001, and you would need to use that port number instead of 5000 in the URL in the browser.


When the Flask receives an incoming request, it examines the path and uses it to determine which function in your program should be executed to handle the request and generate a response. A Flask application can contain one or more of these request handler functions, which are decorated by a line immediately preceding the function that looks like this:


The path in the quotes is matched to the path of the incoming request from the browser. If the incoming path from the browser does not match any of the handler function paths defined by @app.route() decorators, an error occurs. For example, try entering the following URL into your browser when the flaskhello.py program in the last section is running:

You will see an error message appear in the browser, and the log message that appears in the console will have the number 404 after the path, indicating that the path did not match, as shown below: - - [21/Apr/2016 08:02:51] "GET /blah HTTP/1.1" 404 -

Here’s another version of the flaskhello.py program that has two different pages. The first page displays a “Hello world” message and invites the user to click a link to view the time. When the user clicks the link, the time appears.

 1 from flask import Flask
 2 from datetime import datetime
 4 app = Flask(__name__)
 6 @app.route('/')
 7 def hello():
 8     return HELLO_HTML
10 HELLO_HTML = """
11     <html><body>
12         <h1>Hello, world!</h1>
13         Click <a href="/time">here</a> for the time.
14     </body></html>
15     """
17 @app.route('/time')
18 def time():
19     return TIME_HTML.format(datetime.now())
21 TIME_HTML = """
22     <html><body>
23         The time is {0}.
24     </body></html>
25     """
27 if __name__ == "__main__":
28     # Launch the Flask dev server
29     app.run(host="localhost", debug=True)

Here’s how it works:

  1. To begin, the user enters the URL http://localhost:5000, and the browser sends the request to the application. The Flask server matches that path “/” to the hello() function, invokes the function and returns the response to the browser.

  2. The user clicks the link, which triggers the browser to send a request with the URL http://localhost:5000/time to the Flask server. The server matches the path “/time” to the time() function, invokes the function and returns a response containing the time to the browser.

Note that the user does not have to click the link in order to display the time. For example, the user could enter the URL http://localhost:5000/time directly into the browser to bypass the greeting page and get directly to the page showing the time.

The example above used the format() method to build an HTML string. For more information on format(), see String Format Method.

Also, notice how the example above defines separate HELLO_HTML and TIME_HTML variables to hold the HTML. This helps reduce cluttering the handler functions with HTML code, and separating the Python logic from the HTML also improves the overall readability and maintainability of the code.

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