Our application will be a single Python file. Create a directory in your home directory called firstapp and a file inside this called hello.py. In the hello.py file, we'll write code to serve a web page comprising the static string "Hello, World!". The code looks as follows:

from flask import Flask

app = Flask(__name__)

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

if __name__ == '__main__':
    app.run(port=5000, debug=True)

Let's break down what this does. The first line should be familiar; it simply imports Flask from the package flask. The second line creates an instance of the Flask object using our module's name as a parameter. Flask uses this to resolve resources, and in complex cases, one can use something other than __name__ here. For our purposes, we'll always use __name__, which links our module to the Flask object.

Line 3 is a Python decorator. Flask uses decorators for URL routing, so this line of code means that the function directly below it should be called whenever a user visits the main root page of our web application (which is defined by the single forward slash). If you are not familiar with decorators, these are beautiful Python shortcuts that seem a bit like black magic at first. In essence, they call a function that takes the function defined under the decorator (in our case, index()) and returns a modified function.

The next two lines should also seem familiar. They define a very simple function that returns our message. As this function is called by Flask when a user visits our application, the return value of this will be what is sent in response to a user who requests our landing page.

Line 6 is a Python idiom with which you are probably familiar. This is a simple conditional statement that evaluates to True if our application is run directly. It is used to prevent Python scripts from being unintentionally run when they are imported into other Python files.

The final line kicks off Flask's development server on our local machine. We set it to run on port 5000 (we'll use port 80 for production) and set debug to True, which will help us see detailed errors directly in our web browser.

To run our development web server, simply fire up a terminal and run the hello.py file. If you used the same structure outlined in the previous section, the commands will be as follows:

cd firstapp/hello
python hello.py

You should get an output similar to that in the following screenshot:

Also, you should see the process continue to run. This is our web server listening for requests. So, let's make a request!

Fire up a web browser—I use Firefox, which comes packaged with Ubuntu—and navigate to localhost:5000.

The localhost part of the URL is a shortcut to the loopback address, usually 127.0.0.1, which asks your computer to make the web request to itself. The number after the colon (5000) is the port it should make the request to. By default, all HTTP (web) traffic is carried over port 80. For now, we'll use 5000 as it is unlikely to conflict with any existing services, but we'll change over to port 80 in production, which is conventional, so that you won't have to worry about the colon.

You should see the "Hello, World!" string displayed in your browser as in the following screenshot. Congratulations, you've built your first web application using Flask!