21.1. What is Flask

Over the past few chapters, we used HTML and CSS to design static web pages. The content we put on a static page remains the same regardless of what visitors do. The only way to change the content or appearance of the page is to open up the code files and edit the HTML or CSS.

Dynamic web pages respond to user actions and update their appearance based on those actions. For example, think of a page that shows the menu for a local restaurant. A static version lets users scroll through the food choices and read the descriptions and prices. A dynamic version might let users click on items to order, choose how many servings of each, and display a running total for the cost of the order.

Static pages provide information for users to read. Dynamic pages allow users to interact with the content.


Adding a link to an HTML page does NOT make it dynamic. The content on the page itself remains fixed. The link just sends out a new HTTP request to retrieve a different HTML page.

21.1.1. Web Applications

A web application is a program that users run in their browsers. These applications can be simple or complex, but they usually combine code that runs on the client (HTML/CSS) with code that runs at a server (Python, Java, etc.). The front-end part of the application controls what users see in their browsers. The back-end part controls how data gets processed and transmitted.

Examples of web applications include online shopping spaces, email programs, video conferencing, and anything that includes a login page or a Submit button. All of these applications require code that works across multiple browsers AND controls how the server deals with HTTP requests/responses.

Fortunately, a lot of the nuts-and-bolts details behind building a web application have been automated. In our case, modules exist for Python that allow us to quickly set up a new web-based program.

21.1.2. A Web Framework

Flask is an example of a web framework. It is a collection of libraries, tools, and resources that take care of the low-level tasks required to make a web application work. This lets us focus on building our project instead of worrying about how the client and server actually talk to each other.

Flask also lets us test our applications in a browser. It sets up a web server on our personal device. We can then use that local server to try out our dynamic webpages. For example, instead of sending form data to the parrot server, we can use Flask to collect the data, perform actions with it, and then display new information on the page.


When the user submits this form, the webpage displays the original and modified text.

An interactive form that adds text to the screen when the user submits an entry.

Flask helps us develop interactive webpages.

In this chapter, we create some simple Flask applications to get a feel for how to use the framework.


In this course, we go over the basics of using Flask. If you are interested in going further, here are some useful resources to help you take the next steps:

  1. Flask documentation

  2. The Flask Mega-Tutorial

  3. What is Flask (from pythonbasics.org)