Web Apps


Flask is a lightweight and powerful web framework for Python. It’s easy to learn and simple to use, allowing you to build your web app in a short amount of time.

In this toolbox, you’ll how to build a simple website, containing two static pages with a small amount of dynamic content. While Flask can be used for building complex, database-driven websites, starting with mostly static pages will be useful to introduce a workflow, which you can then generalize to make more complex pages in the future. Upon completion, you’ll be able to use this sequence of steps to jump-start your next Flask app.

Get Set

GitHub Classroom invite

In this toolbox, you’ll be learning Flask. To do this, you’ll first need to install Flask. Run the following command:

$ conda install Flask

This toolbox exercise was developed by Patrick Huston.

What is Flask, really?

In the introduction, we defined Flask as a “web framework”, but what does that actually mean? Let’s dig deeper. Before this, let’s develop a better understanding of how the internet works.

When you open up a web page in your browser (e.g. Chrome, Firefox, etc.), it makes an HTTP request to a server somewhere in the world. This could be something like GET me the home page. This server handles this request, sending back data (this can be in the form of HTML, JSON, XML, etc.), which is rendered by your browser.

This is where Flask comes in - it allows you to create the logic to make a web server quickly in Python. You can write logic that will execute when a request is made for one of your routes (e.g. www.mycoolwebsite.com/home)

Hello World in Flask

Let’s write some Flask. A simple Hello World application written in Flask can be as simple as this:

from flask import Flask
app = Flask(__name__)

def hello_world():
    return 'Hello World!'

if __name__ == '__main__':

Write this, save it as something like hello.py, and run it in terminal. Make sure to not call your application flask.py because this would conflict with Flask itself.

$ python hello.py
 * Running on

Now head over to, and you should see your hello world greeting.

In case you’re curious points to your local computer, and 5000 is the port it’s listening on.

What did that actually do? Let’s walk through the steps.

  1. First we imported the Flask class.
  2. Next we create an instance of this class. The first argument is the name of the application’s module or package. If you are using a single module (as in this example), you should use __name__ because depending on if it’s started as application or imported as module the name will be different (‘__main__' versus the actual import name). This is needed so that Flask knows where to look for templates, static files, and so on.
  3. We then use the route() decorator to tell Flask what URL should trigger our function. In this case our route is /, commonly referred to as the index of a page.
  4. The function is given a name which is also used to generate URLs for that particular function, and returns the message we want to display in the user’s browser.
  5. Finally we use the run() function to run the local server with our application. The if __name__== '__main__' makes sure the server only runs if the script is executed directly from the Python interpreter and not used as an imported module.

To stop the server, hit ctrl+c.

What’s this routing business?

In our hello world, example, we had one route denoted by the decorator @app.route('/'). Again, this ‘decorator’ tells the Flask app that any incoming requests for GET / will run the function we called hello_world().

Here are a couple more quick examples -

def index():
    return 'Index Page'

def hello():
    return 'Hello World'

Pretty simple, right? What happens when we want to do something useful - e.g. display something other than text?

Let’s serve some HTML!

HTML (HyperText Markup Language) is the standard markup language used to create web pages. In addition to sending back strings, Flask can send back HTML files to the client, which will be rendered in the browser. Let’s get to work creating a basic HTML document.

Let’s start by creating a file called index.html:

<!DOCTYPE html>
            A Small Hello
    <p>This is very minimal "hello world" HTML document.</p>

Create a new directory called templates, and save this new document as index.html there.

Let’s figure out how to show this document now.

Rendering pages

To render a template you can use the render_template() method. All you have to do is provide the name of the template and the variables you want to pass to the template engine as keyword arguments. Here’s a simple example of how to render a template:

from flask import render_template

def hello(name=None):
    return render_template('hello.html', name=name)

And here is an example template that will work with the above snippet:

<!doctype html>
<title>Hello from Flask</title>
{% if name %}
    <h1>Hello {{ name }}!</h1>
{% else %}
    <h1>Hello World!</h1>
{% endif %}

You may have noticed something really cool that happened here. In our route /hello/<name>, we’re allowing someone to make a request with an additional ‘name’ parameter that can be anything. We can then use this name and render it in our HTML template hello.html. We use the {___} syntax to insert outside variables into the template. Additionally, we can insert pythonic flow logic directly into our HTML page – see {% if name %}. We could go on for years about all of the power of jinja templating, but I’ll leave that joy to this wonderful article.

Getting back to our simple Hello World app, let’s add in a route to display our index.html we created above.

from flask import Flask
app = Flask(__name__)

def hello_world():
    return render_template('index.html')

if __name__ == '__main__':

And that’s it! Again, follow the instructions

$ python hello.py
 * Running on

to run the application, and head over to, and you should see your hello world greeting. It might not look very different, but you’re now working with a much more powerful format of representing information through HTML.

Build your own app: getting input from the user

What use is a web application if you can’t get any data back from the user? Let’s set up a simple app. Here are our end specifications:

  1. Upon visiting the index page at, the user will be greeted by a page that says hello, and includes an input form that requests their name, age, and favorite SoftDes Ninja.
  2. Upon clicking the ‘Submit’ button, the data from the form will be sent via a POST request to the Flask backend at the route POST /login.
  3. The Flask backend will handle the request to POST /login and perform some simple validation on the user input - simply check to see if they exist.
  4. If all the information is present, the app will render a ‘profile’ page for the user - presenting their name and age. Regardless of their input for final question, and regardless of whether Patrick is a SofDes Ninja, the app will display Patrick Huston
  5. If all the information is not present, the app will render a simple error page, which will include some indication that they didn’t include all the required information, in addition to a button that will redirect the user back to the home page.

It will be up to you to make this happen. If you feel confident in your ability to implement this, go for it! If you’d like more scaffolding, continue reading.

Tips and tricks

To complete this exercise, the official Flask documentation will get you pretty far. There is the full documentation and the quickstart guide, both of should be pretty helpful.

  1. HTML Forms To make forms in HTML, check out this resource. For even more information, check this out.
  2. Sending POST Requests To send the data from the form in a POST request, use an input with type submit, and set the action of the form to reflect the destination in your routes.
  3. Handling POST Requests To learn more about handling post requests in Flask, check this resource from the Flask documentation out.
  4. + 5. Accessing the Form Data To access the form data, check out this documentation on using the Flask request utility.

What to turn in

Push the web app you developed in the previous section to GitHub.

Going further

  1. Learn more about Django – an alternative to Flask. They don’t have many major differences other than some small quirks in conventions and style. See here for more analysis.
  2. Want to keep track of some data in your web app? Instead of using a .txt file or a pickle file, it’s common practice in nearly any web app to use a database. A few especially well-known database choices are MySql, SQLite, or PostgreSQL (which all use Structured Query Language to manipulate stored data, as do many other common relational databases) You also may have heard some buzz about MongoDB, which uses an unstructured data format in documents similar to JSON. Mongo is stupidly easy to set up and use, but I’d stop and think first before jumping right in. It may be the easy choice, but representing your data intelligently in a relational table can be much more effective and less of a headache later on.
  3. But HTML is so ugly! HTML alone is very ugly. That’s why we use CSS (Cascading Style Sheets) to add some extra flair and style to our HTML. You can change pretty much anything about HTML - colors, shapes, sizes, placement, etc. with CSS rules. It’s also pretty simple to write. Check this resource out to learn more about CSS.
  4. What about making my website dynamic? SoftDes may be a class in Python, but we can venture out a little and use some jQuery. jQuery might seem scary, but you use it in a way similar to adding/linking CSS styling to your HTML. You write scripts in JavaScript (which isn’t too difficult), which can allow you to add beautiful responsive and dynamic content to your web app.
  5. You can also use D3 to display graphical data in a web page.
  6. Deploying to the cloud. Want to run your application on a server in the cloud, with a public URL? One option to do that easily is Heroku (Note: toolbox available upon request).